rfc9700.original   rfc9700.txt 
Web Authorization Protocol T. Lodderstedt Internet Engineering Task Force (IETF) T. Lodderstedt
Internet-Draft SPRIND Request for Comments: 9700 SPRIND
Updates: 6749, 6750, 6819 (if approved) J. Bradley BCP: 240 J. Bradley
Intended status: Best Current Practice Yubico Updates: 6749, 6750, 6819 Yubico
Expires: 5 December 2024 A. Labunets Category: Best Current Practice A. Labunets
Independent Researcher ISSN: 2070-1721 Independent Researcher
D. Fett D. Fett
Authlete Authlete
3 June 2024 November 2024
OAuth 2.0 Security Best Current Practice OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-29
Abstract Abstract
This document describes best current security practice for OAuth 2.0. This document describes best current security practice for OAuth 2.0.
It updates and extends the threat model and security advice given in It updates and extends the threat model and security advice given in
RFC 6749, RFC 6750, and RFC 6819 to incorporate practical experiences RFCs 6749, 6750, and 6819 to incorporate practical experiences
gathered since OAuth 2.0 was published and covers new threats gathered since OAuth 2.0 was published and covers new threats
relevant due to the broader application of OAuth 2.0. Further, it relevant due to the broader application of OAuth 2.0. Further, it
deprecates some modes of operation that are deemed less secure or deprecates some modes of operation that are deemed less secure or
even insecure. even insecure.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Web Authorization
Protocol Working Group mailing list (oauth@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/oauth/.
Source for this draft and an issue tracker can be found at
https://github.com/oauthstuff/draft-ietf-oauth-security-topics.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This memo documents an Internet Best Current Practice.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 5 December 2024. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9700.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction
1.1. Structure . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Structure
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 5 1.2. Conventions and Terminology
2. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 6 2. Best Practices
2.1. Protecting Redirect-Based Flows . . . . . . . . . . . . . 6 2.1. Protecting Redirect-Based Flows
2.1.1. Authorization Code Grant . . . . . . . . . . . . . . 7 2.1.1. Authorization Code Grant
2.1.2. Implicit Grant . . . . . . . . . . . . . . . . . . . 8 2.1.2. Implicit Grant
2.2. Token Replay Prevention . . . . . . . . . . . . . . . . . 9 2.2. Token Replay Prevention
2.2.1. Access Tokens . . . . . . . . . . . . . . . . . . . . 9 2.2.1. Access Tokens
2.2.2. Refresh Tokens . . . . . . . . . . . . . . . . . . . 9 2.2.2. Refresh Tokens
2.3. Access Token Privilege Restriction . . . . . . . . . . . 9 2.3. Access Token Privilege Restriction
2.4. Resource Owner Password Credentials Grant . . . . . . . . 10 2.4. Resource Owner Password Credentials Grant
2.5. Client Authentication . . . . . . . . . . . . . . . . . . 10 2.5. Client Authentication
2.6. Other Recommendations . . . . . . . . . . . . . . . . . . 10 2.6. Other Recommendations
3. The Updated OAuth 2.0 Attacker Model . . . . . . . . . . . . 11 3. The Updated OAuth 2.0 Attacker Model
4. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 14 4. Attacks and Mitigations
4.1. Insufficient Redirect URI Validation . . . . . . . . . . 14 4.1. Insufficient Redirect URI Validation
4.1.1. Redirect URI Validation Attacks on Authorization Code 4.1.1. Redirect URI Validation Attacks on Authorization Code
Grant . . . . . . . . . . . . . . . . . . . . . . . . 14 Grant
4.1.2. Redirect URI Validation Attacks on Implicit Grant . . 16 4.1.2. Redirect URI Validation Attacks on Implicit Grant
4.1.3. Countermeasures . . . . . . . . . . . . . . . . . . . 17 4.1.3. Countermeasures
4.2. Credential Leakage via Referer Headers . . . . . . . . . 18 4.2. Credential Leakage via Referer Headers
4.2.1. Leakage from the OAuth Client . . . . . . . . . . . . 18 4.2.1. Leakage from the OAuth Client
4.2.2. Leakage from the Authorization Server . . . . . . . . 19 4.2.2. Leakage from the Authorization Server
4.2.3. Consequences . . . . . . . . . . . . . . . . . . . . 19 4.2.3. Consequences
4.2.4. Countermeasures . . . . . . . . . . . . . . . . . . . 19 4.2.4. Countermeasures
4.3. Credential Leakage via Browser History . . . . . . . . . 20 4.3. Credential Leakage via Browser History
4.3.1. Authorization Code in Browser History . . . . . . . . 20 4.3.1. Authorization Code in Browser History
4.3.2. Access Token in Browser History . . . . . . . . . . . 20 4.3.2. Access Token in Browser History
4.4. Mix-Up Attacks . . . . . . . . . . . . . . . . . . . . . 21 4.4. Mix-Up Attacks
4.4.1. Attack Description . . . . . . . . . . . . . . . . . 21 4.4.1. Attack Description
4.4.2. Countermeasures . . . . . . . . . . . . . . . . . . . 23 4.4.2. Countermeasures
4.4.2.1. Mix-Up Defense via Issuer Identification . . . . 24 4.4.2.1. Mix-Up Defense via Issuer Identification
4.4.2.2. Mix-Up Defense via Distinct Redirect URIs . . . . 24 4.4.2.2. Mix-Up Defense via Distinct Redirect URIs
4.5. Authorization Code Injection . . . . . . . . . . . . . . 25 4.5. Authorization Code Injection
4.5.1. Attack Description . . . . . . . . . . . . . . . . . 25 4.5.1. Attack Description
4.5.2. Discussion . . . . . . . . . . . . . . . . . . . . . 26 4.5.2. Discussion
4.5.3. Countermeasures . . . . . . . . . . . . . . . . . . . 27 4.5.3. Countermeasures
4.5.3.1. PKCE . . . . . . . . . . . . . . . . . . . . . . 27 4.5.3.1. PKCE
4.5.3.2. Nonce . . . . . . . . . . . . . . . . . . . . . . 28 4.5.3.2. Nonce
4.5.3.3. Other Solutions . . . . . . . . . . . . . . . . . 28 4.5.3.3. Other Solutions
4.5.4. Limitations . . . . . . . . . . . . . . . . . . . . . 29 4.5.4. Limitations
4.6. Access Token Injection . . . . . . . . . . . . . . . . . 29 4.6. Access Token Injection
4.6.1. Countermeasures . . . . . . . . . . . . . . . . . . . 29 4.6.1. Countermeasures
4.7. Cross-Site Request Forgery . . . . . . . . . . . . . . . 30 4.7. Cross-Site Request Forgery
4.7.1. Countermeasures . . . . . . . . . . . . . . . . . . . 30 4.7.1. Countermeasures
4.8. PKCE Downgrade Attack . . . . . . . . . . . . . . . . . . 31 4.8. PKCE Downgrade Attack
4.8.1. Attack Description . . . . . . . . . . . . . . . . . 31 4.8.1. Attack Description
4.8.2. Countermeasures . . . . . . . . . . . . . . . . . . . 32 4.8.2. Countermeasures
4.9. Access Token Leakage at the Resource Server . . . . . . . 33 4.9. Access Token Leakage at the Resource Server
4.9.1. Access Token Phishing by Counterfeit Resource 4.9.1. Access Token Phishing by Counterfeit Resource Server
Server . . . . . . . . . . . . . . . . . . . . . . . 33 4.9.2. Compromised Resource Server
4.9.2. Compromised Resource Server . . . . . . . . . . . . . 33 4.9.3. Countermeasures
4.9.3. Countermeasures . . . . . . . . . . . . . . . . . . . 34 4.10. Misuse of Stolen Access Tokens
4.10. Misuse of Stolen Access Tokens . . . . . . . . . . . . . 34 4.10.1. Sender-Constrained Access Tokens
4.10.1. Sender-Constrained Access Tokens . . . . . . . . . . 34 4.10.2. Audience-Restricted Access Tokens
4.10.2. Audience-Restricted Access Tokens . . . . . . . . . 36 4.10.3. Discussion: Preventing Leakage via Metadata
4.10.3. Discussion: Preventing Leakage via Metadata . . . . 37 4.11. Open Redirection
4.11. Open Redirection . . . . . . . . . . . . . . . . . . . . 38 4.11.1. Client as Open Redirector
4.11.1. Client as Open Redirector . . . . . . . . . . . . . 38 4.11.2. Authorization Server as Open Redirector
4.11.2. Authorization Server as Open Redirector . . . . . . 39 4.12. 307 Redirect
4.12. 307 Redirect . . . . . . . . . . . . . . . . . . . . . . 40 4.13. TLS Terminating Reverse Proxies
4.13. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 41 4.14. Refresh Token Protection
4.14. Refresh Token Protection . . . . . . . . . . . . . . . . 42 4.14.1. Discussion
4.14.1. Discussion . . . . . . . . . . . . . . . . . . . . . 42 4.14.2. Recommendations
4.14.2. Recommendations . . . . . . . . . . . . . . . . . . 42 4.15. Client Impersonating Resource Owner
4.15. Client Impersonating Resource Owner . . . . . . . . . . . 44 4.15.1. Countermeasures
4.15.1. Countermeasures . . . . . . . . . . . . . . . . . . 44 4.16. Clickjacking
4.16. Clickjacking . . . . . . . . . . . . . . . . . . . . . . 44 4.17. Attacks on In-Browser Communication Flows
4.17. Attacks on In-Browser Communication Flows . . . . . . . . 46 4.17.1. Examples
4.17.1. Examples . . . . . . . . . . . . . . . . . . . . . . 46 4.17.1.1. Insufficient Limitation of Receiver Origins
4.17.1.1. Insufficient Limitation of Receiver Origins . . 46 4.17.1.2. Insufficient URI Validation
4.17.1.2. Insufficient URI Validation . . . . . . . . . . 46
4.17.1.3. Injection after Insufficient Validation of Sender 4.17.1.3. Injection after Insufficient Validation of Sender
Origin . . . . . . . . . . . . . . . . . . . . . . 47 Origin
4.17.2. Recommendations . . . . . . . . . . . . . . . . . . 47 4.17.2. Recommendations
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48 5. IANA Considerations
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 6. Security Considerations
7. Security Considerations . . . . . . . . . . . . . . . . . . . 48 7. References
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 7.1. Normative References
8.1. Normative References . . . . . . . . . . . . . . . . . . 48 7.2. Informative References
8.2. Informative References . . . . . . . . . . . . . . . . . 49 Acknowledgements
Appendix A. Document History . . . . . . . . . . . . . . . . . . 54 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 59
1. Introduction 1. Introduction
Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 (referred Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 (referred
to as simply "OAuth" in the following) has gained massive traction in to as simply "OAuth" in this document) has gained massive traction in
the market and became the standard for API protection and the basis the market and became the standard for API protection and the basis
for federated login using OpenID Connect [OpenID.Core]. While OAuth for federated login using OpenID Connect [OpenID.Core]. While OAuth
is used in a variety of scenarios and different kinds of deployments, is used in a variety of scenarios and different kinds of deployments,
the following challenges can be observed: the following challenges can be observed:
* OAuth implementations are being attacked through known * OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (i.e., well-known implementation weaknesses and anti-patterns (i.e., well-known
patterns that are considered insecure). Although most of these patterns that are considered insecure). Although most of these
threats are discussed in the OAuth 2.0 Threat Model and Security threats are discussed in the OAuth 2.0 Threat Model and Security
Considerations [RFC6819], continued exploitation demonstrates a Considerations [RFC6819], continued exploitation demonstrates a
need for more specific recommendations, easier to implement need for more specific recommendations, easier to implement
mitigations, and more defense in depth. mitigations, and more defense in depth.
* OAuth is being used in environments with higher security * OAuth is being used in environments with higher security
requirements than considered initially, such as Open Banking, requirements than considered initially, such as open banking,
eHealth, eGovernment, and Electronic Signatures. Those use cases eHealth, eGovernment, and electronic signatures. Those use cases
call for stricter guidelines and additional protection. call for stricter guidelines and additional protection.
* OAuth is being used in much more dynamic setups than originally * OAuth is being used in much more dynamic setups than originally
anticipated, creating new challenges with respect to security. anticipated, creating new challenges with respect to security.
Those challenges go beyond the original scope of [RFC6749], Those challenges go beyond the original scope of [RFC6749],
[RFC6750], and [RFC6819]. [RFC6750], and [RFC6819].
OAuth initially assumed static relationships between clients, OAuth initially assumed static relationships between clients,
authorization servers, and resource servers. The URLs of the authorization servers, and resource servers. The URLs of the
servers were known to the client at deployment time and built an servers were known to the client at deployment time and built an
anchor for the trust relationships among those parties. The anchor for the trust relationships among those parties. The
validation of whether the client is talking to a legitimate server validation of whether the client is talking to a legitimate server
was based on TLS server authentication (see [RFC6819], was based on TLS server authentication (see Section 4.5.4 of
Section 4.5.4). With the increasing adoption of OAuth, this [RFC6819]). With the increasing adoption of OAuth, this simple
simple model dissolved and, in several scenarios, was replaced by model dissolved and, in several scenarios, was replaced by a
a dynamic establishment of the relationship between clients on one dynamic establishment of the relationship between clients on one
side and the authorization and resource servers of a particular side and the authorization and resource servers of a particular
deployment on the other side. This way, the same client could be deployment on the other side. This way, the same client could be
used to access services of different providers (in case of used to access services of different providers (in case of
standard APIs, such as e-mail or OpenID Connect) or serve as a standard APIs, such as email or OpenID Connect) or serve as a
front end to a particular tenant in a multi-tenant environment. front end to a particular tenant in a multi-tenant environment.
Extensions of OAuth, such as the OAuth 2.0 Dynamic Client Extensions of OAuth, such as the OAuth 2.0 Dynamic Client
Registration Protocol [RFC7591] and OAuth 2.0 Authorization Server Registration Protocol [RFC7591] and OAuth 2.0 Authorization Server
Metadata [RFC8414] were developed to support the use of OAuth in Metadata [RFC8414] were developed to support the use of OAuth in
dynamic scenarios. dynamic scenarios.
* Technology has changed. For example, the way browsers treat * Technology has changed. For example, the way browsers treat
fragments when redirecting requests has changed, and with it, the fragments when redirecting requests has changed, and with it, the
implicit grant's underlying security model. implicit grant's underlying security model.
This document provides updated security recommendations to address This document provides updated security recommendations to address
these challenges. It introduces new requirements beyond those these challenges. It introduces new requirements beyond those
defined in existing specifications such as OAuth 2.0 [RFC6749] and defined in existing specifications such as OAuth 2.0 [RFC6749] and
OpenID Connect [OpenID.Core] and deprecates some modes of operation OpenID Connect [OpenID.Core] and deprecates some modes of operation
that are deemed less secure or even insecure. However, this document that are deemed less secure or even insecure. However, this document
does not supplant the security advice given in [RFC6749], [RFC6750], does not supplant the security advice given in [RFC6749], [RFC6750],
and [RFC6819], but complements those documents. and [RFC6819], but complements those documents.
Naturally, not all existing ecosystems and implementations are Naturally, not all existing ecosystems and implementations are
compatible with the new requirements and following the best practices compatible with the new requirements, and following the best
described in this document may break interoperability. Nonetheless, practices described in this document may break interoperability.
it is RECOMMENDED that implementers upgrade their implementations and Nonetheless, it is RECOMMENDED that implementers upgrade their
ecosystems as soon as feasible. implementations and ecosystems as soon as feasible.
OAuth 2.1, under developement as [I-D.ietf-oauth-v2-1], will OAuth 2.1, under development as [OAUTH-V2.1], will incorporate
incorporate security recommendations from this document. security recommendations from this document.
1.1. Structure 1.1. Structure
The remainder of this document is organized as follows: The next The remainder of this document is organized as follows: Section 2
section summarizes the most important best practices for every OAuth summarizes the most important best practices for every OAuth
implementor. Afterwards, the updated OAuth attacker model is implementor. Section 3 presents the updated OAuth attacker model.
presented. Subsequently, a detailed analysis of the threats and Section 4 is a detailed analysis of the threats and implementation
implementation issues that can be found in the wild today is given issues that can be found in the wild (at the time of writing) along
along with a discussion of potential countermeasures. with a discussion of potential countermeasures.
1.2. Conventions and Terminology 1.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This specification uses the terms "access token", "authorization This specification uses the terms "access token", "authorization
endpoint", "authorization grant", "authorization server", "client", endpoint", "authorization grant", "authorization server", "client",
"client identifier" (client ID), "protected resource", "refresh "client identifier" (client ID), "protected resource", "refresh
token", "resource owner", "resource server", and "token endpoint" token", "resource owner", "resource server", and "token endpoint"
defined by OAuth 2.0 [RFC6749]. defined by OAuth 2.0 [RFC6749].
An "open redirector" is an endpoint on a web server that forwards a An "open redirector" is an endpoint on a web server that forwards a
user’s browser to an arbitrary URI obtained from a query parameter. user's browser to an arbitrary URI obtained from a query parameter.
2. Best Practices 2. Best Practices
This section describes the core set of security mechanisms and This section describes the core set of security mechanisms and
measures that are considered to be best practices at the time of measures that are considered to be best practices at the time of
writing. Details about these security mechanisms and measures writing. Details about these security mechanisms and measures
(including detailed attack descriptions) and requirements for less (including detailed attack descriptions) and requirements for less
commonly used options are provided in Section 4. commonly used options are provided in Section 4.
2.1. Protecting Redirect-Based Flows 2.1. Protecting Redirect-Based Flows
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leakage of authorization codes and access tokens (see Section 4.1). leakage of authorization codes and access tokens (see Section 4.1).
It can also help to detect mix-up attacks (see Section 4.4). It can also help to detect mix-up attacks (see Section 4.4).
Clients and authorization servers MUST NOT expose URLs that forward Clients and authorization servers MUST NOT expose URLs that forward
the user's browser to arbitrary URIs obtained from a query parameter the user's browser to arbitrary URIs obtained from a query parameter
(open redirectors) as described in Section 4.11. Open redirectors (open redirectors) as described in Section 4.11. Open redirectors
can enable exfiltration of authorization codes and access tokens. can enable exfiltration of authorization codes and access tokens.
Clients MUST prevent Cross-Site Request Forgery (CSRF). In this Clients MUST prevent Cross-Site Request Forgery (CSRF). In this
context, CSRF refers to requests to the redirection endpoint that do context, CSRF refers to requests to the redirection endpoint that do
not originate at the authorization server, but a malicious third not originate at the authorization server, but at a malicious third
party (see Section 4.4.1.8. of [RFC6819] for details). Clients that party (see Section 4.4.1.8 of [RFC6819] for details). Clients that
have ensured that the authorization server supports Proof Key for have ensured that the authorization server supports Proof Key for
Code Exchange (PKCE, [RFC7636]) MAY rely on the CSRF protection Code Exchange (PKCE) [RFC7636] MAY rely on the CSRF protection
provided by PKCE. In OpenID Connect flows, the nonce parameter provided by PKCE. In OpenID Connect flows, the nonce parameter
provides CSRF protection. Otherwise, one-time use CSRF tokens provides CSRF protection. Otherwise, one-time use CSRF tokens
carried in the state parameter that are securely bound to the user carried in the state parameter that are securely bound to the user
agent MUST be used for CSRF protection (see Section 4.7.1). agent MUST be used for CSRF protection (see Section 4.7.1).
When an OAuth client can interact with more than one authorization When an OAuth client can interact with more than one authorization
server, a defense against mix-up attacks (see Section 4.4) is server, a defense against mix-up attacks (see Section 4.4) is
REQUIRED. To this end, clients SHOULD REQUIRED. To this end, clients SHOULD
* use the iss parameter as a countermeasure according to [RFC9207], * use the iss parameter as a countermeasure according to [RFC9207],
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provides CSRF protection. Otherwise, one-time use CSRF tokens provides CSRF protection. Otherwise, one-time use CSRF tokens
carried in the state parameter that are securely bound to the user carried in the state parameter that are securely bound to the user
agent MUST be used for CSRF protection (see Section 4.7.1). agent MUST be used for CSRF protection (see Section 4.7.1).
When an OAuth client can interact with more than one authorization When an OAuth client can interact with more than one authorization
server, a defense against mix-up attacks (see Section 4.4) is server, a defense against mix-up attacks (see Section 4.4) is
REQUIRED. To this end, clients SHOULD REQUIRED. To this end, clients SHOULD
* use the iss parameter as a countermeasure according to [RFC9207], * use the iss parameter as a countermeasure according to [RFC9207],
or or
* use an alternative countermeasure based on an iss value in the * use an alternative countermeasure based on an iss value in the
authorization response (such as the iss Claim in the ID Token in authorization response (such as the iss claim in the ID Token in
[OpenID.Core] or in [OpenID.JARM] responses), processing it as [OpenID.Core] or in [OpenID.JARM] responses), processing it as
described in [RFC9207]. described in [RFC9207].
In the absence of these options, clients MAY instead use distinct In the absence of these options, clients MAY instead use distinct
redirect URIs to identify authorization endpoints and token redirect URIs to identify authorization endpoints and token
endpoints, as described in Section 4.4.2. endpoints, as described in Section 4.4.2.
An authorization server that redirects a request potentially An authorization server that redirects a request potentially
containing user credentials MUST avoid forwarding these user containing user credentials MUST avoid forwarding these user
credentials accidentally (see Section 4.12 for details). credentials accidentally (see Section 4.12 for details).
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2.1.1. Authorization Code Grant 2.1.1. Authorization Code Grant
Clients MUST prevent authorization code injection attacks (see Clients MUST prevent authorization code injection attacks (see
Section 4.5) and misuse of authorization codes using one of the Section 4.5) and misuse of authorization codes using one of the
following options: following options:
* Public clients MUST use PKCE [RFC7636] to this end, as motivated * Public clients MUST use PKCE [RFC7636] to this end, as motivated
in Section 4.5.3.1. in Section 4.5.3.1.
* For confidential clients, the use of PKCE [RFC7636] is * For confidential clients, the use of PKCE [RFC7636] is
RECOMMENDED, as it provides strong protection against misuse and RECOMMENDED, as it provides strong protection against misuse and
injection of authorization codes as described in Section 4.5.3.1 injection of authorization codes as described in Section 4.5.3.1.
and, as a side-effect, prevents CSRF even in the presence of Also, as a side effect, it prevents CSRF even in the presence of
strong attackers as described in Section 4.7.1. strong attackers as described in Section 4.7.1.
* With additional precautions, described in Section 4.5.3.2, * With additional precautions, described in Section 4.5.3.2,
confidential OpenID Connect [OpenID.Core] clients MAY use the confidential OpenID Connect [OpenID.Core] clients MAY use the
nonce parameter and the respective Claim in the ID Token instead. nonce parameter and the respective Claim in the ID Token instead.
In any case, the PKCE challenge or OpenID Connect nonce MUST be In any case, the PKCE challenge or OpenID Connect nonce MUST be
transaction-specific and securely bound to the client and the user transaction-specific and securely bound to the client and the user
agent in which the transaction was started. Authorization servers agent in which the transaction was started. Authorization servers
are encouraged to make a reasonable effort at detecting and are encouraged to make a reasonable effort at detecting and
preventing the use of constant PKCE challenge or OpenID Connect nonce preventing the use of constant values for the PKCE challenge or
values. OpenID Connect nonce.
Note: Although PKCE was designed as a mechanism to protect native Note: Although PKCE was designed as a mechanism to protect native
apps, this advice applies to all kinds of OAuth clients, including apps, this advice applies to all kinds of OAuth clients, including
web applications. web applications.
When using PKCE, clients SHOULD use PKCE code challenge methods that When using PKCE, clients SHOULD use PKCE code challenge methods that
do not expose the PKCE verifier in the authorization request. do not expose the PKCE verifier in the authorization request.
Otherwise, attackers that can read the authorization request (cf. Otherwise, attackers that can read the authorization request (cf.
Attacker A4 in Section 3) can break the security provided by PKCE. attacker (A4) in Section 3) can break the security provided by PKCE.
Currently, S256 is the only such method. Currently, S256 is the only such method.
Authorization servers MUST support PKCE [RFC7636]. Authorization servers MUST support PKCE [RFC7636].
If a client sends a valid PKCE [RFC7636] code_challenge parameter in If a client sends a valid PKCE code_challenge parameter in the
the authorization request, the authorization server MUST enforce the authorization request, the authorization server MUST enforce the
correct usage of code_verifier at the token endpoint. correct usage of code_verifier at the token endpoint.
Authorization servers MUST mitigate PKCE Downgrade Attacks by Authorization servers MUST mitigate PKCE downgrade attacks by
ensuring that a token request containing a code_verifier parameter is ensuring that a token request containing a code_verifier parameter is
accepted only if a code_challenge parameter was present in the accepted only if a code_challenge parameter was present in the
authorization request, see Section 4.8.2 for details. authorization request; see Section 4.8.2 for details.
Authorization servers MUST provide a way to detect their support for Authorization servers MUST provide a way to detect their support for
PKCE. It is RECOMMENDED for authorization servers to publish the PKCE. It is RECOMMENDED for authorization servers to publish the
element code_challenge_methods_supported in their Authorization element code_challenge_methods_supported in their Authorization
Server Metadata ([RFC8414]) containing the supported PKCE challenge Server Metadata [RFC8414] containing the supported PKCE challenge
methods (which can be used by the client to detect PKCE support). methods (which can be used by the client to detect PKCE support).
Authorization servers MAY instead provide a deployment-specific way Authorization servers MAY instead provide a deployment-specific way
to ensure or determine PKCE support by the authorization server. to ensure or determine PKCE support by the authorization server.
2.1.2. Implicit Grant 2.1.2. Implicit Grant
The implicit grant (response type "token") and other response types The implicit grant (response type "token") and other response types
causing the authorization server to issue access tokens in the causing the authorization server to issue access tokens in the
authorization response are vulnerable to access token leakage and authorization response are vulnerable to access token leakage and
access token replay as described in Section 4.1, Section 4.2, access token replay as described in Sections 4.1, 4.2, 4.3, and 4.6.
Section 4.3, and Section 4.6.
Moreover, no standardized method for sender-constraining exists to Moreover, no standardized method for sender-constraining exists to
bind access tokens to a specific client (as recommended in bind access tokens to a specific client (as recommended in
Section 2.2) when the access tokens are issued in the authorization Section 2.2) when the access tokens are issued in the authorization
response. This means that an attacker can use the leaked or stolen response. This means that an attacker can use the leaked or stolen
access token at a resource endpoint. access token at a resource endpoint.
In order to avoid these issues, clients SHOULD NOT use the implicit In order to avoid these issues, clients SHOULD NOT use the implicit
grant (response type "token") or other response types issuing access grant (response type "token") or other response types issuing access
tokens in the authorization response, unless access token injection tokens in the authorization response, unless access token injection
in the authorization response is prevented and the aforementioned in the authorization response is prevented and the aforementioned
token leakage vectors are mitigated. token leakage vectors are mitigated.
Clients SHOULD instead use the response type code (i.e., Clients SHOULD instead use the response type code (i.e.,
authorization code grant type) as specified in Section 2.1.1 or any authorization code grant type) as specified in Section 2.1.1 or any
other response type that causes the authorization server to issue other response type that causes the authorization server to issue
access tokens in the token response, such as the code id_token access tokens in the token response, such as the code id_token
response type. This allows the authorization server to detect replay response type. This allows the authorization server to detect replay
attempts by attackers and generally reduces the attack surface since attempts by attackers and generally reduces the attack surface since
access tokens are not exposed in URLs. It also allows the access tokens are not exposed in URLs. It also allows the
authorization server to sender-constrain the issued tokens (see next authorization server to sender-constrain the issued tokens (see
section). Section 2.2.
2.2. Token Replay Prevention 2.2. Token Replay Prevention
2.2.1. Access Tokens 2.2.1. Access Tokens
A sender-constrained access token scopes the applicability of an A sender-constrained access token scopes the applicability of an
access token to a certain sender. This sender is obliged to access token to a certain sender. This sender is obliged to
demonstrate knowledge of a certain secret as a prerequisite for the demonstrate knowledge of a certain secret as a prerequisite for the
acceptance of that token at the recipient (e.g., a resource server). acceptance of that token at the recipient (e.g., a resource server).
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2.3. Access Token Privilege Restriction 2.3. Access Token Privilege Restriction
The privileges associated with an access token SHOULD be restricted The privileges associated with an access token SHOULD be restricted
to the minimum required for the particular application or use case. to the minimum required for the particular application or use case.
This prevents clients from exceeding the privileges authorized by the This prevents clients from exceeding the privileges authorized by the
resource owner. It also prevents users from exceeding their resource owner. It also prevents users from exceeding their
privileges authorized by the respective security policy. Privilege privileges authorized by the respective security policy. Privilege
restrictions also help to reduce the impact of access token leakage. restrictions also help to reduce the impact of access token leakage.
In particular, access tokens SHOULD be audience-restricted to a In particular, access tokens SHOULD be audience-restricted to a
specific resource server, or, if that is not feasible, to a small set specific resource server or, if that is not feasible, to a small set
of resource servers. To put this into effect, the authorization of resource servers. To put this into effect, the authorization
server associates the access token with certain resource servers and server associates the access token with certain resource servers, and
every resource server is obliged to verify, for every request, every resource server is obliged to verify, for every request,
whether the access token sent with that request was meant to be used whether the access token sent with that request was meant to be used
for that particular resource server. If it was not, the resource for that particular resource server. If it was not, the resource
server MUST refuse to serve the respective request. The aud claim as server MUST refuse to serve the respective request. The aud claim as
defined in [RFC9068] MAY be used to audience-restrict access tokens. defined in [RFC9068] MAY be used to audience-restrict access tokens.
Clients and authorization servers MAY utilize the parameters scope or Clients and authorization servers MAY utilize the parameters scope or
resource as specified in [RFC6749] and [RFC8707], respectively, to resource as specified in [RFC6749] and [RFC8707], respectively, to
determine the resource server they want to access. determine the resource server they want to access.
Additionally, access tokens SHOULD be restricted to certain resources Additionally, access tokens SHOULD be restricted to certain resources
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web origin. web origin.
2.5. Client Authentication 2.5. Client Authentication
Authorization servers SHOULD enforce client authentication if it is Authorization servers SHOULD enforce client authentication if it is
feasible, in the particular deployment, to establish a process for feasible, in the particular deployment, to establish a process for
issuance/registration of credentials for clients and ensuring the issuance/registration of credentials for clients and ensuring the
confidentiality of those credentials. confidentiality of those credentials.
It is RECOMMENDED to use asymmetric cryptography for client It is RECOMMENDED to use asymmetric cryptography for client
authentication, such as mTLS [RFC8705] or signed JWTs ("Private Key authentication, such as mutual TLS (mTLS) [RFC8705] or signed JWTs
JWT") in accordance with [RFC7521] and [RFC7523] (in [OpenID.Core] ("Private Key JWT") in accordance with [RFC7521] and [RFC7523] (in
defined as the client authentication method private_key_jwt). When [OpenID.Core] defined as the client authentication method
asymmetric cryptography for client authentication is used, private_key_jwt). When asymmetric cryptography for client
authorization servers do not need to store sensitive symmetric keys, authentication is used, authorization servers do not need to store
making these methods more robust against leakage of keys. sensitive symmetric keys, making these methods more robust against
leakage of keys.
2.6. Other Recommendations 2.6. Other Recommendations
The use of OAuth Authorization Server Metadata [RFC8414] can help to The use of OAuth Authorization Server Metadata [RFC8414] can help to
improve the security of OAuth deployments: improve the security of OAuth deployments:
* It ensures that security features and other new OAuth features can * It ensures that security features and other new OAuth features can
be enabled automatically by compliant software libraries. be enabled automatically by compliant software libraries.
* It reduces chances for misconfigurations -- for example,
* It reduces chances for misconfigurations, for example
misconfigured endpoint URLs (that might belong to an attacker) or misconfigured endpoint URLs (that might belong to an attacker) or
misconfigured security features. misconfigured security features.
* It can help to facilitate rotation of cryptographic keys and to * It can help to facilitate rotation of cryptographic keys and to
ensure cryptographic agility. ensure cryptographic agility.
It is therefore RECOMMENDED that authorization servers publish OAuth It is therefore RECOMMENDED that authorization servers publish OAuth
Authorization Server Metadata according to [RFC8414] and that clients Authorization Server Metadata according to [RFC8414] and that clients
make use of this Authorization Server Metadata to configure make use of this Authorization Server Metadata (when available) to
themselves when available. configure themselves.
Under the conditions described in Section 4.15.1, authorization Under the conditions described in Section 4.15.1, authorization
servers SHOULD NOT allow clients to influence their client_id or any servers SHOULD NOT allow clients to influence their client_id or any
claim that could cause confusion with a genuine resource owner. claim that could cause confusion with a genuine resource owner.
It is RECOMMENDED to use end-to-end TLS according to [BCP195] between It is RECOMMENDED to use end-to-end TLS according to [BCP195] between
the client and the resource server. If TLS traffic needs to be the client and the resource server. If TLS traffic needs to be
terminated at an intermediary, refer to Section 4.13 for further terminated at an intermediary, refer to Section 4.13 for further
security advice. security advice.
Authorization responses MUST NOT be transmitted over unencrypted Authorization responses MUST NOT be transmitted over unencrypted
network connections. To this end, authorization servers MUST NOT network connections. To this end, authorization servers MUST NOT
allow redirect URIs that use the http scheme except for native allow redirect URIs that use the http scheme except for native
clients that use Loopback Interface Redirection as described in clients that use loopback interface redirection as described in
[RFC8252], Section 7.3. Section 7.3 of [RFC8252].
If the authorization response is sent with in-browser communication If the authorization response is sent with in-browser communication
techniques like postMessage [WHATWG.postmessage_api] instead of HTTP techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
redirects, both the initiator and receiver of the in-browser message redirects, both the initiator and receiver of the in-browser message
MUST be strictly verified as described in Section 4.17. MUST be strictly verified as described in Section 4.17.
To support browser-based clients, endpoints directly accessed by such To support browser-based clients, endpoints directly accessed by such
clients including the Token Endpoint, Authorization Server Metadata clients including the Token Endpoint, Authorization Server Metadata
Endpoint, jwks_uri Endpoint, and the Dynamic Client Registration Endpoint, jwks_uri Endpoint, and Dynamic Client Registration Endpoint
Endpoint MAY support the use of Cross-Origin Resource Sharing (CORS, MAY support the use of Cross-Origin Resource Sharing (CORS)
[WHATWG.CORS]). However, CORS MUST NOT be supported at the [WHATWG.CORS]. However, CORS MUST NOT be supported at the
Authorization Endpoint, as the client does not access this endpoint authorization endpoint, as the client does not access this endpoint
directly; instead, the client redirects the user agent to it. directly; instead, the client redirects the user agent to it.
3. The Updated OAuth 2.0 Attacker Model 3. The Updated OAuth 2.0 Attacker Model
In [RFC6819], a threat model is laid out that describes the threats In [RFC6819], a threat model is laid out that describes the threats
against which OAuth deployments must be protected. While doing so, against which OAuth deployments must be protected. While doing so,
[RFC6819] makes certain assumptions about attackers and their [RFC6819] makes certain assumptions about attackers and their
capabilities, i.e., implicitly establishes an attacker model. In the capabilities, i.e., it implicitly establishes an attacker model. In
following, this attacker model is made explicit and is updated and the following, this attacker model is made explicit and is updated
expanded to account for the potentially dynamic relationships and expanded to account for the potentially dynamic relationships
involving multiple parties (as described in Section 1), to include involving multiple parties (as described in Section 1), to include
new types of attackers and to define the attacker model more clearly. new types of attackers, and to define the attacker model more
clearly.
The goal of this document is to ensure that the authorization of a The goal of this document is to ensure that the authorization of a
resource owner (with a user agent) at an authorization server and the resource owner (with a user agent) at an authorization server and the
subsequent usage of the access token at a resource server is subsequent usage of the access token at a resource server is
protected, as well as practically possible, at least against the protected, as well as practically possible, at least against the
following attackers: following attackers.
* (A1) Web Attackers that can set up and operate an arbitrary number (A1) Web attackers that can set up and operate an arbitrary number
of network endpoints (besides the "honest" ones) including of network endpoints (besides the "honest" ones) including
browsers and servers. Web attackers may set up web sites that are browsers and servers. Web attackers may set up websites that
visited by the resource owner, operate their own user agents, and are visited by the resource owner, operate their own user
participate in the protocol. agents, and participate in the protocol.
Web attackers may, in particular, operate OAuth clients that are In particular, web attackers may operate OAuth clients that are
registered at the authorization server, and operate their own registered at the authorization server, and they may operate
authorization and resource servers that can be used (in parallel their own authorization and resource servers that can be used
to the "honest" ones) by the resource owner and other resource (in parallel to the "honest" ones) by the resource owner and
owners. other resource owners.
It must also be assumed that web attackers can lure the user to It must also be assumed that web attackers can lure the user to
navigate their browser to arbitrary attacker-chosen URIs at any navigate their browser to arbitrary attacker-chosen URIs at any
time. In practice, this can be achieved in many ways, for time. In practice, this can be achieved in many ways, for
example, by injecting malicious advertisements into advertisement example, by injecting malicious advertisements into
networks, or by sending legitimate-looking emails. advertisement networks or by sending legitimate-looking emails.
Web attackers can use their own user credentials to create new Web attackers can use their own user credentials to create new
messages as well as any secrets they learned previously. For messages as well as any secrets they learned previously. For
example, if a web attacker learns an authorization code of a user example, if a web attacker learns an authorization code of a
through a misconfigured redirect URI, the web attacker can then user through a misconfigured redirect URI, the web attacker can
try to redeem that code for an access token. then try to redeem that code for an access token.
They cannot, however, read or manipulate messages that are not They cannot, however, read or manipulate messages that are not
targeted towards them (e.g., sent to a URL controlled by a non- targeted towards them (e.g., sent to a URL controlled by a non-
attacker controlled authorization server). attacker-controlled authorization server).
* (A2) Network Attackers that additionally have full control over (A2) Network attackers that additionally have full control over the
the network over which protocol participants communicate. They network over which protocol participants communicate. They can
can eavesdrop on, manipulate, and spoof messages, except when eavesdrop on, manipulate, and spoof messages, except when these
these are properly protected by cryptographic methods (e.g., TLS). are properly protected by cryptographic methods (e.g., TLS).
Network attackers can also block arbitrary messages. Network attackers can also block arbitrary messages.
While an example for a web attacker would be a customer of an While an example for a web attacker would be a customer of an
internet service provider, network attackers could be the internet internet service provider, network attackers could be the internet
service provider itself, an attacker in a public (Wi-Fi) network service provider itself, an attacker in a public (Wi-Fi) network
using ARP spoofing, or a state-sponsored attacker with access to using ARP spoofing, or a state-sponsored attacker with access to
internet exchange points, for instance. internet exchange points, for instance.
The aforementioned attackers (A1) and (A2) conform to the attacker The aforementioned attackers (A1) and (A2) conform to the attacker
model that was used in formal analysis efforts for OAuth model that was used in formal analysis efforts for OAuth
[arXiv.1601.01229]. This is a minimal attacker model. Implementers [arXiv.1601.01229]. This is a minimal attacker model. Implementers
skipping to change at page 13, line 20 skipping to change at line 574
[arXiv.1901.11520], a very strong attacker model is used that [arXiv.1901.11520], a very strong attacker model is used that
includes attackers that have full control over the token endpoint. includes attackers that have full control over the token endpoint.
This models effects of a possible misconfiguration of endpoints in This models effects of a possible misconfiguration of endpoints in
the ecosystem, which can be avoided by using authorization server the ecosystem, which can be avoided by using authorization server
metadata as described in Section 2.6. Such an attacker is therefore metadata as described in Section 2.6. Such an attacker is therefore
not listed here. not listed here.
However, previous attacks on OAuth have shown that the following However, previous attacks on OAuth have shown that the following
types of attackers are relevant in particular: types of attackers are relevant in particular:
* (A3) Attackers that can read, but not modify, the contents of the (A3) Attackers that can read, but not modify, the contents of the
authorization response (i.e., the authorization response can leak authorization response (i.e., the authorization response can
to an attacker). leak to an attacker).
Examples for such attacks include open redirector attacks, Examples of such attacks include open redirector attacks;
insufficient checking of redirect URIs (see Section 4.1), problems insufficient checking of redirect URIs (see Section 4.1);
existing on mobile operating systems (where different apps can problems existing on mobile operating systems (where different
register themselves on the same URI), mix-up attacks (see apps can register themselves on the same URI); mix-up attacks
Section 4.4), where the client is tricked into sending credentials (see Section 4.4), where the client is tricked into sending
to an attacker-controlled authorization server, and the fact that credentials to an attacker-controlled authorization server; and
URLs are often stored/logged by browsers (history), proxy servers, the fact that URLs are often stored/logged by browsers
and operating systems. (history), proxy servers, and operating systems.
* (A4) Attackers that can read, but not modify, the contents of the (A4) Attackers that can read, but not modify, the contents of the
authorization request (i.e., the authorization request can leak, authorization request (i.e., the authorization request can
in the same manner as above, to an attacker). leak, in the same manner as above, to an attacker).
* (A5) Attackers that can acquire an access token issued by an (A5) Attackers that can acquire an access token issued by an
authorization server. For example, a resource server can be authorization server. For example, a resource server may be
compromised by an attacker, an access token may be sent to an compromised by an attacker, an access token may be sent to an
attacker-controlled resource server due to a misconfiguration, or attacker-controlled resource server due to a misconfiguration,
a resource owner is social-engineered into using an attacker- or social engineering may be used to get a resource owner to
controlled resource server. Also see Section 4.9.2. use an attacker-controlled resource server. Also see
Section 4.9.2.
(A3), (A4) and (A5) typically occur together with either (A1) or (A3), (A4), and (A5) typically occur together with either (A1) or
(A2). Attackers can collaborate to reach a common goal. (A2). Attackers can collaborate to reach a common goal.
Note that an attacker (A1) or (A2) can be a resource owner or act as Note that an attacker (A1) or (A2) can be a resource owner or act as
one. For example, such an attacker can use their own browser to one. For example, such an attacker can use their own browser to
replay tokens or authorization codes obtained by any of the attacks replay tokens or authorization codes obtained by any of the attacks
described above at the client or resource server. described above at the client or resource server.
This document focuses on threats resulting from attackers (A1) to This document focuses on threats resulting from attackers (A1) to
(A5). (A5).
4. Attacks and Mitigations 4. Attacks and Mitigations
This section gives a detailed description of attacks on OAuth This section gives a detailed description of attacks on OAuth
implementations, along with potential countermeasures. Attacks and implementations, along with potential countermeasures. Attacks and
mitigations already covered in [RFC6819] are not listed here, except mitigations already covered in [RFC6819] are not listed here, except
where new recommendations are made. where new recommendations are made.
This section further defines additional requirements beyond those This section further defines additional requirements (beyond those
defined in Section 2 for certain cases and protocol options. defined in Section 2) for certain cases and protocol options.
4.1. Insufficient Redirect URI Validation 4.1. Insufficient Redirect URI Validation
Some authorization servers allow clients to register redirect URI Some authorization servers allow clients to register redirect URI
patterns instead of complete redirect URIs. The authorization patterns instead of complete redirect URIs. The authorization
servers then match the redirect URI parameter value at the servers then match the redirect URI parameter value at the
authorization endpoint against the registered patterns at runtime. authorization endpoint against the registered patterns at runtime.
This approach allows clients to encode transaction state into This approach allows clients to encode transaction state into
additional redirect URI parameters or to register a single pattern additional redirect URI parameters or to register a single pattern
for multiple redirect URIs. for multiple redirect URIs.
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These attacks are shown in detail in the following subsections. These attacks are shown in detail in the following subsections.
4.1.1. Redirect URI Validation Attacks on Authorization Code Grant 4.1.1. Redirect URI Validation Attacks on Authorization Code Grant
For a client using the grant type code, an attack may work as For a client using the grant type code, an attack may work as
follows: follows:
Assume the redirect URL pattern https://*.somesite.example/* is Assume the redirect URL pattern https://*.somesite.example/* is
registered for the client with the client ID s6BhdRkqt3. The registered for the client with the client ID s6BhdRkqt3. The
intention is to allow any subdomain of somesite.example to be a valid intention is to allow any subdomain of somesite.example to be a valid
redirect URI for the client, for example redirect URI for the client, for example,
https://app1.somesite.example/redirect. A naive implementation on https://app1.somesite.example/redirect. However, a naive
the authorization server, however, might interpret the wildcard * as implementation on the authorization server might interpret the
"any character" and not "any character valid for a domain name". The wildcard * as "any character" and not "any character valid for a
authorization server, therefore, might permit domain name". The authorization server, therefore, might permit
https://attacker.example/.somesite.example as a redirect URI, https://attacker.example/.somesite.example as a redirect URI,
although attacker.example is a different domain potentially although attacker.example is a different domain potentially
controlled by a malicious party. controlled by a malicious party.
The attack can then be conducted as follows: The attack can then be conducted as follows:
To begin, the attacker needs to trick the user into opening a To begin, the attacker needs to trick the user into opening a
tampered URL in their browser that launches a page under the tampered URL in their browser that launches a page under the
attacker's control, say https://www.evil.example (see Attacker A1 in attacker's control, say, https://www.evil.example (see attacker A1 in
Section 3). Section 3).
This URL initiates the following authorization request with the This URL initiates the following authorization request with the
client ID of a legitimate client to the authorization endpoint (line client ID of a legitimate client to the authorization endpoint (line
breaks for display only): breaks for display only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13 GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
&redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example &redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example
HTTP/1.1 HTTP/1.1
Host: server.somesite.example Host: server.somesite.example
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attack, the code is issued and immediately sent to the attacker's attack, the code is issued and immediately sent to the attacker's
domain. If an automatic approval of the authorization is enabled domain. If an automatic approval of the authorization is enabled
(which is not recommended for public clients according to [RFC6749]), (which is not recommended for public clients according to [RFC6749]),
the attack can be performed even without user interaction. the attack can be performed even without user interaction.
If the attacker impersonates a public client, the attacker can If the attacker impersonates a public client, the attacker can
exchange the code for tokens at the respective token endpoint. exchange the code for tokens at the respective token endpoint.
This attack will not work as easily for confidential clients, since This attack will not work as easily for confidential clients, since
the code exchange requires authentication with the legitimate the code exchange requires authentication with the legitimate
client's secret. The attacker can, however, use the legitimate client's secret. However, the attacker can use the legitimate
confidential client to redeem the code by performing an authorization confidential client to redeem the code by performing an authorization
code injection attack, see Section 4.5. code injection attack; see Section 4.5.
It is important to note that redirect URI validation vulnerabilities It is important to note that redirect URI validation vulnerabilities
can also exist if the authorization server handles wildcards can also exist if the authorization server handles wildcards
properly. For example, assume that the client registers the redirect properly. For example, assume that the client registers the redirect
URL pattern https://*.somesite.example/* and the authorization server URL pattern https://*.somesite.example/* and the authorization server
interprets this as "allow redirect URIs pointing to any host residing interprets this as "allow redirect URIs pointing to any host residing
in the domain somesite.example". If an attacker manages to establish in the domain somesite.example". If an attacker manages to establish
a host or subdomain in somesite.example, the attacker can impersonate a host or subdomain in somesite.example, the attacker can impersonate
the legitimate client. This could be caused, for example, by a the legitimate client. For example, this could be caused by a
subdomain takeover attack [research.udel], where an outdated CNAME subdomain takeover attack [research.udel], where an outdated CNAME
record (say, external-service.somesite.example) points to an external record (say, external-service.somesite.example) points to an external
DNS name that does no longer exist (say, customer- DNS name that no longer exists (say, customer-abc.service.example)
abc.service.example) and can be taken over by an attacker (e.g., by and can be taken over by an attacker (e.g., by registering as
registering as customer-abc with the external service). customer-abc with the external service).
4.1.2. Redirect URI Validation Attacks on Implicit Grant 4.1.2. Redirect URI Validation Attacks on Implicit Grant
The attack described above works for the implicit grant as well. If The attack described above works for the implicit grant as well. If
the attacker is able to send the authorization response to an the attacker is able to send the authorization response to an
attacker-controlled URI, the attacker will directly get access to the attacker-controlled URI, the attacker will directly get access to the
fragment carrying the access token. fragment carrying the access token.
Additionally, implicit grants (and also other grants when using Additionally, implicit grants (and also other grants when using
response_mode=fragment as defined in [OAuth.Responses]) can be response_mode=fragment as defined in [OAuth.Responses]) can be
subject to a further kind of attack. It utilizes the fact that user subject to a further kind of attack. The attack utilizes the fact
agents re-attach fragments to the destination URL of a redirect if that user agents reattach fragments to the destination URL of a
the location header does not contain a fragment (see [RFC9110], redirect if the location header does not contain a fragment (see
Section 17.11). The attack described here combines this behavior Section 17.11 of [RFC9110]). The attack described here combines this
with the client as an open redirector (see Section 4.11.1) in order behavior with the client as an open redirector (see Section 4.11.1)
to obtain access tokens. This allows circumvention even of very in order to obtain access tokens. This allows circumvention even of
narrow redirect URI patterns, but not strict URL matching. very narrow redirect URI patterns, but not of strict URL matching.
Assume the registered URL pattern for client s6BhdRkqt3 is Assume the registered URL pattern for client s6BhdRkqt3 is
https://client.somesite.example/cb?*, i.e., any parameter is allowed https://client.somesite.example/cb?*, i.e., any parameter is allowed
for redirects to https://client.somesite.example/cb. Unfortunately, for redirects to https://client.somesite.example/cb. Unfortunately,
the client exposes an open redirector. This endpoint supports a the client exposes an open redirector. This endpoint supports a
parameter redirect_to which takes a target URL and will send the parameter redirect_to which takes a target URL and will send the
browser to this URL using an HTTP Location header redirect 303. browser to this URL using an HTTP Location header redirect 303.
The attack can now be conducted as follows: The attack can now be conducted as follows:
To begin, as above, the attacker needs to trick the user into opening To begin, as above, the attacker needs to trick the user into opening
a tampered URL in their browser that launches a page under the a tampered URL in their browser that launches a page under the
attacker's control, say https://www.evil.example. attacker's control, say, https://www.evil.example.
Afterwards, the website initiates an authorization request that is Afterwards, the website initiates an authorization request that is
very similar to the one in the attack on the code flow. Different to very similar to the one in the attack on the code flow. Different to
above, it utilizes the open redirector by encoding above, it utilizes the open redirector by encoding
redirect_to=https://attacker.example into the parameters of the redirect_to=https://attacker.example into the parameters of the
redirect URI and it uses the response type "token" (line breaks for redirect URI, and it uses the response type "token" (line breaks for
display only): display only):
GET /authorize?response_type=token&state=9ad67f13 GET /authorize?response_type=token&state=9ad67f13
&client_id=s6BhdRkqt3 &client_id=s6BhdRkqt3
&redirect_uri=https%3A%2F%2Fclient.somesite.example &redirect_uri=https%3A%2F%2Fclient.somesite.example
%2Fcb%26redirect_to%253Dhttps%253A%252F %2Fcb%26redirect_to%253Dhttps%253A%252F
%252Fattacker.example%252F HTTP/1.1 %252Fattacker.example%252F HTTP/1.1
Host: server.somesite.example Host: server.somesite.example
Now, since the redirect URI matches the registered pattern, the Then, since the redirect URI matches the registered pattern, the
authorization server permits the request and sends the resulting authorization server permits the request and sends the resulting
access token in a 303 redirect (some response parameters omitted for access token in a 303 redirect (some response parameters omitted for
readability): readability):
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://client.somesite.example/cb? Location: https://client.somesite.example/cb?
redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&... #access_token=2YotnFZFEjr1zCsicMWpAA&...
At client.somesite.example, the request arrives at the open At client.somesite.example, the request arrives at the open
redirector. The endpoint will read the redirect parameter and will redirector. The endpoint will read the redirect parameter and will
issue an HTTP 303 Location header redirect to the URL issue an HTTP 303 Location header redirect to the URL
https://attacker.example/. https://attacker.example/.
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://attacker.example/ Location: https://attacker.example/
Since the redirector at client.somesite.example does not include a Since the redirector at client.somesite.example does not include a
fragment in the Location header, the user agent will re-attach the fragment in the Location header, the user agent will reattach the
original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the
URL and will navigate to the following URL: URL and will navigate to the following URL:
https://attacker.example/#access_token=2YotnFZFEjr1z... https://attacker.example/#access_token=2YotnFZFEjr1z...
The attacker's page at attacker.example can now access the fragment The attacker's page at attacker.example can then access the fragment
and obtain the access token. and obtain the access token.
4.1.3. Countermeasures 4.1.3. Countermeasures
The complexity of implementing and managing pattern matching The complexity of implementing and managing pattern matching
correctly obviously causes security issues. This document therefore correctly obviously causes security issues. This document therefore
advises simplifying the required logic and configuration by using advises simplifying the required logic and configuration by using
exact redirect URI matching. This means the authorization server exact redirect URI matching. This means the authorization server
MUST ensure that the two URIs are equal, see [RFC3986], MUST ensure that the two URIs are equal; see Section 6.2.1 of
Section 6.2.1, Simple String Comparison, for details. The only [RFC3986], Simple String Comparison, for details. The only exception
exception is native apps using a localhost URI: In this case, the is native apps using a localhost URI: In this case, the authorization
authorization server MUST allow variable port numbers as described in server MUST allow variable port numbers as described in Section 7.3
[RFC8252], Section 7.3. of [RFC8252].
Additional recommendations: Additional recommendations:
* Web servers on which redirect URIs are hosted MUST NOT expose open * Web servers on which redirect URIs are hosted MUST NOT expose open
redirectors (see Section 4.11). redirectors (see Section 4.11).
* Browsers reattach URL fragments to Location redirection URLs only * Browsers reattach URL fragments to Location redirection URLs only
if the URL in the Location header does not already contain a if the URL in the Location header does not already contain a
fragment. Therefore, servers MAY prevent browsers from fragment. Therefore, servers MAY prevent browsers from
reattaching fragments to redirection URLs by attaching an reattaching fragments to redirection URLs by attaching an
arbitrary fragment identifier, for example #_, to URLs in Location arbitrary fragment identifier, for example #_, to URLs in Location
headers. headers.
* Clients SHOULD use the authorization code response type instead of * Clients SHOULD use the authorization code response type instead of
response types causing access token issuance at the authorization response types that cause access token issuance at the
endpoint. This offers countermeasures against the reuse of leaked authorization endpoint. This offers countermeasures against the
credentials through the exchange process with the authorization reuse of leaked credentials through the exchange process with the
server and token replay through sender-constraining of the access authorization server and against token replay through sender-
tokens. constraining of the access tokens.
If the origin and integrity of the authorization request containing If the origin and integrity of the authorization request containing
the redirect URI can be verified, for example when using [RFC9101] or the redirect URI can be verified, for example, when using [RFC9101]
[RFC9126] with client authentication, the authorization server MAY or [RFC9126] with client authentication, the authorization server MAY
trust the redirect URI without further checks. trust the redirect URI without further checks.
4.2. Credential Leakage via Referer Headers 4.2. Credential Leakage via Referer Headers
The contents of the authorization request URI or the authorization The contents of the authorization request URI or the authorization
response URI can unintentionally be disclosed to attackers through response URI can unintentionally be disclosed to attackers through
the Referer HTTP header (see [RFC9110], Section 10.1.3), by leaking the Referer HTTP header (see Section 10.1.3 of [RFC9110]), by leaking
either from the authorization server's or the client's website, from either the authorization server's or the client's website,
respectively. Most importantly, authorization codes or state values respectively. Most importantly, authorization codes or state values
can be disclosed in this way. Although specified otherwise in can be disclosed in this way. Although specified otherwise in
[RFC9110], Section 10.1.3, the same may happen to access tokens Section 10.1.3 of [RFC9110], the same may happen to access tokens
conveyed in URI fragments due to browser implementation issues, as conveyed in URI fragments due to browser implementation issues, as
illustrated by a (now fixed) issue in the Chromium project illustrated by a (now fixed) issue in the Chromium project
[bug.chromium]. [bug.chromium].
4.2.1. Leakage from the OAuth Client 4.2.1. Leakage from the OAuth Client
Leakage from the OAuth client requires that the client, as a result Leakage from the OAuth client requires that the client, as a result
of a successful authorization request, renders a page that of a successful authorization request, renders a page that
* contains links to other pages under the attacker's control and a * contains links to other pages under the attacker's control and a
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4.2.2. Leakage from the Authorization Server 4.2.2. Leakage from the Authorization Server
In a similar way, an attacker can learn state from the authorization In a similar way, an attacker can learn state from the authorization
request if the authorization endpoint at the authorization server request if the authorization endpoint at the authorization server
contains links or third-party content as above. contains links or third-party content as above.
4.2.3. Consequences 4.2.3. Consequences
An attacker that learns a valid code or access token through a An attacker that learns a valid code or access token through a
Referer header can perform the attacks as described in Section 4.1.1, Referer header can perform the attacks as described in Sections
Section 4.5, and Section 4.6. If the attacker learns state, the CSRF 4.1.1, 4.5 and 4.6. If the attacker learns state, the CSRF
protection achieved by using state is lost, resulting in CSRF attacks protection achieved by using state is lost, resulting in CSRF attacks
as described in [RFC6819], Section 4.4.1.8. as described in Section 4.4.1.8 of [RFC6819].
4.2.4. Countermeasures 4.2.4. Countermeasures
The page rendered as a result of the OAuth authorization response and The page rendered as a result of the OAuth authorization response and
the authorization endpoint SHOULD NOT include third-party resources the authorization endpoint SHOULD NOT include third-party resources
or links to external sites. or links to external sites.
The following measures further reduce the chances of a successful The following measures further reduce the chances of a successful
attack: attack:
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referrer in the response completely suppresses the Referer header referrer in the response completely suppresses the Referer header
in all requests originating from the resulting document. in all requests originating from the resulting document.
* Use authorization code instead of response types causing access * Use authorization code instead of response types causing access
token issuance from the authorization endpoint. token issuance from the authorization endpoint.
* Bind the authorization code to a confidential client or PKCE * Bind the authorization code to a confidential client or PKCE
challenge. In this case, the attacker lacks the secret to request challenge. In this case, the attacker lacks the secret to request
the code exchange. the code exchange.
* As described in [RFC6749], Section 4.1.2, authorization codes MUST * As described in Section 4.1.2 of [RFC6749], authorization codes
be invalidated by the authorization server after their first use MUST be invalidated by the authorization server after their first
at the token endpoint. For example, if an authorization server use at the token endpoint. For example, if an authorization
invalidated the code after the legitimate client redeemed it, the server invalidated the code after the legitimate client redeemed
attacker would fail to exchange this code later. it, the attacker would fail to exchange this code later.
This does not mitigate the attack if the attacker manages to This does not mitigate the attack if the attacker manages to
exchange the code for a token before the legitimate client does exchange the code for a token before the legitimate client does
so. Therefore, [RFC6749] further recommends that, when an attempt so. Therefore, [RFC6749] further recommends that, when an attempt
is made to redeem a code twice, the authorization server SHOULD is made to redeem a code twice, the authorization server SHOULD
revoke all tokens issued previously based on that code. revoke all tokens issued previously based on that code.
* The state value SHOULD be invalidated by the client after its * The state value SHOULD be invalidated by the client after its
first use at the redirection endpoint. If this is implemented, first use at the redirection endpoint. If this is implemented,
and an attacker receives a token through the Referer header from and an attacker receives a token through the Referer header from
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4.3.1. Authorization Code in Browser History 4.3.1. Authorization Code in Browser History
When a browser navigates to client.example/ When a browser navigates to client.example/
redirection_endpoint?code=abcd as a result of a redirect from a redirection_endpoint?code=abcd as a result of a redirect from a
provider's authorization endpoint, the URL including the provider's authorization endpoint, the URL including the
authorization code may end up in the browser's history. An attacker authorization code may end up in the browser's history. An attacker
with access to the device could obtain the code and try to replay it. with access to the device could obtain the code and try to replay it.
Countermeasures: Countermeasures:
* Authorization code replay prevention as described in [RFC6819], * Authorization code replay prevention as described in
Section 4.4.1.1, and Section 4.5. Section 4.4.1.1 of [RFC6819], and Section 4.5.
* Use form post response mode instead of redirect for the * Use the form post response mode instead of redirect for the
authorization response (see [OAuth.Post]). authorization response (see [OAuth.Post]).
4.3.2. Access Token in Browser History 4.3.2. Access Token in Browser History
An access token may end up in the browser history if a client or a An access token may end up in the browser history if a client or a
web site that already has a token deliberately navigates to a page website that already has a token deliberately navigates to a page
like provider.com/get_user_profile?access_token=abcdef. [RFC6750] like provider.com/get_user_profile?access_token=abcdef. [RFC6750]
discourages this practice and advises transferring tokens via a discourages this practice and advises transferring tokens via a
header, but in practice web sites often pass access tokens in query header, but in practice websites often pass access tokens in query
parameters. parameters.
In the case of implicit grant, a URL like client.example/ In the case of implicit grant, a URL like client.example/
redirection_endpoint#access_token=abcdef may also end up in the redirection_endpoint#access_token=abcdef may also end up in the
browser history as a result of a redirect from a provider's browser history as a result of a redirect from a provider's
authorization endpoint. authorization endpoint.
Countermeasures: Countermeasures:
* Clients MUST NOT pass access tokens in a URI query parameter in * Clients MUST NOT pass access tokens in a URI query parameter in
the way described in Section 2.3 of [RFC6750]. The authorization the way described in Section 2.3 of [RFC6750]. The authorization
code grant or alternative OAuth response modes like the form post code grant or alternative OAuth response modes like the form post
response mode [OAuth.Post] can be used to this end. response mode [OAuth.Post] can be used to this end.
4.4. Mix-Up Attacks 4.4. Mix-Up Attacks
Mix-up is an attack on scenarios where an OAuth client interacts with Mix-up attacks are in scenarios where an OAuth client interacts with
two or more authorization servers and at least one authorization two or more authorization servers and at least one authorization
server is under the control of the attacker. This can be the case, server is under the control of the attacker. This can be the case,
for example, if the attacker uses dynamic registration to register for example, if the attacker uses dynamic registration to register
the client at their own authorization server or if an authorization the client at their own authorization server or if an authorization
server becomes compromised. server becomes compromised.
The goal of the attack is to obtain an authorization code or an The goal of the attack is to obtain an authorization code or an
access token for an uncompromised authorization server. This is access token for an uncompromised authorization server. This is
achieved by tricking the client into sending those credentials to the achieved by tricking the client into sending those credentials to the
compromised authorization server (the attacker) instead of using them compromised authorization server (the attacker) instead of using them
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authorization servers of which one is considered "honest" (H-AS) authorization servers of which one is considered "honest" (H-AS)
and one is operated by the attacker (A-AS), and and one is operated by the attacker (A-AS), and
* the client stores the authorization server chosen by the user in a * the client stores the authorization server chosen by the user in a
session bound to the user's browser and uses the same redirection session bound to the user's browser and uses the same redirection
endpoint URI for each authorization server. endpoint URI for each authorization server.
In the following, it is further assumed that the client is registered In the following, it is further assumed that the client is registered
with H-AS (URI: https://honest.as.example, client ID: 7ZGZldHQ) and with H-AS (URI: https://honest.as.example, client ID: 7ZGZldHQ) and
with A-AS (URI: https://attacker.example, client ID: 666RVZJTA). with A-AS (URI: https://attacker.example, client ID: 666RVZJTA).
URLs shown in the following example are shortened for presentation to URLs shown in the following example are shortened for presentation to
only include parameters relevant to the attack. include only parameters relevant to the attack.
Attack on the authorization code grant: Attack on the authorization code grant:
1. The user selects to start the grant using A-AS (e.g., by clicking 1. The user selects to start the grant using A-AS (e.g., by clicking
on a button on the client's website). on a button on the client's website).
2. The client stores in the user's session that the user selected 2. The client stores in the user's session that the user selected
"A-AS" and redirects the user to A-AS's authorization endpoint "A-AS" and redirects the user to A-AS's authorization endpoint
with a Location header containing the URL with a Location header containing the URL
https://attacker.example/ https://attacker.example/
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5. Since the client still assumes that the code was issued by A-AS, 5. Since the client still assumes that the code was issued by A-AS,
it will try to redeem the code at A-AS's token endpoint. it will try to redeem the code at A-AS's token endpoint.
6. The attacker therefore obtains code and can either exchange the 6. The attacker therefore obtains code and can either exchange the
code for an access token (for public clients) or perform an code for an access token (for public clients) or perform an
authorization code injection attack as described in Section 4.5. authorization code injection attack as described in Section 4.5.
Variants: Variants:
* Mix-Up With Interception: This variant works only if the attacker * Mix-Up with Interception: This variant works only if the attacker
can intercept and manipulate the first request/response pair from can intercept and manipulate the first request/response pair from
a user's browser to the client (in which the user selects a a user's browser to the client (in which the user selects a
certain authorization server and is then redirected by the client certain authorization server and is then redirected by the client
to that authorization server), as in Attacker A2 (see Section 3). to that authorization server), as in Attacker (A2) (see
This capability can, for example, be the result of a attacker-in- Section 3). This capability can, for example, be the result of a
the-middle attack on the user's connection to the client. In the attacker-in-the-middle attack on the user's connection to the
attack, the user starts the flow with H-AS. The attacker client. In the attack, the user starts the flow with H-AS. The
intercepts this request and changes the user's selection to A-AS. attacker intercepts this request and changes the user's selection
The rest of the attack proceeds as in Steps 2 and following above. to A-AS. The rest of the attack proceeds as in Step 2 and
following above.
* Implicit Grant: In the implicit grant, the attacker receives an * Implicit Grant: In the implicit grant, the attacker receives an
access token instead of the code in Step 4. The attacker's access token instead of the code in Step 4. The attacker's
authorization server receives the access token when the client authorization server receives the access token when the client
makes a request to the A-AS userinfo endpoint, or since the client makes either a request to the A-AS userinfo endpoint or a request
believes it has completed the flow with A-AS, a request to the to the attacker's resource server (since the client believes it
attacker's resource server. has completed the flow with A-AS).
* Per-AS Redirect URIs: If clients use different redirect URIs for * Per-AS Redirect URIs: If clients use different redirect URIs for
different authorization servers, do not store the selected different authorization servers, clients do not store the selected
authorization server in the user's session, and authorization authorization server in the user's session, and authorization
servers do not check the redirect URIs properly, attackers can servers do not check the redirect URIs properly, attackers can
mount an attack called "Cross-Social Network Request Forgery". mount an attack called "Cross-Social Network Request Forgery".
These attacks have been observed in practice. Refer to These attacks have been observed in practice. Refer to
[research.jcs_14] for details. [research.jcs_14] for details.
* OpenID Connect: Some variants can be used to attack OpenID * OpenID Connect: Some variants can be used to attack OpenID
Connect. In these attacks, the attacker misuses features of the Connect. In these attacks, the attacker misuses features of the
OpenID Connect Discovery [OpenID.Discovery] mechanism or replays OpenID Connect Discovery [OpenID.Discovery] mechanism or replays
access tokens or ID Tokens to conduct a mix-up attack. The access tokens or ID Tokens to conduct a mix-up attack. The
attacks are described in detail in [arXiv.1704.08539], Appendix A, attacks are described in detail in Appendix A of
and [arXiv.1508.04324v2], Section 6 ("Malicious Endpoints [arXiv.1704.08539] and Section 6 of [arXiv.1508.04324v2]
Attacks"). ("Malicious Endpoints Attacks").
4.4.2. Countermeasures 4.4.2. Countermeasures
When an OAuth client can only interact with one authorization server, When an OAuth client can only interact with one authorization server,
a mix-up defense is not required. In scenarios where an OAuth client a mix-up defense is not required. In scenarios where an OAuth client
interacts with two or more authorization servers, however, clients interacts with two or more authorization servers, however, clients
MUST prevent mix-up attacks. Two different methods are discussed in MUST prevent mix-up attacks. Two different methods are discussed
the following. below.
For both defenses, clients MUST store, for each authorization For both defenses, clients MUST store, for each authorization
request, the issuer they sent the authorization request to and bind request, the issuer they sent the authorization request to and bind
this information to the user agent. The issuer serves, via the this information to the user agent. The issuer serves, via the
associated metadata, as an abstract identifier for the combination of associated metadata, as an abstract identifier for the combination of
the authorization endpoint and token endpoint that are to be used in the authorization endpoint and token endpoint that are to be used in
the flow. If an issuer identifier is not available, for example, if the flow. If an issuer identifier is not available (for example, if
neither OAuth Authorization Server Metadata [RFC8414] nor OpenID neither OAuth Authorization Server Metadata [RFC8414] nor OpenID
Connect Discovery [OpenID.Discovery] is used, a different unique Connect Discovery [OpenID.Discovery] is used), a different unique
identifier for this tuple or the tuple itself can be used instead. identifier for this tuple or the tuple itself can be used instead.
For brevity of presentation, such a deployment-specific identifier For brevity of presentation, such a deployment-specific identifier
will be subsumed under the issuer (or issuer identifier) in the will be subsumed under the issuer (or issuer identifier) in the
following. following.
It is important to note that just storing the authorization server It is important to note that just storing the authorization server
URL is not sufficient to identify mix-up attacks. An attacker might URL is not sufficient to identify mix-up attacks. An attacker might
declare an uncompromised authorization server's authorization declare an uncompromised authorization server's authorization
endpoint URL as "their" authorization server URL, but declare a token endpoint URL as "their" authorization server URL, but declare a token
endpoint under their own control. endpoint under their own control.
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to the attacker's authorization server. The attacker could then run to the attacker's authorization server. The attacker could then run
the attack as described above, replacing the client ID with the the attack as described above, replacing the client ID with the
client ID of their newly created client. client ID of their newly created client.
This defense SHOULD therefore only be used if other options are not This defense SHOULD therefore only be used if other options are not
available. available.
4.5. Authorization Code Injection 4.5. Authorization Code Injection
An attacker who has gained access to an authorization code contained An attacker who has gained access to an authorization code contained
in an authorization response (see Attacker A3 in Section 3) can try in an authorization response (see Attacker (A3) in Section 3) can try
to redeem the authorization code for an access token or otherwise to redeem the authorization code for an access token or otherwise
make use of the authorization code. make use of the authorization code.
In the case that the authorization code was created for a public In the case that the authorization code was created for a public
client, the attacker can send the authorization code to the token client, the attacker can send the authorization code to the token
endpoint of the authorization server and thereby get an access token. endpoint of the authorization server and thereby get an access token.
This attack was described in Section 4.4.1.1 of [RFC6819]. This attack was described in Section 4.4.1.1. of [RFC6819].
For confidential clients, or in some special situations, the attacker For confidential clients, or in some special situations, the attacker
can execute an authorization code injection attack, as described in can execute an authorization code injection attack, as described in
the following. the following.
In an authorization code injection attack, the attacker attempts to In an authorization code injection attack, the attacker attempts to
inject a stolen authorization code into the attacker's own session inject a stolen authorization code into the attacker's own session
with the client. The aim is to associate the attacker's session at with the client. The aim is to associate the attacker's session at
the client with the victim's resources or identity, thereby giving the client with the victim's resources or identity, thereby giving
the attacker at least limited access to the victim's resources. the attacker at least limited access to the victim's resources.
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Except in these special cases, authorization code injection is Except in these special cases, authorization code injection is
usually not interesting when the code is created for a public client, usually not interesting when the code is created for a public client,
as sending the code to the token endpoint is a simpler and more as sending the code to the token endpoint is a simpler and more
powerful attack, as described above. powerful attack, as described above.
4.5.1. Attack Description 4.5.1. Attack Description
The authorization code injection attack works as follows: The authorization code injection attack works as follows:
1. The attacker obtains an authorization code (see attacker A3 in 1. The attacker obtains an authorization code (see Attacker (A3) in
Section 3). For the rest of the attack, only the capabilities of Section 3). For the rest of the attack, only the capabilities of
a web attacker (A1) are required. a web attacker (A1) are required.
2. From the attacker's device, the attacker starts a regular OAuth 2. From the attacker's device, the attacker starts a regular OAuth
authorization process with the legitimate client. authorization process with the legitimate client.
3. In the response of the authorization server to the legitimate 3. In the response of the authorization server to the legitimate
client, the attacker replaces the newly created authorization client, the attacker replaces the newly created authorization
code with the stolen authorization code. Since this response is code with the stolen authorization code. Since this response is
passing through the attacker's device, the attacker can use any passing through the attacker's device, the attacker can use any
tool that can intercept and manipulate the authorization response tool that can intercept and manipulate the authorization response
to this end. The attacker does not need to control the network. to this end. The attacker does not need to control the network.
4. The legitimate client sends the code to the authorization 4. The legitimate client sends the code to the authorization
server's token endpoint, along with the redirect_uri and the server's token endpoint, along with the redirect_uri and the
client's client ID and client secret (or other means of client client's client ID and client secret (or other means of client
authentication). authentication).
5. The authorization server checks the client secret, whether the 5. The authorization server checks the client secret, whether the
code was issued to the particular client, and whether the actual code was issued to the particular client, and whether the actual
redirect URI matches the redirect_uri parameter (see [RFC6749]). redirect URI matches the redirect_uri parameter (see [RFC6749]).
6. All checks succeed and the authorization server issues access and 6. All checks succeed and the authorization server issues access and
other tokens to the client. The attacker has now associated other tokens to the client. The attacker has now associated
their session with the legitimate client with the victim's their session with the legitimate client with the victim's
resources and/or identity. resources and/or identity.
4.5.2. Discussion 4.5.2. Discussion
Obviously, the check-in step (5.) will fail if the code was issued to Obviously, the check-in step (Step 5) will fail if the code was
another client ID, e.g., a client set up by the attacker. The check issued to another client ID, e.g., a client set up by the attacker.
will also fail if the authorization code was already redeemed by the The check will also fail if the authorization code was already
legitimate user and was one-time use only. redeemed by the legitimate user and was one-time use only.
An attempt to inject a code obtained via a manipulated redirect URI An attempt to inject a code obtained via a manipulated redirect URI
should also be detected if the authorization server stored the should also be detected if the authorization server stored the
complete redirect URI used in the authorization request and compares complete redirect URI used in the authorization request and compares
it with the redirect_uri parameter. it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the authorization server to "... Section 4.1.3 of [RFC6749] requires the authorization server to
ensure that the redirect_uri parameter is present if the redirect_uri
parameter was included in the initial authorization request as | ensure that the "redirect_uri" parameter is present if the
described in Section 4.1.1, and if included ensure that their values | "redirect_uri" parameter was included in the initial authorization
are identical.". In the attack scenario described above, the | request as described in Section 4.1.1, and if included ensure that
legitimate client would use the correct redirect URI it always uses | their values are identical.
for authorization requests. But this URI would not match the
tampered redirect URI used by the attacker (otherwise, the redirect In the attack scenario described in Section 4.5.1, the legitimate
would not land at the attacker's page). So the authorization server client would use the correct redirect URI it always uses for
would detect the attack and refuse to exchange the code. authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not
land at the attacker's page). So, the authorization server would
detect the attack and refuse to exchange the code.
This check could also detect attempts to inject an authorization code This check could also detect attempts to inject an authorization code
that had been obtained from another instance of the same client on that had been obtained from another instance of the same client on
another device if certain conditions are fulfilled: another device if certain conditions are fulfilled:
* the redirect URI itself needs to contain a nonce or another kind * the redirect URI itself contains a nonce or another kind of one-
of one-time use, secret data and time use, secret data and
* the client has bound this data to this particular instance of the * the client has bound this data to this particular instance of the
client. client.
But this approach conflicts with the idea of enforcing exact redirect But, this approach conflicts with the idea of enforcing exact
URI matching at the authorization endpoint. Moreover, it has been redirect URI matching at the authorization endpoint. Moreover, it
observed that providers very often ignore the redirect_uri check has been observed that providers very often ignore the redirect_uri
requirement at this stage, maybe because it doesn't seem to be check requirement at this stage, maybe because it doesn't seem to be
security-critical from reading the specification. security-critical from reading the specification.
Other providers just pattern match the redirect_uri parameter against Other providers just pattern match the redirect_uri parameter against
the registered redirect URI pattern. This saves the authorization the registered redirect URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the server from storing the link between the actual redirect URI and the
respective authorization code for every transaction. But this kind respective authorization code for every transaction. However, this
of check obviously does not fulfill the intent of the specification, kind of check obviously does not fulfill the intent of the
since the tampered redirect URI is not considered. So any attempt to specification, since the tampered redirect URI is not considered.
inject an authorization code obtained using the client_id of a So, any attempt to inject an authorization code obtained using the
legitimate client or by utilizing the legitimate client on another client_id of a legitimate client or by utilizing the legitimate
device will not be detected in the respective deployments. client on another device will not be detected in the respective
deployments.
It is also assumed that the requirements defined in [RFC6749], It is also assumed that the requirements defined in Section 4.1.3 of
Section 4.1.3, increase client implementation complexity as clients [RFC6749] increase client implementation complexity as clients need
need to store or re-construct the correct redirect URI for the call to store or reconstruct the correct redirect URI for the call to the
to the token endpoint. token endpoint.
Asymmetric methods for client authentication do not stop this attack, Asymmetric methods for client authentication do not stop this attack,
as the legitimate client authenticates at the token endpoint. as the legitimate client authenticates at the token endpoint.
This document therefore recommends instead binding every This document therefore recommends instead binding every
authorization code to a certain client instance on a certain device authorization code to a certain client instance on a certain device
(or in a certain user agent) in the context of a certain transaction (or in a certain user agent) in the context of a certain transaction
using one of the mechanisms described next. using one of the mechanisms described next.
4.5.3. Countermeasures 4.5.3. Countermeasures
There are two good technical solutions to binding authorization codes There are two good technical solutions to binding authorization codes
to client instances, outlined in the following. to client instances, as follows.
4.5.3.1. PKCE 4.5.3.1. PKCE
The PKCE mechanism specified in [RFC7636] can be used as a The PKCE mechanism specified in [RFC7636] can be used as a
countermeasure (even though it was originally designed to secure countermeasure (even though it was originally designed to secure
native apps). When the attacker attempts to inject an authorization native apps). When the attacker attempts to inject an authorization
code, the check of the code_verifier fails: the client uses its code, the check of the code_verifier fails: the client uses its
correct verifier, but the code is associated with a code_challenge correct verifier, but the code is associated with a code_challenge
that does not match this verifier. that does not match this verifier.
PKCE does not only protect against the authorization code injection PKCE not only protects against the authorization code injection
attack but also protects authorization codes created for public attack but also protects authorization codes created for public
clients: PKCE ensures that an attacker cannot redeem a stolen clients: PKCE ensures that an attacker cannot redeem a stolen
authorization code at the token endpoint of the authorization server authorization code at the token endpoint of the authorization server
without knowledge of the code_verifier. without knowledge of the code_verifier.
4.5.3.2. Nonce 4.5.3.2. Nonce
OpenID Connect's existing nonce parameter can protect against OpenID Connect's existing nonce parameter can protect against
authorization code injection attacks. The nonce value is one-time authorization code injection attacks. The nonce value is one-time
use and is created by the client. The client is supposed to bind it use and is created by the client. The client is supposed to bind it
to the user agent session and send it with the initial request to the to the user agent session and send it with the initial request to the
OpenID Provider (OP). The OP puts the received nonce value into the OpenID Provider (OP). The OP puts the received nonce value into the
ID Token that is issued as part of the code exchange at the token ID Token that is issued as part of the code exchange at the token
endpoint. If an attacker injects an authorization code in the endpoint. If an attacker injects an authorization code in the
authorization response, the nonce value in the client session and the authorization response, the nonce value in the client session and the
nonce value in the ID token received from the token endpoint will not nonce value in the ID Token received from the token endpoint will not
match and the attack is detected. The assumption is that an attacker match, and the attack is detected. The assumption is that an
cannot get hold of the user agent state on the victim's device (from attacker cannot get hold of the user agent state on the victim's
which the attacker has stolen the respective authorization code). device (from which the attacker has stolen the respective
authorization code).
It is important to note that this countermeasure only works if the It is important to note that this countermeasure only works if the
client properly checks the nonce parameter in the ID Token obtained client properly checks the nonce parameter in the ID Token obtained
from the token endpoint and does not use any issued token until this from the token endpoint and does not use any issued token until this
check has succeeded. More precisely, a client protecting itself check has succeeded. More precisely, a client protecting itself
against code injection using the nonce parameter against code injection using the nonce parameter
1. MUST validate the nonce in the ID Token obtained from the token 1. MUST validate the nonce in the ID Token obtained from the token
endpoint, even if another ID Token was obtained from the endpoint, even if another ID Token was obtained from the
authorization response (e.g., response_type=code+id_token), and authorization response (e.g., response_type=code+id_token), and
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tokens (ID Tokens and the access token) are disregarded and not tokens (ID Tokens and the access token) are disregarded and not
used for any other purpose. used for any other purpose.
It is important to note that nonce does not protect authorization It is important to note that nonce does not protect authorization
codes of public clients, as an attacker does not need to execute an codes of public clients, as an attacker does not need to execute an
authorization code injection attack. Instead, an attacker can authorization code injection attack. Instead, an attacker can
directly call the token endpoint with the stolen authorization code. directly call the token endpoint with the stolen authorization code.
4.5.3.3. Other Solutions 4.5.3.3. Other Solutions
Other solutions, like binding state to the code, sender-constraining Other solutions like binding state to the code, sender-constraining
the code using cryptographic means, or per-instance client the code using cryptographic means, or per-instance client
credentials are conceivable, but lack support and bring new security credentials are conceivable, but lack support and bring new security
requirements. requirements.
PKCE is the most obvious solution for OAuth clients as it is PKCE is the most obvious solution for OAuth clients, as it is
available today, while nonce is appropriate for OpenID Connect available at the time of writing, while nonce is appropriate for
clients. OpenID Connect clients.
4.5.4. Limitations 4.5.4. Limitations
An attacker can circumvent the countermeasures described above if he An attacker can circumvent the countermeasures described above if
can modify the nonce or code_challenge values that are used in the they can modify the nonce or code_challenge values that are used in
victim's authorization request. The attacker can modify these values the victim's authorization request. The attacker can modify these
to be the same ones as those chosen by the client in their own values to be the same ones as those chosen by the client in their own
session in Step 2 of the attack above. (This requires that the session in Step 2 of the attack above. (This requires that the
victim's session with the client begins after the attacker started victim's session with the client begins after the attacker started
their session with the client.) If the attacker is then able to their session with the client.) If the attacker is then able to
capture the authorization code from the victim, the attacker will be capture the authorization code from the victim, the attacker will be
able to inject the stolen code in Step 3 even if PKCE or nonce are able to inject the stolen code in Step 3 even if PKCE or nonce are
used. used.
This attack is complex and requires a close interaction between the This attack is complex and requires a close interaction between the
attacker and the victim's session. Nonetheless, measures to prevent attacker and the victim's session. Nonetheless, measures to prevent
attackers from reading the contents of the authorization response attackers from reading the contents of the authorization response
still need to be taken, as described in Section 4.1, Section 4.2, still need to be taken, as described in Sections 4.1, 4.2, 4.3, 4.4,
Section 4.3, Section 4.4, and Section 4.11. and 4.11.
4.6. Access Token Injection 4.6. Access Token Injection
In an access token injection attack, the attacker attempts to inject In an access token injection attack, the attacker attempts to inject
a stolen access token into a legitimate client (that is not under the a stolen access token into a legitimate client (that is not under the
attacker's control). This will typically happen if the attacker attacker's control). This will typically happen if the attacker
wants to utilize a leaked access token to impersonate a user in a wants to utilize a leaked access token to impersonate a user in a
certain client. certain client.
To conduct the attack, the attacker starts an OAuth flow with the To conduct the attack, the attacker starts an OAuth flow with the
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There is no way to detect such an injection attack in pure-OAuth There is no way to detect such an injection attack in pure-OAuth
flows since the token is issued without any binding to the flows since the token is issued without any binding to the
transaction or the particular user agent. transaction or the particular user agent.
In OpenID Connect, the attack can be mitigated, as the authorization In OpenID Connect, the attack can be mitigated, as the authorization
response additionally contains an ID Token containing the at_hash response additionally contains an ID Token containing the at_hash
claim. The attacker therefore needs to replace both the access token claim. The attacker therefore needs to replace both the access token
as well as the ID Token in the response. The attacker cannot forge as well as the ID Token in the response. The attacker cannot forge
the ID Token, as it is signed or encrypted with authentication. The the ID Token, as it is signed or encrypted with authentication. The
attacker also cannot inject a leaked ID Token matching the stolen attacker also cannot inject a leaked ID Token matching the stolen
access token, as the nonce claim in the leaked ID Token will (with a access token, as the nonce claim in the leaked ID Token will contain
very high probability) contain a different value than the one (with a very high probability) a different value than the one
expected in the authorization response. expected in the authorization response.
Note that further protection, like sender-constrained access tokens, Note that further protection, like sender-constrained access tokens,
is still required to prevent attackers from using the access token at is still required to prevent attackers from using the access token at
the resource endpoint directly. the resource endpoint directly.
The recommendations in Section 2.1.2 follow from this. The recommendations in Section 2.1.2 follow from this.
4.7. Cross-Site Request Forgery 4.7. Cross-Site Request Forgery
An attacker might attempt to inject a request to the redirect URI of An attacker might attempt to inject a request to the redirect URI of
the legitimate client on the victim's device, e.g., to cause the the legitimate client on the victim's device, e.g., to cause the
client to access resources under the attacker's control. This is a client to access resources under the attacker's control. This is a
variant of an attack known as Cross-Site Request Forgery (CSRF). variant of an attack known as Cross-Site Request Forgery (CSRF).
4.7.1. Countermeasures 4.7.1. Countermeasures
The long-established countermeasure is that clients pass a random The long-established countermeasure is that clients pass a random
value, also known as a CSRF Token, in the state parameter that links value, also known as a CSRF Token, in the state parameter that links
the request to the redirect URI to the user agent session as the request to the redirect URI to the user agent session as
described. This countermeasure is described in detail in [RFC6819], described. This countermeasure is described in detail in
Section 5.3.5. The same protection is provided by PKCE or the OpenID Section 5.3.5 of [RFC6819]. The same protection is provided by PKCE
Connect nonce value. or the OpenID Connect nonce value.
When using PKCE instead of state or nonce for CSRF protection, it is When using PKCE instead of state or nonce for CSRF protection, it is
important to note that: important to note that:
* Clients MUST ensure that the authorization server supports PKCE * Clients MUST ensure that the authorization server supports PKCE
before using PKCE for CSRF protection. If an authorization server before using PKCE for CSRF protection. If an authorization server
does not support PKCE, state or nonce MUST be used for CSRF does not support PKCE, state or nonce MUST be used for CSRF
protection. protection.
* If state is used for carrying application state, and the integrity * If state is used for carrying application state, and the integrity
of its contents is a concern, clients MUST protect state against of its contents is a concern, clients MUST protect state against
tampering and swapping. This can be achieved by binding the tampering and swapping. This can be achieved by binding the
contents of state to the browser session and/or signed/encrypted contents of state to the browser session and/or signed/encrypted
state values. One example of this is discussed in the now-expired state values. One example of this is discussed in the expired
draft [I-D.bradley-oauth-jwt-encoded-state]. Internet-Draft [JWT-ENCODED-STATE].
The authorization server therefore MUST provide a way to detect their The authorization server therefore MUST provide a way to detect their
support for PKCE. Using Authorization Server Metadata according to support for PKCE. Using Authorization Server Metadata according to
[RFC8414] is RECOMMENDED, but authorization servers MAY instead [RFC8414] is RECOMMENDED, but authorization servers MAY instead
provide a deployment-specific way to ensure or determine PKCE provide a deployment-specific way to ensure or determine PKCE
support. support.
PKCE provides robust protection against CSRF attacks even in presence PKCE provides robust protection against CSRF attacks even in the
of an attacker that can read the authorization response (see Attacker presence of an attacker that can read the authorization response (see
A3 in Section 3). When state is used or an ID Token is returned in Attacker (A3) in Section 3). When state is used or an ID Token is
the authorization response (e.g., response_type=code+id_token), the returned in the authorization response (e.g.,
attacker either learns the state value and can replay it into the response_type=code+id_token), the attacker either learns the state
forged authorization response, or can extract the nonce from the ID value and can replay it into the forged authorization response, or
Token and use it in a new request to the authorization server to mint can extract the nonce from the ID Token and use it in a new request
an ID Token with the same nonce. The new ID Token can then be used to the authorization server to mint an ID Token with the same nonce.
for the CSRF attack. The new ID Token can then be used for the CSRF attack.
4.8. PKCE Downgrade Attack 4.8. PKCE Downgrade Attack
An authorization server that supports PKCE but does not make its use An authorization server that supports PKCE but does not make its use
mandatory for all flows can be susceptible to a PKCE downgrade mandatory for all flows can be susceptible to a PKCE downgrade
attack. attack.
The first prerequisite for this attack is that there is an attacker- The first prerequisite for this attack is that there is an attacker-
controllable flag in the authorization request that enables or controllable flag in the authorization request that enables or
disables PKCE for the particular flow. The presence or absence of disables PKCE for the particular flow. The presence or absence of
the code_challenge parameter lends itself for this purpose, i.e., the the code_challenge parameter lends itself for this purpose, i.e., the
authorization server enables and enforces PKCE if this parameter is authorization server enables and enforces PKCE if this parameter is
present in the authorization request, but does not enforce PKCE if present in the authorization request, but it does not enforce PKCE if
the parameter is missing. the parameter is missing.
The second prerequisite for this attack is that the client is not The second prerequisite for this attack is that the client is not
using state at all (e.g., because the client relies on PKCE for CSRF using state at all (e.g., because the client relies on PKCE for CSRF
prevention) or that the client is not checking state correctly. prevention) or that the client is not checking state correctly.
Roughly speaking, this attack is a variant of a CSRF attack. The Roughly speaking, this attack is a variant of a CSRF attack. The
attacker achieves the same goal as in the attack described in attacker achieves the same goal as in the attack described in
Section 4.7: The attacker injects an authorization code (and with Section 4.7: The attacker injects an authorization code (and with
that, an access token) that is bound to the attacker's resources into that, an access token) that is bound to the attacker's resources into
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a session between their victim and the client. a session between their victim and the client.
4.8.1. Attack Description 4.8.1. Attack Description
1. The user has started an OAuth session using some client at an 1. The user has started an OAuth session using some client at an
authorization server. In the authorization request, the client authorization server. In the authorization request, the client
has set the parameter code_challenge=hash(abc) as the PKCE code has set the parameter code_challenge=hash(abc) as the PKCE code
challenge (with the hash function and parameter encoding as challenge (with the hash function and parameter encoding as
defined in [RFC7636]). The client is now waiting to receive the defined in [RFC7636]). The client is now waiting to receive the
authorization response from the user's browser. authorization response from the user's browser.
2. To conduct the attack, the attacker uses their own device to 2. To conduct the attack, the attacker uses their own device to
start an authorization flow with the targeted client. The client start an authorization flow with the targeted client. The client
now uses another PKCE code challenge, say now uses another PKCE code challenge, say,
code_challenge=hash(xyz), in the authorization request. The code_challenge=hash(xyz), in the authorization request. The
attacker intercepts the request and removes the entire attacker intercepts the request and removes the entire
code_challenge parameter from the request. Since this step is code_challenge parameter from the request. Since this step is
performed on the attacker's device, the attacker has full access performed on the attacker's device, the attacker has full access
to the request contents, for example using browser debug tools. to the request contents, for example, using browser debug tools.
3. If the authorization server allows for flows without PKCE, it 3. If the authorization server allows for flows without PKCE, it
will create a code that is not bound to any PKCE code challenge. will create a code that is not bound to any PKCE code challenge.
4. The attacker now redirects the user's browser to an authorization 4. The attacker now redirects the user's browser to an authorization
response URL that contains the code for the attacker's session response URL that contains the code for the attacker's session
with the authorization server. with the authorization server.
5. The user's browser sends the authorization code to the client, 5. The user's browser sends the authorization code to the client,
which will now try to redeem the code for an access token at the which will now try to redeem the code for an access token at the
authorization server. The client will send code_verifier=abc as authorization server. The client will send code_verifier=abc as
the PKCE code verifier in the token request. the PKCE code verifier in the token request.
6. Since the authorization server sees that this code is not bound 6. Since the authorization server sees that this code is not bound
to any PKCE code challenge, it will not check the presence or to any PKCE code challenge, it will not check the presence or
contents of the code_verifier parameter. It will issue an access contents of the code_verifier parameter. It will issue an access
token that belongs to the attacker's resource to the client under token (which belongs to the attacker's resource) to the client
the user's control. under the user's control.
4.8.2. Countermeasures 4.8.2. Countermeasures
Using state properly would prevent this attack. However, practice Using state properly would prevent this attack. However, practice
has shown that many OAuth clients do not use or check state properly. has shown that many OAuth clients do not use or check state properly.
Therefore, authorization servers MUST mitigate this attack. Therefore, authorization servers MUST mitigate this attack.
Note that from the view of the authorization server, in the attack Note that from the view of the authorization server, in the attack
described above, a code_verifier parameter is received at the token described above, a code_verifier parameter is received at the token
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* when a code arrives at the token endpoint, and there was a * when a code arrives at the token endpoint, and there was a
code_challenge in the authorization request for which this code code_challenge in the authorization request for which this code
was issued, there must be a valid code_verifier in the token was issued, there must be a valid code_verifier in the token
request. request.
Beyond this, to prevent PKCE downgrade attacks, the authorization Beyond this, to prevent PKCE downgrade attacks, the authorization
server MUST ensure that if there was no code_challenge in the server MUST ensure that if there was no code_challenge in the
authorization request, a request to the token endpoint containing a authorization request, a request to the token endpoint containing a
code_verifier is rejected. code_verifier is rejected.
Authorization servers that mandate the use of PKCE in general or for Authorization servers that mandate the use of PKCE (in general or for
particular clients implicitly implement this security measure. particular clients) implicitly implement this security measure.
4.9. Access Token Leakage at the Resource Server 4.9. Access Token Leakage at the Resource Server
Access tokens can leak from a resource server under certain Access tokens can leak from a resource server under certain
circumstances. circumstances.
4.9.1. Access Token Phishing by Counterfeit Resource Server 4.9.1. Access Token Phishing by Counterfeit Resource Server
An attacker may set up their own resource server and trick a client An attacker may set up their own resource server and trick a client
into sending access tokens to it that are valid for other resource into sending access tokens to it that are valid for other resource
servers (see Attackers A1 and A5 in Section 3). If the client sends servers (see Attackers (A1) and (A5) in Section 3). If the client
a valid access token to this counterfeit resource server, the sends a valid access token to this counterfeit resource server, the
attacker in turn may use that token to access other services on attacker in turn may use that token to access other services on
behalf of the resource owner. behalf of the resource owner.
This attack assumes the client is not bound to one specific resource This attack assumes the client is not bound to one specific resource
server (and its URL) at development time, but client instances are server (and its URL) at development time, but client instances are
provided with the resource server URL at runtime. This kind of late provided with the resource server URL at runtime. This kind of late
binding is typical in situations where the client uses a service binding is typical in situations where the client uses a service
implementing a standardized API (e.g., for e-mail, calendar, health, implementing a standardized API (e.g., for email, calendar, health,
or banking) and where the client is configured by a user or or banking) and where the client is configured by a user or
administrator for a service that this user or company uses. administrator for a service that this user or company uses.
4.9.2. Compromised Resource Server 4.9.2. Compromised Resource Server
An attacker may compromise a resource server to gain access to the An attacker may compromise a resource server to gain access to the
resources of the respective deployment. Such a compromise may range resources of the respective deployment. Such a compromise may range
from partial access to the system, e.g., its log files, to full from partial access to the system, e.g., its log files, to full
control over the respective server, in which case all controls can be control over the respective server, in which case all controls can be
circumvented and all resources can be accessed. The attacker would circumvented and all resources can be accessed. The attacker would
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* Sender-constrained access tokens, as described in Section 4.10.1, * Sender-constrained access tokens, as described in Section 4.10.1,
SHOULD be used to prevent the attacker from replaying the access SHOULD be used to prevent the attacker from replaying the access
tokens on other resource servers. If an attacker has only partial tokens on other resource servers. If an attacker has only partial
access to the compromised system, like a read-only access to web access to the compromised system, like a read-only access to web
server logs, sender-constrained access tokens may also prevent server logs, sender-constrained access tokens may also prevent
replay on the compromised system. replay on the compromised system.
* Audience restriction as described in Section 4.10.2 SHOULD be used * Audience restriction as described in Section 4.10.2 SHOULD be used
to prevent replay of captured access tokens on other resource to prevent replay of captured access tokens on other resource
servers. servers.
* The resource server MUST treat access tokens like other sensitive * The resource server MUST treat access tokens like other sensitive
secrets and not store or transfer them in plain text. secrets and not store or transfer them in plaintext.
The first and second recommendations also apply to other scenarios The first and second recommendations also apply to other scenarios
where access tokens leak (see Attacker A5 in Section 3). where access tokens leak (see Attacker (A5) in Section 3).
4.10. Misuse of Stolen Access Tokens 4.10. Misuse of Stolen Access Tokens
Access tokens can be stolen by an attacker in various ways, for Access tokens can be stolen by an attacker in various ways, for
example, via the attacks described in Section 4.1, Section 4.2, example, via the attacks described in Sections 4.1, 4.2, 4.3, 4.4,
Section 4.3, Section 4.4 and Section 4.9. Some of these attacks can and 4.9. Some of these attacks can be mitigated by specific security
be mitigated by specific security measures, as described in the measures, as described in the respective sections. However, in some
respective sections. However, in some cases, these measures are not cases, these measures are not sufficient or are not implemented
sufficient or are not implemented correctly. Authorization servers correctly. Authorization servers therefore SHOULD ensure that access
therefore SHOULD ensure that access tokens are sender-constrained and tokens are sender-constrained and audience-restricted as described in
audience-restricted as described in the following. Architecture and the following. Architecture and performance reasons may prevent the
performance reasons may prevent the use of these measures in some use of these measures in some deployments.
deployments.
4.10.1. Sender-Constrained Access Tokens 4.10.1. Sender-Constrained Access Tokens
As the name suggests, sender-constrained access tokens scope the As the name suggests, sender-constrained access tokens scope the
applicability of an access token to a certain sender. This sender is applicability of an access token to a certain sender. This sender is
obliged to demonstrate knowledge of a certain secret as a obliged to demonstrate knowledge of a certain secret as a
prerequisite for the acceptance of that token at a resource server. prerequisite for the acceptance of that token at a resource server.
A typical flow looks like this: A typical flow looks like this:
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obliged to demonstrate knowledge of a certain secret as a obliged to demonstrate knowledge of a certain secret as a
prerequisite for the acceptance of that token at a resource server. prerequisite for the acceptance of that token at a resource server.
A typical flow looks like this: A typical flow looks like this:
1. The authorization server associates data with the access token 1. The authorization server associates data with the access token
that binds this particular token to a certain client. The that binds this particular token to a certain client. The
binding can utilize the client's identity, but in most cases, the binding can utilize the client's identity, but in most cases, the
authorization server utilizes key material (or data derived from authorization server utilizes key material (or data derived from
the key material) known to the client. the key material) known to the client.
2. This key material must be distributed somehow. Either the key 2. This key material must be distributed somehow. Either the key
material already exists before the authorization server creates material already exists before the authorization server creates
the binding or the authorization server creates ephemeral keys. the binding or the authorization server creates ephemeral keys.
The way pre-existing key material is distributed varies among the The way preexisting key material is distributed varies among the
different approaches. For example, X.509 Certificates can be different approaches. For example, X.509 certificates can be
used, in which case the distribution happens explicitly during used, in which case the distribution happens explicitly during
the enrollment process. Or the key material is created and the enrollment process. Or, the key material is created and
distributed at the TLS layer, in which case it might distributed at the TLS layer, in which case it might
automatically happen during the setup of a TLS connection. automatically happen during the setup of a TLS connection.
3. The resource server must implement the actual proof of possession 3. The resource server must implement the actual proof-of-possession
check. This is typically done on the application level, often check. This is typically done on the application level, often
tied to specific material provided by transport layer (e.g., tied to specific material provided by the transport layer (e.g.,
TLS). The resource server must also ensure that a replay of the TLS). The resource server must also ensure that a replay of the
proof of possession is not possible. proof of possession is not possible.
Two methods for sender-constrained access tokens using proof-of- Two methods for sender-constrained access tokens using proof of
possession have been defined by the OAuth working group and are in possession have been defined by the OAuth working group and are in
use in practice: use in practice:
* OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound * "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
Access Tokens ([RFC8705]): The approach specified in this document Access Tokens" [RFC8705]: The approach specified in this) document
allows the use of mutual TLS (mTLS) for both client authentication allows the use of mutual TLS (mTLS) for both client authentication
and sender-constrained access tokens. For the purpose of sender- and sender-constrained access tokens. For the purpose of sender-
constrained access tokens, the client is identified towards the constrained access tokens, the client is identified towards the
resource server by the fingerprint of its public key. During the resource server by the fingerprint of its public key. During the
processing of an access token request, the authorization server processing of an access token request, the authorization server
obtains the client's public key from the TLS stack and associates obtains the client's public key from the TLS stack and associates
its fingerprint with the respective access tokens. The resource its fingerprint with the respective access tokens. The resource
server in the same way obtains the public key from the TLS stack server in the same way obtains the public key from the TLS stack
and compares its fingerprint with the fingerprint associated with and compares its fingerprint with the fingerprint associated with
the access token. the access token.
* OAuth 2.0 Demonstrating Proof of Possession (DPoP) ([RFC9449]): * "OAuth 2.0 Demonstrating Proof of Possession (DPoP)" [RFC9449]:
DPoP outlines an application-level sender-constraining for access DPoP outlines an application-level sender-constraining for access
and refresh tokens. It uses proof-of-possession based on a and refresh tokens. It uses proof-of-possession based on a
public/private key pair and application-level signing. DPoP can public/private key pair and application-level signing. DPoP can
be used with public clients and, in the case of confidential be used with public clients and, in the case of confidential
clients, can be combined with any client authentication method. clients, can be combined with any client authentication method.
Note that the security of sender-constrained tokens is undermined Note that the security of sender-constrained tokens is undermined
when an attacker gets access to the token and the key material. This when an attacker gets access to the token and the key material. This
is, in particular, the case for corrupted client software and cross- is, in particular, the case for corrupted client software and cross-
site scripting attacks (when the client is running in the browser). site scripting attacks (when the client is running in the browser).
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module or only indirectly accessible (like in a TLS stack), sender- module or only indirectly accessible (like in a TLS stack), sender-
constrained tokens at least protect against the use of the token when constrained tokens at least protect against the use of the token when
the client is offline, i.e., when the security module or interface is the client is offline, i.e., when the security module or interface is
not available to the attacker. This applies to access tokens as well not available to the attacker. This applies to access tokens as well
as to refresh tokens (see Section 4.14). as to refresh tokens (see Section 4.14).
4.10.2. Audience-Restricted Access Tokens 4.10.2. Audience-Restricted Access Tokens
Audience restriction essentially restricts access tokens to a Audience restriction essentially restricts access tokens to a
particular resource server. The authorization server associates the particular resource server. The authorization server associates the
access token with the particular resource server and the resource access token with the particular resource server, and the resource
server is then supposed to verify the intended audience. If the server is then supposed to verify the intended audience. If the
access token fails the intended audience validation, the resource access token fails the intended audience validation, the resource
server refuses to serve the respective request. server refuses to serve the respective request.
In general, audience restriction limits the impact of token leakage. In general, audience restriction limits the impact of token leakage.
In the case of a counterfeit resource server, it may (as described In the case of a counterfeit resource server, it may (as described
below) also prevent abuse of the phished access token at the below) also prevent abuse of the phished access token at the
legitimate resource server. legitimate resource server.
The audience can be expressed using logical names or physical The audience can be expressed using logical names or physical
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legitimate resource server's URL by using a valid TLS certificate legitimate resource server's URL by using a valid TLS certificate
obtained from a different CA. It might also be considered a privacy obtained from a different CA. It might also be considered a privacy
benefit to hide the resource server URL from the authorization benefit to hide the resource server URL from the authorization
server. server.
Audience restriction may seem easier to use since it does not require Audience restriction may seem easier to use since it does not require
any cryptography on the client side. Still, since every access token any cryptography on the client side. Still, since every access token
is bound to a specific resource server, the client also needs to is bound to a specific resource server, the client also needs to
obtain a single resource server-specific access token when accessing obtain a single resource server-specific access token when accessing
several resource servers. (Resource indicators, as specified in several resource servers. (Resource indicators, as specified in
[RFC8707], can help to achieve this.) [I-D.ietf-oauth-token-binding] [RFC8707], can help to achieve this.) [TOKEN-BINDING] had the same
had the same property since different token-binding IDs must be property since different token-binding IDs must be associated with
associated with the access token. Using [RFC8705], on the other the access token. Using Mutual TLS for OAuth 2.0 [RFC8705], on the
hand, allows a client to use the access token at multiple resource other hand, allows a client to use the access token at multiple
servers. resource servers.
It should be noted that audience restrictions, or generally speaking It should be noted that audience restrictions -- or, generally
an indication by the client to the authorization server where it speaking, an indication by the client to the authorization server
wants to use the access token, have additional benefits beyond the where it wants to use the access token -- have additional benefits
scope of token leakage prevention. It allows the authorization beyond the scope of token leakage prevention. They allow the
server to create a different access token whose format and content authorization server to create a different access token whose format
are specifically minted for the respective server. This has huge and content are specifically minted for the respective server. This
functional and privacy advantages in deployments using structured has huge functional and privacy advantages in deployments using
access tokens. structured access tokens.
4.10.3. Discussion: Preventing Leakage via Metadata 4.10.3. Discussion: Preventing Leakage via Metadata
An authorization server could provide the client with additional An authorization server could provide the client with additional
information about the locations where it is safe to use its access information about the locations where it is safe to use its access
tokens. This approach, and why it is not recommended, is discussed tokens. This approach, and why it is not recommended, is discussed
in the following. in the following.
In the simplest form, this would require the authorization server to In the simplest form, this would require the authorization server to
publish a list of its known resource servers, illustrated in the publish a list of its known resource servers, illustrated in the
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"issuer":"https://server.somesite.example", "issuer":"https://server.somesite.example",
"authorization_endpoint": "authorization_endpoint":
"https://server.somesite.example/authorize", "https://server.somesite.example/authorize",
"resource_servers":[ "resource_servers":[
"email.somesite.example", "email.somesite.example",
"storage.somesite.example", "storage.somesite.example",
"video.somesite.example" "video.somesite.example"
] ]
... ...
} }
The authorization server could also return the URL(s) an access token The authorization server could also return the URL(s) an access token
is good for in the token response, illustrated by the example and is good for in the token response, illustrated by the example and
non-standard return parameter access_token_resource_server: non-standard return parameter access_token_resource_server:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8 Content-Type: application/json;charset=UTF-8
Cache-Control: no-store Cache-Control: no-store
Pragma: no-cache Pragma: no-cache
{ {
"access_token":"2YotnFZFEjr1zCsicMWpAA", "access_token":"2YotnFZFEjr1zCsicMWpAA",
"access_token_resource_server": "access_token_resource_server":
"https://hostedresource.somesite.example/path1", "https://hostedresource.somesite.example/path1",
... ...
} }
This mitigation strategy would rely on the client to enforce the This mitigation strategy would rely on the client to enforce the
security policy and to only send access tokens to legitimate security policy and to only send access tokens to legitimate
destinations. Results of OAuth-related security research (see for destinations. Results of OAuth-related security research (see, for
example [research.ubc] and [research.cmu]) indicate a large portion example, [research.ubc] and [research.cmu]) indicate a large portion
of client implementations do not or fail to properly implement of client implementations do not or fail to properly implement
security controls, like state checks. So relying on clients to security controls, like state checks. So, relying on clients to
prevent access token phishing is likely to fail as well. Moreover, prevent access token phishing is likely to fail as well. Moreover,
given the ratio of clients to authorization and resource servers, it given the ratio of clients to authorization and resource servers, it
is considered the more viable approach to move as much as possible is considered the more viable approach to move as much as possible
security-related logic to those entities. Clearly, the client has to security-related logic to those entities. Clearly, the client has to
contribute to the overall security. However, there are alternative contribute to the overall security. However, there are alternative
countermeasures, as described before, that provide a better balance countermeasures, as described before, that provide a better balance
between the involved parties. between the involved parties.
4.11. Open Redirection 4.11. Open Redirection
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In order to prevent open redirection, clients should only redirect if In order to prevent open redirection, clients should only redirect if
the target URLs are allowed or if the origin and integrity of a the target URLs are allowed or if the origin and integrity of a
request can be authenticated. Countermeasures against open request can be authenticated. Countermeasures against open
redirection are described by OWASP [owasp.redir]. redirection are described by OWASP [owasp.redir].
4.11.2. Authorization Server as Open Redirector 4.11.2. Authorization Server as Open Redirector
Just as with clients, attackers could try to utilize a user's trust Just as with clients, attackers could try to utilize a user's trust
in the authorization server (and its URL in particular) for in the authorization server (and its URL in particular) for
performing phishing attacks. OAuth authorization servers regularly performing phishing attacks. OAuth authorization servers regularly
redirect users to other websites (the clients), but must do so redirect users to other websites (the clients), but they must do so
safely. safely.
[RFC6749], Section 4.1.2.1, already prevents open redirects by Section 4.1.2.1 of [RFC6749] already prevents open redirects by
stating that the authorization server MUST NOT automatically redirect stating that the authorization server MUST NOT automatically redirect
the user agent in case of an invalid combination of client_id and the user agent in case of an invalid combination of client_id and
redirect_uri. redirect_uri.
However, an attacker could also utilize a correctly registered However, an attacker could also utilize a correctly registered
redirect URI to perform phishing attacks. The attacker could, for redirect URI to perform phishing attacks. The attacker could, for
example, register a client via dynamic client registration [RFC7591] example, register a client via dynamic client registration [RFC7591]
and execute one of the following attacks: and execute one of the following attacks:
1. Intentionally send an erroneous authorization request, e.g., by 1. Intentionally send an erroneous authorization request, e.g., by
using an invalid scope value, thus instructing the authorization using an invalid scope value, thus instructing the authorization
server to redirect the user-agent to its phishing site. server to redirect the user agent to its phishing site.
2. Intentionally send a valid authorization request with client_id 2. Intentionally send a valid authorization request with client_id
and redirect_uri controlled by the attacker. After the user and redirect_uri controlled by the attacker. After the user
authenticates, the authorization server prompts the user to authenticates, the authorization server prompts the user to
provide consent to the request. If the user notices an issue provide consent to the request. If the user notices an issue
with the request and declines the request, the authorization with the request and declines the request, the authorization
server still redirects the user agent to the phishing site. In server still redirects the user agent to the phishing site. In
this case, the user agent will be redirected to the phishing site this case, the user agent will be redirected to the phishing site
regardless of the action taken by the user. regardless of the action taken by the user.
3. Intentionally send a valid silent authentication request 3. Intentionally send a valid silent authentication request
(prompt=none) with client_id and redirect_uri controlled by the (prompt=none) with client_id and redirect_uri controlled by the
attacker. In this case, the authorization server will attacker. In this case, the authorization server will
automatically redirect the user agent to the phishing site. automatically redirect the user agent to the phishing site.
The authorization server MUST take precautions to prevent these The authorization server MUST take precautions to prevent these
threats. The authorization server MUST always authenticate the user threats. The authorization server MUST always authenticate the user
first and, with the exception of the silent authentication use case, first and, with the exception of the silent authentication use case,
prompt the user for credentials when needed, before redirecting the prompt the user for credentials when needed, before redirecting the
user. Based on its risk assessment, the authorization server needs user. Based on its risk assessment, the authorization server needs
to decide whether it can trust the redirect URI or not. It could to decide whether or not it can trust the redirect URI. It could
take into account URI analytics done internally or through some take into account URI analytics done internally or through some
external service to evaluate the credibility and trustworthiness of external service to evaluate the credibility and trustworthiness of
content behind the URI, and the source of the redirect URI and other content behind the URI, and the source of the redirect URI and other
client data. client data.
The authorization server SHOULD only automatically redirect the user The authorization server SHOULD only automatically redirect the user
agent if it trusts the redirect URI. If the URI is not trusted, the agent if it trusts the redirect URI. If the URI is not trusted, the
authorization server MAY inform the user and rely on the user to make authorization server MAY inform the user and rely on the user to make
the correct decision. the correct decision.
4.12. 307 Redirect 4.12. 307 Redirect
At the authorization endpoint, a typical protocol flow is that the At the authorization endpoint, a typical protocol flow is that the
authorization server prompts the user to enter their credentials in a authorization server prompts the user to enter their credentials in a
form that is then submitted (using the HTTP POST method) back to the form that is then submitted (using the HTTP POST method) back to the
authorization server. The authorization server checks the authorization server. The authorization server checks the
credentials and, if successful, redirects the user agent to the credentials and, if successful, redirects the user agent to the
client's redirection endpoint. client's redirection endpoint.
In [RFC6749], the HTTP status code 302 is used for this purpose, but In [RFC6749], the HTTP status code 302 (Found) is used for this
"any other method available via the user-agent to accomplish this purpose, but "any other method available via the user-agent to
redirection is allowed". When the status code 307 is used for accomplish this redirection is allowed". When the status code 307 is
redirection instead, the user agent will send the user's credentials used for redirection instead, the user agent will send the user's
via HTTP POST to the client. credentials via HTTP POST to the client.
This discloses the sensitive credentials to the client. If the This discloses the sensitive credentials to the client. If the
client is malicious, it can use the credentials to impersonate the client is malicious, it can use the credentials to impersonate the
user at the authorization server. user at the authorization server.
The behavior might be unexpected for developers but is defined in The behavior might be unexpected for developers but is defined in
[RFC9110], Section 15.4.8. This status code does not require the Section 15.4.8 of [RFC9110]. This status code (307) does not require
user agent to rewrite the POST request to a GET request and thereby the user agent to rewrite the POST request to a GET request and
drop the form data in the POST request body. thereby drop the form data in the POST request body.
In the HTTP standard [RFC9110], only the status code 303 In the HTTP standard [RFC9110], only the status code 303
unambiguously enforces rewriting the HTTP POST request to an HTTP GET unambiguously enforces rewriting the HTTP POST request to an HTTP GET
request. For all other status codes, including the popular 302, user request. For all other status codes, including the popular 302, user
agents can opt not to rewrite POST to GET requests and therefore to agents can opt not to rewrite POST to GET requests and therefore to
reveal the user's credentials to the client. (In practice, however, reveal the user's credentials to the client. (In practice, however,
most user agents will only show this behaviour for 307 redirects.) most user agents will only show this behavior for 307 redirects.)
Authorization servers that redirect a request that potentially Authorization servers that redirect a request that potentially
contains the user's credentials therefore MUST NOT use the HTTP 307 contains the user's credentials therefore MUST NOT use the HTTP 307
status code for redirection. If an HTTP redirection (and not, for status code for redirection. If an HTTP redirection (and not, for
example, JavaScript) is used for such a request, the authorization example, JavaScript) is used for such a request, the authorization
server SHOULD use HTTP status code 303 (See Other). server SHOULD use HTTP status code 303 (See Other).
4.13. TLS Terminating Reverse Proxies 4.13. TLS Terminating Reverse Proxies
A common deployment architecture for HTTP applications is to hide the A common deployment architecture for HTTP applications is to hide the
application server behind a reverse proxy that terminates the TLS application server behind a reverse proxy that terminates the TLS
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logic performed in the application server, the attacker could simply logic performed in the application server, the attacker could simply
add an allowed IP address to the header and render the protection add an allowed IP address to the header and render the protection
useless. useless.
A reverse proxy MUST therefore sanitize any inbound requests to A reverse proxy MUST therefore sanitize any inbound requests to
ensure the authenticity and integrity of all header values relevant ensure the authenticity and integrity of all header values relevant
for the security of the application servers. for the security of the application servers.
If an attacker were able to get access to the internal network If an attacker were able to get access to the internal network
between the proxy and application server, the attacker could also try between the proxy and application server, the attacker could also try
to circumvent security controls in place. It is, therefore, to circumvent security controls in place. Therefore, it is essential
essential to ensure the authenticity of the communicating entities. to ensure the authenticity of the communicating entities.
Furthermore, the communication link between the reverse proxy and Furthermore, the communication link between the reverse proxy and
application server MUST be protected against eavesdropping, application server MUST be protected against eavesdropping,
injection, and replay of messages. injection, and replay of messages.
4.14. Refresh Token Protection 4.14. Refresh Token Protection
Refresh tokens are a convenient and user-friendly way to obtain new Refresh tokens are a convenient and user-friendly way to obtain new
access tokens. They also add to the security of OAuth, since they access tokens. They also add to the security of OAuth, since they
allow the authorization server to issue access tokens with a short allow the authorization server to issue access tokens with a short
lifetime and reduced scope, thus reducing the potential impact of lifetime and reduced scope, thus reducing the potential impact of
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Authorization servers MUST utilize one of these methods to detect Authorization servers MUST utilize one of these methods to detect
refresh token replay by malicious actors for public clients: refresh token replay by malicious actors for public clients:
* *Sender-constrained refresh tokens:* the authorization server * *Sender-constrained refresh tokens:* the authorization server
cryptographically binds the refresh token to a certain client cryptographically binds the refresh token to a certain client
instance, e.g., by utilizing [RFC8705] or [RFC9449]. instance, e.g., by utilizing [RFC8705] or [RFC9449].
* *Refresh token rotation:* the authorization server issues a new * *Refresh token rotation:* the authorization server issues a new
refresh token with every access token refresh response. The refresh token with every access token refresh response. The
previous refresh token is invalidated but information about the previous refresh token is invalidated, but information about the
relationship is retained by the authorization server. If a relationship is retained by the authorization server. If a
refresh token is compromised and subsequently used by both the refresh token is compromised and subsequently used by both the
attacker and the legitimate client, one of them will present an attacker and the legitimate client, one of them will present an
invalidated refresh token, which will inform the authorization invalidated refresh token, which will inform the authorization
server of the breach. The authorization server cannot determine server of the breach. The authorization server cannot determine
which party submitted the invalid refresh token, but it will which party submitted the invalid refresh token, but it will
revoke the active refresh token. This stops the attack at the revoke the active refresh token. This stops the attack at the
cost of forcing the legitimate client to obtain a fresh cost of forcing the legitimate client to obtain a fresh
authorization grant. authorization grant.
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may be encoded into the refresh token itself. This can enable an may be encoded into the refresh token itself. This can enable an
authorization server to efficiently determine the grant to which a authorization server to efficiently determine the grant to which a
refresh token belongs, and by extension, all refresh tokens that refresh token belongs, and by extension, all refresh tokens that
need to be revoked. Authorization servers MUST ensure the need to be revoked. Authorization servers MUST ensure the
integrity of the refresh token value in this case, for example, integrity of the refresh token value in this case, for example,
using signatures. using signatures.
Authorization servers MAY revoke refresh tokens automatically in case Authorization servers MAY revoke refresh tokens automatically in case
of a security event, such as: of a security event, such as:
* password change * password change or
* logout at the authorization server * logout at the authorization server.
Refresh tokens SHOULD expire if the client has been inactive for some Refresh tokens SHOULD expire if the client has been inactive for some
time, i.e., the refresh token has not been used to obtain fresh time, i.e., the refresh token has not been used to obtain fresh
access tokens for some time. The expiration time is at the access tokens for some time. The expiration time is at the
discretion of the authorization server. It might be a global value discretion of the authorization server. It might be a global value
or determined based on the client policy or the grant associated with or determined based on the client policy or the grant associated with
the refresh token (and its sensitivity). the refresh token (and its sensitivity).
4.15. Client Impersonating Resource Owner 4.15. Client Impersonating Resource Owner
Resource servers may make access control decisions based on the Resource servers may make access control decisions based on the
identity of a resource owner for which an access token was issued, or identity of a resource owner for which an access token was issued, or
based on the identity of a client in the client credentials grant. based on the identity of a client in the client credentials grant.
For example, [RFC9068] (JSON Web Token (JWT) Profile for OAuth 2.0 For example, [RFC9068] (JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens) describes a data structure for access tokens Access Tokens) describes a data structure for access tokens
containing a sub claim defined as follows: containing a sub claim defined as follows:
| In cases of access tokens obtained through grants where a resource | In cases of access tokens obtained through grants where a resource
| owner is involved, such as the authorization code grant, the value | owner is involved, such as the authorization code grant, the value
| of sub SHOULD correspond to the subject identifier of the resource | of "sub" SHOULD correspond to the subject identifier of the
| owner. In cases of access tokens obtained through grants where no | resource owner. In cases of access tokens obtained through grants
| resource owner is involved, such as the client credentials grant, | where no resource owner is involved, such as the client
| the value of sub SHOULD correspond to an identifier the | credentials grant, the value of "sub" SHOULD correspond to an
| authorization server uses to indicate the client application. | identifier the authorization server uses to indicate the client
| application.
If both options are possible, a resource server may mistake a If both options are possible, a resource server may mistake a
client's identity for the identity of a resource owner. For example, client's identity for the identity of a resource owner. For example,
if a client is able to choose its own client_id during registration if a client is able to choose its own client_id during registration
with the authorization server, a malicious client may set it to a with the authorization server, a malicious client may set it to a
value identifying a resource owner (e.g., a sub value if OpenID value identifying a resource owner (e.g., a sub value if OpenID
Connect is used). If the resource server cannot properly distinguish Connect is used). If the resource server cannot properly distinguish
between access tokens obtained with involvement of the resource owner between access tokens obtained with involvement of the resource owner
and those without, the client may accidentally be able to access and those without, the client may accidentally be able to access
resources belonging to the resource owner. resources belonging to the resource owner.
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request is susceptible to clickjacking attacks, also called user request is susceptible to clickjacking attacks, also called user
interface redressing. In such an attack, an attacker embeds the interface redressing. In such an attack, an attacker embeds the
authorization endpoint user interface in an innocuous context. A authorization endpoint user interface in an innocuous context. A
user believing to interact with that context, for example, by user believing to interact with that context, for example, by
clicking on buttons, inadvertently interacts with the authorization clicking on buttons, inadvertently interacts with the authorization
endpoint user interface instead. The opposite can be achieved as endpoint user interface instead. The opposite can be achieved as
well: A user believing to interact with the authorization endpoint well: A user believing to interact with the authorization endpoint
might inadvertently type a password into an attacker-provided input might inadvertently type a password into an attacker-provided input
field overlaid over the original user interface. Clickjacking field overlaid over the original user interface. Clickjacking
attacks can be designed such that users can hardly notice the attack, attacks can be designed such that users can hardly notice the attack,
for example using almost invisible iframes overlaid on top of other for example, using almost invisible iframes overlaid on top of other
elements. elements.
An attacker can use this vector to obtain the user's authentication An attacker can use this vector to obtain the user's authentication
credentials, change the scope of access granted to the client, and credentials, change the scope of access granted to the client, and
potentially access the user's resources. potentially access the user's resources.
Authorization servers MUST prevent clickjacking attacks. Multiple Authorization servers MUST prevent clickjacking attacks. Multiple
countermeasures are described in [RFC6819], including the use of the countermeasures are described in [RFC6819], including the use of the
X-Frame-Options HTTP response header field and frame-busting X-Frame-Options HTTP response header field and frame-busting
JavaScript. In addition to those, authorization servers SHOULD also JavaScript. In addition to those, authorization servers SHOULD also
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authorize the client (e.g., the device authorization endpoint, login authorize the client (e.g., the device authorization endpoint, login
pages, error pages, etc.). This prevents framing by unauthorized pages, error pages, etc.). This prevents framing by unauthorized
origins in user agents that support CSP. The client MAY permit being origins in user agents that support CSP. The client MAY permit being
framed by some other origin than the one used in its redirection framed by some other origin than the one used in its redirection
endpoint. For this reason, authorization servers SHOULD allow endpoint. For this reason, authorization servers SHOULD allow
administrators to configure allowed origins for particular clients administrators to configure allowed origins for particular clients
and/or for clients to register these dynamically. and/or for clients to register these dynamically.
Using CSP allows authorization servers to specify multiple origins in Using CSP allows authorization servers to specify multiple origins in
a single response header field and to constrain these using flexible a single response header field and to constrain these using flexible
patterns (see [W3C.CSP-2] for details). Level 2 of this standard patterns (see [W3C.CSP-2] for details). Level 2 of CSP provides a
provides a robust mechanism for protecting against clickjacking by robust mechanism for protecting against clickjacking by using
using policies that restrict the origin of frames (using frame- policies that restrict the origin of frames (by using frame-
ancestors) together with those that restrict the sources of scripts ancestors) together with those that restrict the sources of scripts
allowed to execute on an HTML page (by using script-src). A non- allowed to execute on an HTML page (by using script-src). A non-
normative example of such a policy is shown in the following listing: normative example of such a policy is shown in the following listing:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Security-Policy: frame-ancestors https://ext.example.org:8000 Content-Security-Policy: frame-ancestors https://ext.example.org:8000
Content-Security-Policy: script-src 'self' Content-Security-Policy: script-src 'self'
X-Frame-Options: ALLOW-FROM https://ext.example.org:8000 X-Frame-Options: ALLOW-FROM https://ext.example.org:8000
... ...
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4.17. Attacks on In-Browser Communication Flows 4.17. Attacks on In-Browser Communication Flows
If the authorization response is sent with in-browser communication If the authorization response is sent with in-browser communication
techniques like postMessage [WHATWG.postmessage_api] instead of HTTP techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
redirects, messages may inadvertently be sent to malicious origins or redirects, messages may inadvertently be sent to malicious origins or
injected from malicious origins. injected from malicious origins.
4.17.1. Examples 4.17.1. Examples
The following non-normative pseudocode examples of attacks using in- The following non-normative pseudocode examples of attacks using in-
browser communication are described in [research.rub]: browser communication are described in [research.rub].
4.17.1.1. Insufficient Limitation of Receiver Origins 4.17.1.1. Insufficient Limitation of Receiver Origins
When sending the authorization response or token response via When sending the authorization response or token response via
postMessage, the authorization server sends the response to the postMessage, the authorization server sends the response to the
wildcard origin "*" instead of the client's origin. When the window wildcard origin "*" instead of the client's origin. When the window
to which the response is sent is controlled by an attacker, the to which the response is sent is controlled by an attacker, the
attacker can read the response. attacker can read the response.
window.opener.postMessage( window.opener.postMessage(
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code: "ABC", code: "ABC",
state: "123" state: "123"
}, },
"*" // any website in the opener window can receive the message "*" // any website in the opener window can receive the message
) )
4.17.1.2. Insufficient URI Validation 4.17.1.2. Insufficient URI Validation
When sending the authorization response or token response via When sending the authorization response or token response via
postMessage, the authorization server may not check the receiver postMessage, the authorization server may not check the receiver
origin against the redirect URI and instead, for example, send the origin against the redirect URI and instead, for example, may send
response to an origin provided by an attacker. This is analogous to the response to an origin provided by an attacker. This is analogous
the attack described in Section 4.1. to the attack described in Section 4.1.
window.opener.postMessage( window.opener.postMessage(
{ {
code: "ABC", code: "ABC",
state: "123" state: "123"
}, },
"https://attacker.example" // attacker-provided value "https://attacker.example" // attacker-provided value
) )
4.17.1.3. Injection after Insufficient Validation of Sender Origin 4.17.1.3. Injection after Insufficient Validation of Sender Origin
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token response into the client. token response into the client.
In the case of a maliciously injected authorization response, the In the case of a maliciously injected authorization response, the
attack is a variant of the CSRF attacks described in Section 4.7. attack is a variant of the CSRF attacks described in Section 4.7.
The countermeasures described in Section 4.7 apply to this attack as The countermeasures described in Section 4.7 apply to this attack as
well. well.
In the case of a maliciously injected token response, sender- In the case of a maliciously injected token response, sender-
constrained access tokens as described in Section 4.10.1 may prevent constrained access tokens as described in Section 4.10.1 may prevent
the attack under some circumstances, but additional countermeasures the attack under some circumstances, but additional countermeasures
as described next are generally required. as described in Section 4.17.2 are generally required.
4.17.2. Recommendations 4.17.2. Recommendations
When comparing client receiver origins against pre-registered When comparing client receiver origins against pre-registered
origins, authorization servers MUST utilize exact string matching as origins, authorization servers MUST utilize exact string matching as
described in Section 4.1.3. Authorization servers MUST send described in Section 4.1.3. Authorization servers MUST send
postMessages to trusted client receiver origins, as shown in the postMessages to trusted client receiver origins, as shown in the
following, non-normative example: following, non-normative example:
window.opener.postMessage( window.opener.postMessage(
{ {
code: "ABC", code: "ABC",
state: "123" state: "123"
}, },
"https://client.example" // use explicit client origin "https://client.example" // use explicit client origin
) )
Wildcard origins like "*" in postMessage MUST NOT be used as Wildcard origins like "*" in postMessage MUST NOT be used, as
attackers can use them to leak a victim's in-browser message to attackers can use them to leak a victim's in-browser message to
malicious origins. Both measures contribute to the prevention of malicious origins. Both measures contribute to the prevention of
leakage of authorization codes and access tokens (see Section 4.1). leakage of authorization codes and access tokens (see Section 4.1).
Clients MUST prevent injection of in-browser messages on the client Clients MUST prevent injection of in-browser messages on the client
receiver endpoint. Clients MUST utilize exact string matching to receiver endpoint. Clients MUST utilize exact string matching to
compare the initiator origin of an in-browser message with the compare the initiator origin of an in-browser message with the
authorization server origin, as shown in the following, non-normative authorization server origin, as shown in the following, non-normative
example: example:
skipping to change at page 48, line 17 skipping to change at line 2210
if (e.origin === "https://honest.as.example") { if (e.origin === "https://honest.as.example") {
// process e.data.code and e.data.state // process e.data.code and e.data.state
} }
}) })
Since in-browser communication flows only apply a different Since in-browser communication flows only apply a different
communication technique (i.e., postMessage instead of HTTP redirect), communication technique (i.e., postMessage instead of HTTP redirect),
all measures protecting the authorization response listed in all measures protecting the authorization response listed in
Section 2.1 MUST be applied equally. Section 2.1 MUST be applied equally.
5. Acknowledgements 5. IANA Considerations
We would like to thank Brock Allen, Annabelle Richard Backman, This document has no IANA actions.
Dominick Baier, Vittorio Bertocci, Brian Campbell, Bruno Crispo,
William Dennis, George Fletcher, Matteo Golinelli, Dick Hardt, Joseph
Heenan, Pedram Hosseyni, Phil Hunt, Tommaso Innocenti, Louis Jannett,
Jared Jennings, Michael B. Jones, Engin Kirda, Konstantin Lapine,
Neil Madden, Christian Mainka, Jim Manico, Nov Matake, Doug McDorman,
Ali Mirheidari, Vladislav Mladenov, Karsten Meyer zu Selhausen, Kaan
Onarioglu, Aaron Parecki, Michael Peck, Johan Peeters, Nat Sakimura,
Guido Schmitz, Jörg Schwenk, Rifaat Shekh-Yusef, Travis Spencer,
Petteri Stenius, Tomek Stojecki, Tim Wuertele, David Waite and Hans
Zandbelt for their valuable feedback.
6. IANA Considerations 6. Security Considerations
This draft makes no requests to IANA. Security considerations are described in Sections 2, 3, and 4.
7. Security Considerations 7. References
Security considerations are described in Section 2, Section 3, and 7.1. Normative References
Section 4.
8. References [BCP195] Best Current Practice 195,
<https://www.rfc-editor.org/info/bcp195>.
At the time of writing, this BCP comprises the following:
8.1. Normative References Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
[BCP195] IETF, "BCP195", <https://www.rfc-editor.org/info/bcp195>. Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>. <https://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012, RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>. <https://www.rfc-editor.org/info/rfc6749>.
skipping to change at page 49, line 48 skipping to change at line 2284
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T. [RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705, and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020, DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>. <https://www.rfc-editor.org/info/rfc8705>.
[RFC9068] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0 [RFC9068] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
2021, <https://www.rfc-editor.org/info/rfc9068>. 2021, <https://www.rfc-editor.org/info/rfc9068>.
8.2. Informative References 7.2. Informative References
[I-D.bradley-oauth-jwt-encoded-state] [JWT-ENCODED-STATE]
Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
claims in the OAuth 2 state parameter using a JWT", Work claims in the OAuth 2 state parameter using a JWT", Work
in Progress, Internet-Draft, draft-bradley-oauth-jwt- in Progress, Internet-Draft, draft-bradley-oauth-jwt-
encoded-state-09, 4 November 2018, encoded-state-09, 4 November 2018,
<https://datatracker.ietf.org/doc/html/draft-bradley- <https://datatracker.ietf.org/doc/html/draft-bradley-
oauth-jwt-encoded-state-09>. oauth-jwt-encoded-state-09>.
[I-D.ietf-oauth-token-binding] [TOKEN-BINDING]
Jones, M., Campbell, B., Bradley, J., and W. Denniss, Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", Work in Progress, Internet- "OAuth 2.0 Token Binding", Work in Progress, Internet-
Draft, draft-ietf-oauth-token-binding-08, 19 October 2018, Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth- <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
token-binding-08>. token-binding-08>.
[I-D.ietf-oauth-v2-1] [OAUTH-V2.1]
Hardt, D., Parecki, A., and T. Lodderstedt, "The OAuth 2.1 Hardt, D., Parecki, A., and T. Lodderstedt, "The OAuth 2.1
Authorization Framework", Work in Progress, Internet- Authorization Framework", Work in Progress, Internet-
Draft, draft-ietf-oauth-v2-1-11, 14 May 2024, Draft, draft-ietf-oauth-v2-1-12, 15 November 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth- <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
v2-1-11>. v2-1-12>.
[OAuth.Post] [OAuth.Post]
Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
Mode", 27 April 2015, <https://openid.net/specs/oauth-v2- Mode", The OpenID Foundation, 27 April 2015,
form-post-response-mode-1_0.html>. <https://openid.net/specs/oauth-v2-form-post-response-
mode-1_0.html>.
[OAuth.Responses] [OAuth.Responses]
de Medeiros, B., Scurtescu, M., Tarjan, P., and M. Jones, de Medeiros, B., Ed., Scurtescu, M., Tarjan, P., and M.
"OAuth 2.0 Multiple Response Type Encoding Practices", 25 Jones, "OAuth 2.0 Multiple Response Type Encoding
February 2014, <https://openid.net/specs/oauth-v2- Practices", The OpenID Foundation, 25 February 2014,
multiple-response-types-1_0.html>. <https://openid.net/specs/oauth-v2-multiple-response-
types-1_0.html>.
[OpenID.Core] [OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 2", 15 December 2023, errata set 2", The OpenID Foundation, 15 December 2023,
<https://openid.net/specs/openid-connect-core-1_0.html>. <https://openid.net/specs/openid-connect-core-1_0.html>.
[OpenID.Discovery] [OpenID.Discovery]
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0 incorporating errata set 2", 15 Connect Discovery 1.0 incorporating errata set 2", The
December 2023, <https://openid.net/specs/openid-connect- OpenID Foundation, 15 December 2023,
discovery-1_0.html>. <https://openid.net/specs/openid-connect-discovery-
1_0.html>.
[OpenID.JARM] [OpenID.JARM]
Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
Secured Authorization Response Mode for OAuth 2.0 (JARM)", Secured Authorization Response Mode for OAuth 2.0 (JARM)",
17 October 2018, The OpenID Foundation, 17 October 2018,
<https://openid.net/specs/openid-financial-api-jarm.html>. <https://openid.net/specs/openid-financial-api-jarm.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015, RFC 7591, DOI 10.17487/RFC7591, July 2015,
skipping to change at page 52, line 16 skipping to change at line 2399
Field", RFC 9440, DOI 10.17487/RFC9440, July 2023, Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
<https://www.rfc-editor.org/info/rfc9440>. <https://www.rfc-editor.org/info/rfc9440>.
[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T., [RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449, Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/info/rfc9449>. September 2023, <https://www.rfc-editor.org/info/rfc9449>.
[W3C.CSP-2] [W3C.CSP-2]
West, M., Barth, A., and D. Veditz, "Content Security West, M., Barth, A., and D. Veditz, "Content Security
Policy Level 2", July 2015, <https://www.w3.org/TR/CSP2>. Policy Level 2", W3C Recommendation, December 2016,
<https://www.w3.org/TR/2016/REC-CSP2-20161215/>. Latest
version available at <https://www.w3.org/TR/CSP2/>.
[W3C.WebAuthn] [W3C.WebAuthn]
Hodges, J., Jones, J.C., Jones, M.B., Kumar, A., and E. Hodges, J., Jones, J.C., Jones, M.B., Kumar, A., and E.
Lundberg, "Web Authentication: An API for accessing Public Lundberg, "Web Authentication: An API for accessing Public
Key Credentials Level 2", 8 April 2021, Key Credentials Level 2", W3C Recommendation, 8 April
2021,
<https://www.w3.org/TR/2021/REC-webauthn-2-20210408/>. <https://www.w3.org/TR/2021/REC-webauthn-2-20210408/>.
Latest version available at
<https://www.w3.org/TR/webauthn-2/>.
[W3C.WebCrypto] [W3C.WebCrypto]
Watson, M., "Web Cryptography API", 26 January 2017, Watson, M., Ed., "Web Cryptography API", W3C
Recommendation, 26 January 2017,
<https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>. <https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>.
Latest version available at
<https://www.w3.org/TR/WebCryptoAPI/>.
[W3C.webappsec-referrer-policy] [W3C.webappsec-referrer-policy]
Eisinger, J. and E. Stark, "Referrer Policy", 20 April Eisinger, J. and E. Stark, "Referrer Policy", 26 January
2017, <https://w3c.github.io/webappsec-referrer-policy>. 2017,
<https://www.w3.org/TR/2017/CR-referrer-policy-20170126/>.
Latest version available at
<https://www.w3.org/TR/referrer-policy/>.
[WHATWG.CORS] [WHATWG.CORS]
"Fetch Standard: CORS protocol", WHATWG, "CORS protocol", Fetch: Living Standard,
Section 3.2, 17 June 2024,
<https://fetch.spec.whatwg.org/#http-cors-protocol>. <https://fetch.spec.whatwg.org/#http-cors-protocol>.
[WHATWG.postmessage_api] [WHATWG.postmessage_api]
"HTML Living Standard: Cross-document messaging", WHATWG, "Cross-document messaging", HTML: Living Standard,
Section 9.3, 19 August 2024,
<https://html.spec.whatwg.org/multipage/web- <https://html.spec.whatwg.org/multipage/web-
messaging.html#web-messaging>. messaging.html#web-messaging>.
[arXiv.1508.04324v2] [arXiv.1508.04324v2]
Mladenov, V., Mainka, C., and J. Schwenk, "On the security Mladenov, V., Mainka, C., and J. Schwenk, "On the security
of modern Single Sign-On Protocols: Second-Order of modern Single Sign-On Protocols: Second-Order
Vulnerabilities in OpenID Connect", arXiv 1508.04324v2, 7 Vulnerabilities in OpenID Connect", arXiv:1508.04324v2,
January 2016, <https://arxiv.org/abs/1508.04324v2/>. DOI 10.48550/arXiv.1508.04324, 7 January 2016,
<https://arxiv.org/abs/1508.04324v2/>.
[arXiv.1601.01229] [arXiv.1601.01229]
Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0", arXiv 1601.01229, Formal Security Analysis of OAuth 2.0", arXiv:1601.01229,
6 January 2016, <https://arxiv.org/abs/1601.01229/>. DOI 10.48550/arXiv.1601.01229, 6 January 2016,
<https://arxiv.org/abs/1601.01229/>.
[arXiv.1704.08539] [arXiv.1704.08539]
Fett, D., Küsters, R., and G. Schmitz, "The Web SSO Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
Standard OpenID Connect: In-Depth Formal Security Analysis Standard OpenID Connect: In-Depth Formal Security Analysis
and Security Guidelines", arXiv 1704.08539, 27 April 2017, and Security Guidelines", arXiv:1704.08539,
DOI 10.48550/arXiv.1704.08539, 27 April 2017,
<https://arxiv.org/abs/1704.08539/>. <https://arxiv.org/abs/1704.08539/>.
[arXiv.1901.11520] [arXiv.1901.11520]
Fett, D., Hosseyni, P., and R. Küsters, "An Extensive Fett, D., Hosseyni, P., and R. Küsters, "An Extensive
Formal Security Analysis of the OpenID Financial-grade Formal Security Analysis of the OpenID Financial-grade
API", arXiv 1901.11520, 31 January 2019, API", arXiv:1901.11520, DOI 10.48550/arXiv.1901.11520, 31
<https://arxiv.org/abs/1901.11520/>. January 2019, <https://arxiv.org/abs/1901.11520/>.
[bug.chromium] [bug.chromium]
"Referer header includes URL fragment when opening link "Referer header includes URL fragment when opening link
using New Tab", using New Tab", Chromium Issue Tracker, Issue ID:
<https://issues.chromium.org/issues/40076763>. 40076763, <https://issues.chromium.org/issues/40076763>.
[owasp.redir] [owasp.redir]
"OWASP Cheat Sheet Series - Unvalidated Redirects and OWASP Foundation, "Unvalidated Redirects and Forwards
Forwards", Cheat Sheet", OWASP Cheat Sheet Series,
<https://cheatsheetseries.owasp.org/cheatsheets/ <https://cheatsheetseries.owasp.org/cheatsheets/
Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>. Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.
[research.cmu] [research.cmu]
Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P. Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
Tague, "OAuth Demystified for Mobile Application Tague, "OAuth Demystified for Mobile Application
Developers", November 2014, Developers", November 2014,
<https://css.csail.mit.edu/6.858/2012/readings/oauth- <https://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>. sso.pdf>.
[research.jcs_14] [research.jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S. Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
Maffeis, "Discovering concrete attacks on website Maffeis, "Discovering concrete attacks on website
authorization by formal analysis", 23 April 2014, authorization by formal analysis", Journal of Computer
Security, vol. 22, no. 4, pp. 601-657, DOI 10.3233/JCS-
140503, 23 April 2014,
<https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>. <https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.
[research.rub] [research.rub]
Jannett, L., Mladenov, V., Mainka, C., and J. Schwenk, Jannett, L., Mladenov, V., Mainka, C., and J. Schwenk,
"DISTINCT: Identity Theft using In-Browser Communications "DISTINCT: Identity Theft using In-Browser Communications
in Dual-Window Single Sign-On", in Dual-Window Single Sign-On", CCS '22: Proceedings of
DOI 10.1145/3548606.3560692, 7 November 2022, the 2022 ACM SIGSAC Conference on Computer and
<https://distinct-sso.com/paper.pdf>. Communications Security, DOI 10.1145/3548606.3560692, 7
November 2022,
<https://dl.acm.org/doi/pdf/10.1145/3548606.3560692>.
[research.rub2] [research.rub2]
Fries, C., "Security Analysis of Real-Life OpenID Connect Fries, C., "Security Analysis of Real-Life OpenID Connect
Implementations", 20 December 2020, Implementations", Master's thesis, Ruhr-Universität Bochum
(RUB), 20 December 2020,
<https://www.nds.rub.de/media/ei/arbeiten/2021/05/03/ <https://www.nds.rub.de/media/ei/arbeiten/2021/05/03/
masterthesis.pdf>. masterthesis.pdf>.
[research.ubc] [research.ubc]
Sun, S.-T. and K. Beznosov, "The Devil is in the Sun, S.-T. and K. Beznosov, "The Devil is in the
(Implementation) Details: An Empirical Analysis of OAuth (Implementation) Details: An Empirical Analysis of OAuth
SSO Systems", October 2012, SSO Systems", Proceedings of the 2012 ACM conference on
<https://passwordresearch.com/papers/paper267.html>. Computer and communications security (CCS '12), pp.
378-390, DOI 10.1145/2382196.2382238, October 2012,
<https://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>.
[research.udel] [research.udel]
Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
to Us: Understanding the Security Threats of Dangling DNS to Us: Understanding the Security Threats of Dangling DNS
Records", 24 October 2016, Records", CCS '16: Proceedings of the 2016 ACM SIGSAC
<https://www.eecis.udel.edu/~hnw/paper/ccs16a.pdf>. Conference on Computer and Communications Security, pp.
1414-1425, DOI 10.1145/2976749.2978387, 24 October 2016,
Appendix A. Document History <https://dl.acm.org/doi/pdf/10.1145/2976749.2978387>.
[[ To be removed from the final specification ]]
-29
* Fix broken reference
-28
* Various editorial fixes
* Address feedback from IESG ballot
-27
* Mostly editorial feedback from Microsoft incorporated
* Feedback from SECDIR review incorporated
-26
* Feedback from ARTART review incorporated
* Gen-ART review (typo fixes)
-25
* Shepherd's writeup feedback: Removed discussion on outdated POP
approaches
* Shepherd's writeup feedback: Clarify relationship to other
document.
* Shepherd's writeup feedback: Expand abbreviations
* Shepherd's writeup feedback: Better explain attacker model
* Shepherd's writeup feedback: Various editorial changes
* AD review: Mention updated documents in abstract
* AD review: Fix HTTP reference
* AD review: Clarification in the attacker model
* AD review: Various editorial and minor changes
-24
* Some feedback from shepherd's writeup incorporated
* Cleaned up references
* Clarification on mix-up attack
* Add researcher names to acknowledgements
* Removed sentence stating that only MTLS is standardized; DPoP is
now as well
-23
* Added CORS considerations
* Reworded Section 4.15.1 to be more in line with OAuth 2.1
* Editorial changes
* Clarifications and updated references
-22
* Added section on securing in-browser communication
* Merged section on phishing via AS into existing section on open
redirectors
* Restructure and move section on sender-constrained tokens
* Mention RFCs for Private Key JWK method
-21
* Improved wording on phishing via AS
-20
* Improved description of authorization code injection attacks and
PKCE protection
* Removed recommendation for MTLS in discussion (not reflected in
actual Recommendations section)
* Reworded "placeholder" text in security considerations.
* Alphabetized list of names and fixed unicode problem
* Explained Clickjacking
* Explained Open Redirectors
* Clarified references to attacker model by including a link to
Section 3
* Clarified description of "CSRF tokens" and reference to RFC6819
* Described that OIDC can prevent access token injection
* Updated references
-19
* Changed affiliation of Andrey Labunets
* Editorial change to clarify the new recommendations for refresh
tokens
-18
* Fix editorial and spelling issues.
* Change wording for disallowing HTTP redirect URIs.
-17
* Make the use of metadata RECOMMENDED for both servers and clients
* Make announcing PKCE support in metadata the RECOMMENDED way
(before: either metadata or deployment-specific way)
* AS also MUST NOT expose open redirectors.
* Mention that attackers can collaborate.
* Update recommendations regarding mix-up defense, building upon
[RFC9207].
* Improve description of mix-up attack.
* Make HTTPS mandatory for most redirect URIs.
-16
* Make MTLS a suggestion instead of RECOMMENDED.
* Add important requirements when using nonce for code injection
protection.
* Highlight requirements for refresh token sender-constraining.
* Make PKCE a MUST for public clients.
* Describe PKCE Downgrade Attacks and countermeasures.
* Allow variable port numbers in localhost redirect URIs as in
RFC8252, Section 7.3.
-15
* Update reference to DPoP
* Fix reference to RFC8414
* Move to xml2rfcv3
-14
* Added info about using CSP to prevent clickjacking
* Changes from WGLC feedback
* Editorial changes
* AS MUST announce PKCE support either in metadata or using
deployment-specific ways (before: SHOULD)
-13
* Discourage use of Resource Owner Password Credentials Grant
* Added text on client impersonating resource owner
* Recommend asymmetric methods for client authentication
* Encourage use of PKCE mode "S256"
* PKCE may replace state for CSRF protection
* AS SHOULD publish PKCE support
* Cleaned up discussion on auth code injection
* AS MUST support PKCE
-12
* Added updated attacker model
-11
* Adapted section 2.1.2 to outcome of consensus call
* more text on refresh token inactivity and implementation note on
refresh token replay detection via refresh token rotation
-10
* incorporated feedback by Joseph Heenan
* changed occurrences of SHALL to MUST
* added text on lack of token/cert binding support tokens issued in
the authorization response as justification to not recommend
issuing tokens there at all
* added requirement to authenticate clients during code exchange
(PKCE or client credential) to 2.1.1.
* added section on refresh tokens
* editorial enhancements to 2.1.2 based on feedback
-09
* changed text to recommend not to use implicit but code
* added section on access token injection
* reworked sections 3.1 through 3.3 to be more specific on implicit
grant issues
-08
* added recommendations re implicit and token injection
* uppercased key words in Section 2 according to RFC 2119
-07
* incorporated findings of Doug McDorman
* added section on HTTP status codes for redirects
* added new section on access token privilege restriction based on
comments from Johan Peeters
-06
* reworked section 3.8.1
* incorporated Phil Hunt's feedback
* reworked section on mix-up
* extended section on code leakage via referrer header to also cover
state leakage
* added Daniel Fett as author
* replaced text intended to inform WG discussion by recommendations
to implementors
* modified example URLs to conform to RFC 2606
-05
* Completed sections on code leakage via referrer header, attacks in
browser, mix-up, and CSRF
* Reworked Code Injection Section
* Added reference to OpenID Connect spec
* removed refresh token leakage as respective considerations have
been given in section 10.4 of RFC 6749
* first version on open redirection
* incorporated Christian Mainka's review feedback
-04
* Restructured document for better readability
* Added best practices on Token Leakage prevention
-03
* Added section on Access Token Leakage at Resource Server
* incorporated Brian Campbell's findings
-02
* Folded Mix up and Access Token leakage through a bad AS into new
section for dynamic OAuth threats
* reworked dynamic OAuth section
-01
* Added references to mitigation methods for token leakage Acknowledgements
* Added reference to Token Binding for Authorization Code
* incorporated feedback of Phil Hunt
* fixed numbering issue in attack descriptions in section 2
-00 (WG document) We would like to thank Brock Allen, Annabelle Richard Backman,
* turned the ID into a WG document and a BCP Dominick Baier, Vittorio Bertocci, Brian Campbell, Bruno Crispo,
* Added federated app login as topic in Other Topics William Dennis, George Fletcher, Matteo Golinelli, Dick Hardt, Joseph
Heenan, Pedram Hosseyni, Phil Hunt, Tommaso Innocenti, Louis Jannett,
Jared Jennings, Michael B. Jones, Engin Kirda, Konstantin Lapine,
Neil Madden, Christian Mainka, Jim Manico, Nov Matake, Doug McDorman,
Karsten Meyer zu Selhausen, Ali Mirheidari, Vladislav Mladenov, Kaan
Onarioglu, Aaron Parecki, Michael Peck, Johan Peeters, Nat Sakimura,
Guido Schmitz, Jörg Schwenk, Rifaat Shekh-Yusef, Travis Spencer,
Petteri Stenius, Tomek Stojecki, David Waite, Tim Würtele, and Hans
Zandbelt for their valuable feedback.
Authors' Addresses Authors' Addresses
Torsten Lodderstedt Torsten Lodderstedt
SPRIND SPRIND
Email: torsten@lodderstedt.net Email: torsten@lodderstedt.net
John Bradley John Bradley
Yubico Yubico
Email: ve7jtb@ve7jtb.com Email: ve7jtb@ve7jtb.com
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