Updates to OAuth 2.0 Security Best Current Practice
draft-wuertele-oauth-security-topics-update-02

Web Authorization Protocol                                    T. Würtele
Internet-Draft                                               P. Hosseyni
Updates: 6749, 6750, 7521, 7522, 7523, 9700      University of Stuttgart
         (if approved)                                            K. Luo
Intended status: Best Current PracticeThe Chinese University of Hong Kong
Expires: 2 April 2026                                            A. Fung
                                                Samsung Research America
                                                       29 September 2025

          Updates to OAuth 2.0 Security Best Current Practice
             draft-wuertele-oauth-security-topics-update-02

Abstract

   This document updates the set of best current security practices for
   OAuth 2.0 by extending the security advice given in RFC 6749, RFC
   6750, and RFC 9700, to cover new threats that have been discovered
   since the former documents have been published.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://SECtim.github.io/draft-wuertele-oauth-security-topics-update/
   draft-wuertele-oauth-security-topics-update.html.  Status information
   for this document may be found at https://datatracker.ietf.org/doc/
   draft-wuertele-oauth-security-topics-update/.

   Discussion of this document takes place on the Web Authorization
   Protocol mailing list (mailto:oauth@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/oauth/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/SECtim/draft-wuertele-oauth-security-topics-
   update.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .   3
     2.1.  Audience Injection Attacks  . . . . . . . . . . . . . . .   4
       2.1.1.  Attack Description  . . . . . . . . . . . . . . . . .   4
       2.1.2.  Countermeasures . . . . . . . . . . . . . . . . . . .   6
     2.2.  Cross-tool OAuth Account Takeover . . . . . . . . . . . .   8
       2.2.1.  Attack Description  . . . . . . . . . . . . . . . . .   9
       2.2.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  11
     2.3.  Cross-user OAuth Session Fixation . . . . . . . . . . . .  12
       2.3.1.  Attack Description  . . . . . . . . . . . . . . . . .  13
       2.3.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  14
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Document History  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   Since the publication of the first OAuth 2.0 Security Best Practices
   document [RFC9700], new threats to OAuth 2.0 ecosystems have been
   identified.  This document therefore serves as an extension of the
   original [RFC9700] and is to be read in conjunction with it.

   Like [RFC9700] before, this document provides important security
   recommendations and it is RECOMMENDED that implementers upgrade their
   implementations and ecosystems as soon as feasible.

1.1.  Structure

   The remainder of this document is organized as follows: Section 2 is
   a detailed analysis of the threats and implementation issues that can
   be found in the wild (at the time of writing) along with a discussion
   of potential countermeasures.

1.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the terms "access token", "authorization
   endpoint", "authorization grant", "authorization server", "client",
   "client identifier" (client ID), "protected resource", "refresh
   token", "resource owner", "resource server", and "token endpoint"
   defined by OAuth 2.0 [RFC6749].

   // Make sure to update this list once the technical sections below
   // are completed.
   //
   // -- Tim W.

2.  Attacks and Mitigations

   This section gives a detailed description of new attacks on OAuth
   implementations, along with potential countermeasures.  Attacks and
   mitigations already covered in [RFC9700] are not listed here, except
   where clarifications or new recommendations are made.

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2.1.  Audience Injection Attacks

   When using signature-based client authentication methods such as
   private_key_jwt as defined in [OpenID.Core] or signed JWTs as defined
   in [RFC7521] and [RFC7523], a malicious authorization server may be
   able to obtain and use a client's authentication credential, enabling
   them to impersonate a client towards another honest authorization
   server.

2.1.1.  Attack Description

   The descriptions here follow [research.ust], where additional details
   of the attack are laid out.  Audience injection attacks require a
   client to interact with at least two authorization servers, one of
   which is malicious, and to authenticate to both with a signature-
   based authentication method using the same key pair.  The following
   description uses the jwt-bearer client authentication from [RFC7523],
   see Section 2.1.1.3 for other affected client authentication methods.
   Furthermore, the client needs to be willing to authenticate at an
   endpoint other than the token endpoint at the attacker authorization
   server (see Section 2.1.1.2).

2.1.1.1.  Core Attack Steps

   In the following, let H-AS be an honest authorization server and let
   A-AS be an attacker-controlled authorization server.

   Assume that the authorization servers publish the following URIs for
   their token endpoints, for example via mechanisms such as
   authorization server metadata [RFC8414] or OpenID Discovery
   [OpenID.Discovery].  The exact publication mechanism is not relevant,
   as audience injection attacks are also possible on clients with
   manually configured authorization server metadata.

   Excerpt from H-AS' metadata:

   "issuer": "https://honest.com",
   "token_endpoint": "https://honest.com/token",
   ...

   Excerpt from A-AS' metadata:

   "issuer": "https://attacker.com",
   "token_endpoint": "https://honest.com/token",
   ...

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   Therefore, the attacker authorization server claims to use the honest
   authorization server's token endpoint.  Note that the attacker
   authorization server does not control this endpoint.  The attack then
   commences as follows:

   1.  Client registers at H-AS, and gets assigned a client ID cid.

   2.  Client registers at A-AS, and gets assigned the same client ID
       cid.  Note that the client ID is not a secret (Section 2.2 of
       [RFC6749]).

   Now, whenever the client creates a client assertion for
   authentication to A-AS, the assertion consists of a JSON Web Token
   (JWT) that is signed by the client and contains, among others, the
   following claims:

   "iss": "cid",
   "sub": "cid",
   "aud": "https://honest.com/token"

   Due to the malicious use of H-AS' token endpoint in A-AS'
   authorization server metadata, the aud claim contains H-AS' token
   endpoint.  Recall that both A-AS and H-AS registered the client with
   client ID cid, and that the client uses the same key pair for
   authentication at both authorization servers.  Hence, this client
   assertion is a valid authentication credential for the client at
   H-AS.

   The use of the token endpoint to identify the authorization server as
   a client assertion's audience even for client assertions that are not
   sent to the token endpoint is encouraged, or at least allowed by many
   standards, including [RFC7521], [RFC7522], [RFC7523], [RFC9126],
   [OpenID.Core], [OpenID.CIBA], and all standards referencing the IANA
   registry for OAuth Token Endpoint Authentication Methods for
   available client authentication methods.

   As described in [research.ust], the attacker can then utilize the
   obtained client authentication assertion to impersonate the client
   and, for example, obtain access tokens.

2.1.1.2.  Endpoints Requiring Client Authentication

   As mentioned above, the attack is only possible if the client
   authenticates to an endpoint other than the token endpoint at A-AS.
   This is because if the client sends a token request to A-AS, it will
   use A-AS' token endpoint as published by A-AS and hence, send the
   token request to H-AS, i.e., the attacker cannot obtain the client
   assertion.

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   As detailed in [research.ust], the attack is confirmed to be possible
   if the client authenticates with such client assertions at the
   following endpoints of A-AS:

   *  Pushed Authorization Endpoint (see [RFC9126])

   *  Token Revocation Endpoint (see [RFC7009])

   *  CIBA Backchannel Authentication Endpoint (see [OpenID.CIBA])

   *  Device Authorization Endpoint (see [RFC8628])

   Note that this list of examples is not exhaustive.  Hence, any client
   that might authenticate at any endpoint other than the token endpoint
   SHOULD employ countermeasures as described in Section 2.1.2.

2.1.1.3.  Affected Client Authentication Methods

   The same attacks are possible for the private_key_jwt client
   authentication method defined in [OpenID.Core], as well as
   instantiations of client authentication assertions defined in
   [RFC7521], including the SAML assertions defined in [RFC7522].

   Furthermore, a similar attack is possible for jwt-bearer
   authorization grants as defined in Section 2.1 of [RFC7523], albeit
   under additional assumptions (see [research.ust] for details).

2.1.2.  Countermeasures

   At its core, audience injection attacks exploit the fact that, from
   the client's point of view, an authorization server's token endpoint
   is a mostly opaque value and does not uniquely identify an
   authorization server.  Therefore, an attacker authorization server
   may claim any URI as its token endpoint, including, for example, an
   honest authorization server's issuer identifier.  Hence, as long as a
   client uses the token endpoint as an audience value when
   authenticating to the attacker authorization server, audience
   injection attacks are possible.  Therefore, audience injection
   attacks need to be prevented by the client.

   Note that the following countermeasures mandate the use of single
   audience value (as opposed to multiple audiences in array).  This is
   because Section 4.1.3 of [RFC7519] allows the receiver of an
   audience-restricted JWT to accept the JWT even if the receiver
   identifies with only one of the values in such an array.

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   Clients that interact with more than one authorization server and
   authenticate with signature-based client authentication methods MUST
   employ one of the following countermeasures, unless audience
   injection attacks are mitigated by other means, such as using fresh
   key material for each authorization server.

   Note that the countermeasures described in Section 2.1.2.1 and
   Section 2.1.2.2 do not imply any normative changes to the
   authorization server: Section 4.1.3 of [RFC7519] requires the
   authorization server to only accept a JWT if the authorization server
   can identify itself with (at least one of the elements in) the JWT's
   audience value.  Authentication JWTs produced by a client
   implementing one of these countermeasures meet this condition.  Of
   course, an authorization server MAY still decide to only accept its
   issuer identifier (Section 2.1.2.1) or the endpoint that received the
   JWT (Section 2.1.2.2) as an audience value, for example, to force its
   clients to adopt the respective countermeasure.

2.1.2.1.  Authorization Server Issuer Identifier

   Clients MUST use the authorization server's issuer identifier as
   defined in [RFC8414]/[OpenID.Discovery] as the sole audience value in
   client assertions.  Clients MUST retrieve and validate this value as
   described in Section 3.3 of [RFC8414]/Section 4.3 of
   [OpenID.Discovery].

   For jwt-bearer client assertions as defined by [RFC7523], this
   mechanism is also described in [OAUTH-7523bis].

   Note that "issuer identifier" here does not refer to the term
   "issuer" as defined in Section 4.4 of [RFC9700], but to the issuer
   identifier defined in [RFC8414] and [OpenID.Discovery].  In
   particular, the issuer identifier is not just "an abstract identifier
   for the combination the authorization endpoint and token endpoint".

2.1.2.2.  Exact Target Endpoint URI

   Clients MUST use the exact endpoint URI to which a client assertion
   is sent as that client assertion's sole audience value.

   This countermeasure can be used for authorization servers that do not
   use authorization server metadata [RFC8414] or OpenID Discovery
   [OpenID.Discovery].

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2.2.  Cross-tool OAuth Account Takeover

   It is increasingly common and observed that a single OAuth client
   supports multiple tools, and each of which is mapped to an OAuth
   provider configuration (which includes at least the authorization
   server (AS) endpoints and client registration).  A successful OAuth
   connection is established when the OAuth client obtains an access
   token for a tool based on its corresponding OAuth provider
   configuration.  The tool can then use the access token to access the
   user's resource at a resource server (RS).

   Multiple OAuth connections can be linked to some form of user's
   identity based on these common deployment scenarios:

   *  Platform Integrations: The OAuth connections made with different
      tools are linked to a platform's user account or session (e.g.,
      represented by a platform's user identifier or a short-lived
      anonymous session).  This is common where a user authorizes a
      platform (e.g., agentic AI service) to orchestrate multiple tools,
      of which some of them together with their OAuth providers can be
      contributed by the public.

   *  Multi-tenant OAuth-as-a-Service (OaaS): In cases when the OAuth
      client is managed by a multi-tenant OAuth-as-a-Service provider, a
      successful OAuth connection is linked to a tenant's user
      identifier in addition to the tenant identifier.  This is a
      generalization of the last deployment scenario, where a platform
      using this OAuth-as-a-Service is becoming a tenant.  A tenant can
      usually choose some off-the-shelf tools using
      (partially-)completed OAuth providers, if not adding their own
      with custom OAuth providers to support the tenant's service.

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   When controlled by an attacker, the open configurations of OAuth
   providers have posed a new threat to this centralized OAuth client
   design.  If the client fails to properly identify, track, and isolate
   which proper OAuth connection context (representing a combination of
   OAuth provider, tool, and tenant) is in use during an authorization
   flow, an attacker can exploit this to mount the Cross-tool OAuth
   Account Takeover (COAT) attacks (see [research.cuhk] and
   [research.cuhk3]).  The COAT attacker uses a malicious tool to steal
   a victim's authorization code issued by an honest OAuth provider of
   an honest tool, and apply the authorization code injection (as
   defined in Section 4.5 of [RFC9700]) against a new OAuth connection
   with the attacker's identity.  This results in a compromised OAuth
   connection between the attacker's platform identity and the victim's
   tool account.  The impact is equivalent to an account takeover: the
   attacker can operate the honest tool using the victim's tool account
   (hijacked either under the same platform, or even cross-tenant that
   shares a vulnerable OAuth-as-a-service).

2.2.1.  Attack Description

   Preconditions: It is assumed that

   *  the implicit or authorization code grant is used with multiple
      OAuth connection contexts, of which one combination is considered
      "honest" (H-Tool using H-AuthProvider with H-AS) and one is
      operated by the attacker (A-Tool using A-AuthProvider with A-AS),
      and

   *  the client stores the connection context chosen by the user in a
      session bound to the user's browser, and

   *  the client issues redirection URIs which do not depend on all
      variables in the connection context (e.g., auth provider, tool,
      tenant), and

   *  the authorization servers properly check the redirection URI by
      enforcing exact redirection URI matching (otherwise, see Cross
      Social-Network Request Forgery in [research.jcs_14] for details).

   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 A-AS (URI: https://attacker.example, client ID: 666RVZJTA).
   Assume that the client issues the redirection URI https://client.com/
   honest-cb for the honest tool and https://client.com/attack-cb for
   the attacker's.  URLs shown in the following example are shortened
   for presentation to include only parameters relevant to the attack.

   Attack on the authorization code grant:

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   1.  A victim user selects to start the grant using A-AS of A-Tool
       (e.g., by initiating a tool use on an agentic AI service).

   2.  The client stores in the user's session that the user has
       selected such OAuth connection context and redirects the user to
       A-AS's authorization endpoint with a Location header containing
       the URL https://attacker.example/
       authorize?response_type=code&client_id=666RVZJTA&state=[state]
       &redirect_uri=https%3A%2F%2Fclient.com%2Fattack-cb.

   3.  When the user's browser navigates to the A-AS, the attacker
       immediately redirects the browser to the authorization endpoint
       of H-AS.  In the authorization request, the attacker uses the
       honest authorization URL and replaces the state with the one
       freshly received.  Therefore, the browser receives a redirection
       with a Location header pointing to https://honest.as.example/
       authorize?response_type=code&client_id=7ZGZldHQ&state=[state]
       &redirect_uri=https%3A%2F%2Fclient.com%2Fhonest-cb.

   4.  Due to implicit or prior approvals, the user might not be
       prompted for a re-authorization (re-consent).  H-AS issues a code
       and sends it (via the browser) back with the state to the client.

   5.  Since the client still assumes that the code was issued by
       A-Tool, as stored in the user's session (with state verified), it
       will try to redeem the code at A-AS's token endpoint.

   6.  The attacker therefore obtains code and can either exchange the
       code for an access token (for public clients) or perform an
       authorization code injection attack as described in Section 4.5
       of [RFC9700].

   This Cross-tool OAuth Account Takeover (COAT) attack is a
   generalization of the Cross-app OAuth Account Takeover as defined in
   [research.cuhk] and the mix-up attack as defined in Section 4.4 of
   [RFC9700].  This COAT exploits confusion between the OAuth connection
   context (i.e., a combination of OAuth provider, tool, tenant) of a
   centralized client rather than limited to confusion between two
   distinct authorization servers.

   Variants:

   *  COAT under the OaaS context: the attack above can be launched with
      a malicious tenant (1) simply using a shared off-the-shelf tool
      that comes with pre-built OAuth providers (with client
      registration included), if so allowed; or (2) adding a custom tool
      with an OAuth provider targeting an honest AS used by another
      tenant's tool.

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   *  Implicit Grant: In the implicit grant, the attacker receives an
      access token instead of the code in Step 4.  The attacker's
      authorization server receives the access token when the client
      makes either a request to the A-AS userinfo endpoint (defined in
      [OpenID.Core]) or a request to the attacker's resource server
      (since the client believes it has completed the flow with A-AS).

   *  Cross-tool OAuth Request Forgery (CORF): If clients do not store
      the selected OAuth connection context in the user's session, but
      in the redirection URI instead, attackers can mount an attack
      called Cross-tool OAuth Request Forgery (CORF).  This results in a
      compromised OAuth connection between the victim's platform
      identity and the attacker's tool account.  The goal of this
      specific attack variant is not to obtain an authorization code or
      access token, but to force the client to use an attacker's
      authorization code or access token for H-AS.  This Cross-tool
      OAuth Request Forgery attack is a generalization of the Cross-app
      OAuth Request Forgery as defined in [research.cuhk] and the Naïve
      RP Session Integrity Attack when the OAuth connection context is
      limited to AS, and is detailed in Section 3.4 of
      [arXiv.1601.01229].

   *  OpenID Connect: Some variants can be used to attack OpenID
      Connect.  In these attacks, the attacker misuses features of the
      OpenID Connect Discovery [OpenID.Discovery] mechanism or replays
      access tokens or ID Tokens to conduct a mix-up attack.  The
      attacks are described in detail in Appendix A of
      [arXiv.1704.08539] and Section 6 of [arXiv.1508.04324v2]
      ("Malicious Endpoints Attacks").

2.2.2.  Countermeasures

   The client MUST NOT share OAuth providers with completed client
   registrations across tools and tenants belonging to different owners.

   The client MUST use all variables in its supported OAuth connection
   context to form a unique connection context identifier, which always
   includes the unique tool identifier.  Additionally,

   *  a client allowing each tool to use multiple OAuth providers, of
      which one AS may get compromised as assumed in Section 4.4 of
      [RFC9700], MUST also include the OAuth provider identifier;

   *  a cross-tenant client MUST also include the tenant identifier, if
      the tool identifier is not globally unique.

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   Unless otherwise specified as follows, the client MUST issue per-
   context distinct redirection URI that incorporates this unique
   connection context identifier.  When initiating an authorization
   request, the client MUST store this identifier in the user's session.
   When an authorization response was received on the redirection URI
   endpoint, clients MUST also check that the context identifier from
   the URI matches with the one in the distinct redirection URI.  If
   there is a mismatch, the client MUST abort the flow.

   Existing mix-up countermeasures Section 4.4 of [RFC9700] can be a
   replacement under the following conditions:

   *  the client has entirely dropped the support to implicit grant, and

   *  the OAuth provider specifies an AS not by individual AS endpoints
      but instead replaced with an abstract issuer identifier
      representing the endpoints, and

   *  the issuer identifier is used either in place of the connection
      context identifier or is separately returned according to
      [RFC9207], and

   *  an additional runtime resolution is used to resolve the issuer to
      retrieve the associated AS endpoints (e.g., with the authorization
      server metadata [RFC8414]).  Clients using such resolution solely
      to populate an OAuth provider defined with individual AS endpoints
      and lack the connection context identifier defense will remain
      vulnerable.

2.3.  Cross-user OAuth Session Fixation

   Based on similar deployment needs as outlined in Section 2.2,
   multiple OAuth connections can be linked to some form of user's
   identity (e.g., a platform's user identifier).  This identity
   information is supposedly maintained in a session established and
   already bound to the user agent.  In real-world deployments, however,
   this prerequisite can be broken for various reasons.  For instance,
   in cross-user-agent OAuth deployments, where an authenticated native
   app with its backend acting as a confidential OAuth client, the
   client opens a tool linking URL in an external user agent (a browser)
   that has no authenticated sessions with the client.  As a workaround,
   the client introduces a session fixation vulnerability: it encodes a
   session identifier into the URL, which fixates a dedicated
   authorization session to complete the OAuth connection with the user
   at the client.

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   The Cross-user OAuth Session Fixation exploits this session fixation
   attack vector.  The attacker attempts to trick a victim into
   completing an OAuth flow that the attacker has initiated at the
   client.  As a result, the attacker's session will be used to
   establish an OAuth connection with the victim's tool resources or
   identity, hence resulting in the same impact of COAT.  However, this
   attack exploits confusion over the intended user bound to that
   connection context during the OAuth flow, contrasting with COAT,
   which exploits confusion within the OAuth connection context (OAuth
   provider, tool, tenant).

   In general, this session fixation vulnerability may be viewed as
   violating the requirement of "binding the contents of state to the
   browser (more precisely, the initiating user agent) session" to
   defend against CSRF (Section 4.7 of [RFC9700]).  However, CSRF
   defenses, including PKCE [RFC7636], cannot mitigate this new attack,
   since the entire OAuth flow including the authorization request and
   the access token request are completed by the same victim user.  The
   impact of the new attack is also more severe from that of typical
   CSRF attacks.

   Note that this section focuses on the authorization code grant.  For
   similar attacks in cross-device OAuth flows, see Section 4 of [CDFS].

2.3.1.  Attack Description

   Preconditions: It is assumed that the client has maintained a user's
   session.  But it does not want to or cannot authenticate the user at
   the redirection endpoint for usability reasons, before completing the
   OAuth connection.

   Example Attack:

   1.  From a vulnerable client, the attacker initiates OAuth against a
       tool and obtains an authorization request URL, in which the state
       has encoded a newly fixable authorization session of the
       attacker.

   2.  The attacker sends this authorization request URL to a victim.

   3.  The victim visits the URL and (automatically, due to prior or
       implicit approvals,) authorizes the client to access their
       resources.

   4.  Upon receiving the state at the redirection endpoint, the client
       fixates the attacker's authorization session and completes the
       OAuth connection.

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   5.  The attacker's account at the client now gains access to the
       victim's resources.

   Variant:

   The client may first generate a pre-authorization URL for the purpose
   of fixating a session before redirecting to the authorization
   endpoint.

   Non-normative example request:

GET /oauth?auth_session_id=6064f11c-f73e-425b-b9b9-4a36088cdb2b HTTP/1.1
Host: client.com

   Non-normative example response:

   HTTP/1.1 303 See Other
   Location: https://as.example/authorize?
             response_type=code&client_id=K9dTpWzqL7&state=b1d8f043
             &redirect_uri=https%3A%2F%2Fclient.com%2Fcb
   Set-Cookie: auth_session_id=6064f11c-f73e-425b-b9b9-4a36088cdb2b

   This attack differs from the above only by obtaining and using the
   pre-authorization URL instead, which will first fixate the attacker's
   authorization session (rather than in Step 4).

2.3.2.  Countermeasures

   Defending against the Cross-user OAuth Session Fixation attack
   requires ensuring that an OAuth connection flow initiated by one user
   MUST only be completed by the same user.

   The most straightforward countermeasure is to re-authenticate the
   user instead of trying to fixate a session if usability condition
   permits.  It is also understandable that the session fixation vector
   cannot be eliminated due to application needs.  For instance, the
   client's user session and the OAuth client responsible for making
   OAuth connections are handled by separate entities (e.g., separate
   services hosted and isolated under different origins, or when the
   OAuth client is outsourced to an OAuth-as-a-Service provider), as
   observed in practice by [research.cuhk2] and [research.cuhk3].

   Hence, the client MUST bind any _newly fixated session_ (conveyed via
   state or the preauthorization URL during an OAuth flow to establish
   the OAuth connection) with the _existing session_ (maintained at the
   user agent) which initiates the OAuth flow, before proceeding with
   the access token request.  Depending on the specific current
   settings:

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   *  When the endpoint with existing user session and the redirection
      endpoint are hosted in the same origin and same user agent, the
      client MUST validate the binding between the newly fixated session
      and the existing session before the access token request.

   *  In case the redirection endpoint is hosted elsewhere (a different
      origin or user agent), the countermeasure requires:

      -  an implementation change to co-locate the endpoint with user
         session and the redirection endpoint in the same origin and
         user agent (see above), or

      -  at the current redirection endpoint, further redirect, using
         HTTP Location or native app redirection as detailed in
         Section 7 of [RFC8252], back to (the starting origin and/or
         user agent) where the existing session is available.  The
         location of this further redirection MUST NOT be controllable
         by an attacker, or it will result in Open Redirection
         (Section 4.11 of [RFC9700]).  The client MUST validate the
         binding between the sessions before the access token request.

3.  Security Considerations

   Security considerations are described in Section 2.

4.  IANA Considerations

   This document has no IANA actions.

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6749>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/rfc/rfc7521>.

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   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/rfc/rfc7523>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8252>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/rfc/rfc8414>.

   [RFC9700]  Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "Best Current Practice for OAuth 2.0 Security", BCP 240,
              RFC 9700, DOI 10.17487/RFC9700, January 2025,
              <https://www.rfc-editor.org/rfc/rfc9700>.

5.2.  Informative References

   [arXiv.1508.04324v2]
              Mladenov, V., Mainka, C., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", arXiv:1508.04324v2,
              January 2016, <https://arxiv.org/abs/1508.04324v2/>.

   [arXiv.1601.01229]
              Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0", arXiv:1601.01229,
              January 2016, <https://arxiv.org/abs/1601.01229/>.

   [arXiv.1704.08539]
              Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
              Standard OpenID Connect: In-Depth Formal Security Analysis
              and Security Guidelines", arXiv:1704.08539, April 2017,
              <https://arxiv.org/abs/1704.08539/>.

   [CDFS]     Kasselman, P., Fett, D., and F. Skokan, "Cross-Device
              Flows: Security Best Current Practice", Work in Progress,
              Internet-Draft, draft-ietf-oauth-cross-device-security-12,
              5 September 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-oauth-cross-device-security-12>.

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   [MCP-Spec] Anthropic, "Model Context Protocol (MCP) Specification",
              June 2025, <https://modelcontextprotocol.io/
              specification/2025-06-18>.

   [OAUTH-7523bis]
              Jones, M. B., Campbell, B., and C. Mortimore, "JSON Web
              Token (JWT) Profile for OAuth 2.0 Client Authentication
              and Authorization Grants", Work in Progress, Internet-
              Draft, draft-ietf-oauth-rfc7523bis-00, 21 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              rfc7523bis-00>.

   [OpenID.CIBA]
              Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B.
              Campbell, "OpenID Connect Client-Initiated Backchannel
              Authentication Flow - Core 1.0", September 2021,
              <https://openid.net/specs/openid-client-initiated-
              backchannel-authentication-core-1_0.html>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 2", December 2023,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [OpenID.Discovery]
              Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
              Connect Discovery 1.0 incorporating errata set 2",
              December 2023, <https://openid.net/specs/openid-connect-
              discovery-1_0.html>.

   [research.cuhk]
              Luo, K., Wang, X., Fung, P. H. A., Lau, W. C., and J.
              Lecomte, "Universal Cross-app Attacks: Exploiting and
              Securing OAuth 2.0 in Integration Platforms", 34th USENIX
              Security Symposium (USENIX Security 25), August 2025,
              <https://www.usenix.org/system/files/usenixsecurity25-luo-
              kaixuan.pdf>.

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   [research.cuhk2]
              Luo, K., Wang, X., Fung, A., Lecomte, J., and W. C. Lau,
              "One Hack to Rule Them All: Pervasive Account Takeovers in
              Integration Platforms for Workflow Automation, Virtual
              Voice Assistant, IoT, & LLM Services", Black Hat USA 2024,
              August 2024, <https://www.blackhat.com/us-24/briefings/
              schedule/#one-hack-to-rule-them-all-pervasive-account-
              takeovers-in-integration-platforms-for-workflow-
              automation-virtual-voice-assistant-iot-38-llm-services-
              38994>.

   [research.cuhk3]
              Luo, K., Wang, X., Fung, A., Bi, Y., and W. C. Lau, "Back
              to the Future: Hacking and Securing Connection-based OAuth
              Architectures in Agentic AI and Integration Platforms",
              Black Hat USA 2025, August 2025,
              <https://www.blackhat.com/us-25/briefings/schedule/
              index.html#back-to-the-future-hacking-and-securing-
              connection-based-oauth-architectures-in-agentic-ai-and-
              integration-platforms-44686>.

   [research.jcs_14]
              Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
              Maffeis, "Discovering concrete attacks on website
              authorization by formal analysis", Journal of Computer
              Security, vol. 22, no. 4, pp. 601-657, April 2014,
              <https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.

   [research.ust]
              Hosseyni, P., Küsters, R., and T. Würtele, "Audience
              Injection Attacks: A New Class of Attacks on Web-Based
              Authorization and Authentication Standards", April 2025,
              <https://eprint.iacr.org/2025/629>.

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/rfc/rfc7009>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [RFC7522]  Campbell, B., Mortimore, C., and M. Jones, "Security
              Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0
              Client Authentication and Authorization Grants", RFC 7522,
              DOI 10.17487/RFC7522, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7522>.

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   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7636>.

   [RFC8628]  Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Device Authorization Grant", RFC 8628,
              DOI 10.17487/RFC8628, August 2019,
              <https://www.rfc-editor.org/rfc/rfc8628>.

   [RFC9126]  Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
              and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
              RFC 9126, DOI 10.17487/RFC9126, September 2021,
              <https://www.rfc-editor.org/rfc/rfc9126>.

   [RFC9207]  Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0
              Authorization Server Issuer Identification", RFC 9207,
              DOI 10.17487/RFC9207, March 2022,
              <https://www.rfc-editor.org/rfc/rfc9207>.

Acknowledgments

   We would like to thank
   // TODO add names, sort by last name.
   //
   // -- Tim W.  Daniel Fett, Wing Cheong Lau, Julien Lecomte, Aaron
   Parecki, Guido Schmitz, and Xianbo Wang

   for their valuable feedback and contributions to this document.

Document History

   [[ To be removed from the final specification ]]

   -02

   *  Rewrote Mix-up related sections

   *  Added section on Session Fixation attack

   -01

   *  Updated temporary title

   *  Added introductory paragraphs, replaced placeholders

   *  Clarified issuer does not uniquely identify client config

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   *  Cleaned up acknowledgement list

   -00

   *  Initial version

Authors' Addresses

   Tim Würtele
   University of Stuttgart
   Germany
   Email: tim.wuertele@sec.uni-stuttgart.de

   Pedram Hosseyni
   University of Stuttgart
   Germany
   Email: pedram.hosseyni@sec.uni-stuttgart.de

   Kaixuan Luo
   The Chinese University of Hong Kong
   Hong Kong
   Email: kaixuan@ie.cuhk.edu.hk

   Adonis Fung
   Samsung Research America
   United States of America
   Email: adonis.fung@samsung.com

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