



Internet Engineering Task Force (IETF)                    T. Lodderstedt
Request for Comments: 9700                                        SPRIND
BCP: 240                                                      J. Bradley
Updates: 6749, 6750, 6819                                         Yubico
Category: Best Current Practice                              A. Labunets
ISSN: 2070-1721                                   Independent Researcher
                                                                 D. Fett
                                                                Authlete
                                                            January 2025


              Best Current Practice for OAuth 2.0 Security

Abstract

   This document describes best current security practice for OAuth 2.0.
   It updates and extends the threat model and security advice given in
   RFCs 6749, 6750, and 6819 to incorporate practical experiences
   gathered since OAuth 2.0 was published and covers new threats
   relevant due to the broader application of OAuth 2.0.  Further, it
   deprecates some modes of operation that are deemed less secure or
   even insecure.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   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 (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Structure
     1.2.  Conventions and Terminology
   2.  Best Practices
     2.1.  Protecting Redirect-Based Flows
       2.1.1.  Authorization Code Grant
       2.1.2.  Implicit Grant
     2.2.  Token Replay Prevention
       2.2.1.  Access Tokens
       2.2.2.  Refresh Tokens
     2.3.  Access Token Privilege Restriction
     2.4.  Resource Owner Password Credentials Grant
     2.5.  Client Authentication
     2.6.  Other Recommendations
   3.  The Updated OAuth 2.0 Attacker Model
   4.  Attacks and Mitigations
     4.1.  Insufficient Redirection URI Validation
       4.1.1.  Redirect URI Validation Attacks on Authorization Code
               Grant
       4.1.2.  Redirect URI Validation Attacks on Implicit Grant
       4.1.3.  Countermeasures
     4.2.  Credential Leakage via Referer Headers
       4.2.1.  Leakage from the OAuth Client
       4.2.2.  Leakage from the Authorization Server
       4.2.3.  Consequences
       4.2.4.  Countermeasures
     4.3.  Credential Leakage via Browser History
       4.3.1.  Authorization Code in Browser History
       4.3.2.  Access Token in Browser History
     4.4.  Mix-Up Attacks
       4.4.1.  Attack Description
       4.4.2.  Countermeasures
         4.4.2.1.  Mix-Up Defense via Issuer Identification
         4.4.2.2.  Mix-Up Defense via Distinct Redirect URIs
     4.5.  Authorization Code Injection
       4.5.1.  Attack Description
       4.5.2.  Discussion
       4.5.3.  Countermeasures
         4.5.3.1.  PKCE
         4.5.3.2.  Nonce
         4.5.3.3.  Other Solutions
       4.5.4.  Limitations
     4.6.  Access Token Injection
       4.6.1.  Countermeasures
     4.7.  Cross-Site Request Forgery
       4.7.1.  Countermeasures
     4.8.  PKCE Downgrade Attack
       4.8.1.  Attack Description
       4.8.2.  Countermeasures
     4.9.  Access Token Leakage at the Resource Server
       4.9.1.  Access Token Phishing by Counterfeit Resource Server
       4.9.2.  Compromised Resource Server
       4.9.3.  Countermeasures
     4.10. Misuse of Stolen Access Tokens
       4.10.1.  Sender-Constrained Access Tokens
       4.10.2.  Audience-Restricted Access Tokens
       4.10.3.  Discussion: Preventing Leakage via Metadata
     4.11. Open Redirection
       4.11.1.  Client as Open Redirector
       4.11.2.  Authorization Server as Open Redirector
     4.12. 307 Redirect
     4.13. TLS Terminating Reverse Proxies
     4.14. Refresh Token Protection
       4.14.1.  Discussion
       4.14.2.  Recommendations
     4.15. Client Impersonating Resource Owner
       4.15.1.  Countermeasures
     4.16. Clickjacking
     4.17. Attacks on In-Browser Communication Flows
       4.17.1.  Examples
         4.17.1.1.  Insufficient Limitation of Receiver Origins
         4.17.1.2.  Insufficient URI Validation
         4.17.1.3.  Injection after Insufficient Validation of Sender
                 Origin
       4.17.2.  Recommendations
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 (referred
   to as simply "OAuth" in this document) has gained massive traction in
   the market and became the standard for API protection and the basis
   for federated login using OpenID Connect [OpenID.Core].  While OAuth
   is used in a variety of scenarios and different kinds of deployments,
   the following challenges can be observed:

   *  OAuth implementations are being attacked through known
      implementation weaknesses and anti-patterns (i.e., well-known
      patterns that are considered insecure).  Although most of these
      threats are discussed in the OAuth 2.0 Threat Model and Security
      Considerations [RFC6819], continued exploitation demonstrates a
      need for more specific recommendations, easier to implement
      mitigations, and more defense in depth.

   *  OAuth is being used in environments with higher security
      requirements than considered initially, such as open banking,
      eHealth, eGovernment, and electronic signatures.  Those use cases
      call for stricter guidelines and additional protection.

   *  OAuth is being used in much more dynamic setups than originally
      anticipated, creating new challenges with respect to security.
      Those challenges go beyond the original scope of [RFC6749],
      [RFC6750], and [RFC6819].

      OAuth initially assumed static relationships between clients,
      authorization servers, and resource servers.  The URLs of the
      servers were known to the client at deployment time and built an
      anchor for the trust relationships among those parties.  The
      validation of whether the client is talking to a legitimate server
      was based on TLS server authentication (see Section 4.5.4 of
      [RFC6819]).  With the increasing adoption of OAuth, this simple
      model dissolved and, in several scenarios, was replaced by a
      dynamic establishment of the relationship between clients on one
      side and the authorization and resource servers of a particular
      deployment on the other side.  This way, the same client could be
      used to access services of different providers (in case of
      standard APIs, such as email or OpenID Connect) or serve as a
      front end to a particular tenant in a multi-tenant environment.
      Extensions of OAuth, such as the OAuth 2.0 Dynamic Client
      Registration Protocol [RFC7591] and OAuth 2.0 Authorization Server
      Metadata [RFC8414] were developed to support the use of OAuth in
      dynamic scenarios.

   *  Technology has changed.  For example, the way browsers treat
      fragments when redirecting requests has changed, and with it, the
      implicit grant's underlying security model.

   This document provides updated security recommendations to address
   these challenges.  It introduces new requirements beyond those
   defined in existing specifications such as OAuth 2.0 [RFC6749] and
   OpenID Connect [OpenID.Core] and deprecates some modes of operation
   that are deemed less secure or even insecure.  However, this document
   does not supplant the security advice given in [RFC6749], [RFC6750],
   and [RFC6819], but complements those documents.

   Naturally, not all existing ecosystems and implementations are
   compatible with the new requirements, and following the best
   practices described in this document may break interoperability.
   Nonetheless, it is RECOMMENDED that implementers upgrade their
   implementations and ecosystems as soon as feasible.

   OAuth 2.1, under development as [OAUTH-V2.1], will incorporate
   security recommendations from this document.

1.1.  Structure

   The remainder of this document is organized as follows: Section 2
   summarizes the most important best practices for every OAuth
   implementer.  Section 3 presents the updated OAuth attacker model.
   Section 4 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].

   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.

2.  Best Practices

   This section describes the core set of security mechanisms and
   measures that are considered to be best practices at the time of
   writing.  Details about these security mechanisms and measures
   (including detailed attack descriptions) and requirements for less
   commonly used options are provided in Section 4.

2.1.  Protecting Redirect-Based Flows

   When comparing client redirection URIs against pre-registered URIs,
   authorization servers MUST utilize exact string matching except for
   port numbers in localhost redirection URIs of native apps (see
   Section 4.1.3).  This measure contributes to the prevention of
   leakage of authorization codes and access tokens (see Section 4.1).
   It can also help to detect mix-up attacks (see Section 4.4).

   Clients and authorization servers MUST NOT expose URLs that forward
   the user's browser to arbitrary URIs obtained from a query parameter
   (open redirectors) as described in Section 4.11.  Open redirectors
   can enable exfiltration of authorization codes and access tokens.

   Clients MUST prevent Cross-Site Request Forgery (CSRF).  In this
   context, CSRF refers to requests to the redirection endpoint that do
   not originate at the authorization server, but at a malicious third
   party (see Section 4.4.1.8 of [RFC6819] for details).  Clients that
   have ensured that the authorization server supports Proof Key for
   Code Exchange (PKCE) [RFC7636] MAY rely on the CSRF protection
   provided by PKCE.  In OpenID Connect flows, the nonce parameter
   provides CSRF protection.  Otherwise, one-time use CSRF tokens
   carried in the state parameter that are securely bound to the user
   agent MUST be used for CSRF protection (see Section 4.7.1).

   When an OAuth client can interact with more than one authorization
   server, a defense against mix-up attacks (see Section 4.4) is
   REQUIRED.  To this end, clients SHOULD

   *  use the iss parameter as a countermeasure according to [RFC9207],
      or
   *  use an alternative countermeasure based on an iss value in the
      authorization response (such as the iss claim in the ID Token in
      [OpenID.Core] or in [OpenID.JARM] responses), processing that
      value as described in [RFC9207].

   In the absence of these options, clients MAY instead use distinct
   redirection URIs to identify authorization endpoints and token
   endpoints, as described in Section 4.4.2.

   An authorization server that redirects a request potentially
   containing user credentials MUST avoid forwarding these user
   credentials accidentally (see Section 4.12 for details).

2.1.1.  Authorization Code Grant

   Clients MUST prevent authorization code injection attacks (see
   Section 4.5) and misuse of authorization codes using one of the
   following options:

   *  Public clients MUST use PKCE [RFC7636] to this end, as motivated
      in Section 4.5.3.1.
   *  For confidential clients, the use of PKCE [RFC7636] is
      RECOMMENDED, as it provides strong protection against misuse and
      injection of authorization codes as described in Section 4.5.3.1.
      Also, as a side effect, it prevents CSRF even in the presence of
      strong attackers as described in Section 4.7.1.
   *  With additional precautions, described in Section 4.5.3.2,
      confidential OpenID Connect [OpenID.Core] clients MAY use the
      nonce parameter and the respective Claim in the ID Token instead.

   In any case, the PKCE challenge or OpenID Connect nonce MUST be
   transaction-specific and securely bound to the client and the user
   agent in which the transaction was started.  Authorization servers
   are encouraged to make a reasonable effort at detecting and
   preventing the use of constant values for the PKCE challenge or
   OpenID Connect nonce.

   Note: Although PKCE was designed as a mechanism to protect native
   apps, this advice applies to all kinds of OAuth clients, including
   web applications.

   When using PKCE, clients SHOULD use PKCE code challenge methods that
   do not expose the PKCE verifier in the authorization request.
   Otherwise, attackers that can read the authorization request (cf.
   Attacker (A4) in Section 3) can break the security provided by PKCE.
   Currently, S256 is the only such method.

   Authorization servers MUST support PKCE [RFC7636].

   If a client sends a valid PKCE code_challenge parameter in the
   authorization request, the authorization server MUST enforce the
   correct usage of code_verifier at the token endpoint.

   Authorization servers MUST mitigate PKCE downgrade attacks by
   ensuring that a token request containing a code_verifier parameter is
   accepted only if a code_challenge parameter was present in the
   authorization request; see Section 4.8.2 for details.

   Authorization servers MUST provide a way to detect their support for
   PKCE.  It is RECOMMENDED for authorization servers to publish the
   element code_challenge_methods_supported in their Authorization
   Server Metadata [RFC8414] containing the supported PKCE challenge
   methods (which can be used by the client to detect PKCE support).
   Authorization servers MAY instead provide a deployment-specific way
   to ensure or determine PKCE support by the authorization server.

2.1.2.  Implicit Grant

   The implicit grant (response type token) and other response types
   causing the authorization server to issue access tokens in the
   authorization response are vulnerable to access token leakage and
   access token replay as described in Sections 4.1, 4.2, 4.3, and 4.6.

   Moreover, no standardized method for sender-constraining exists to
   bind access tokens to a specific client (as recommended in
   Section 2.2) when the access tokens are issued in the authorization
   response.  This means that an attacker can use the leaked or stolen
   access token at a resource endpoint.

   In order to avoid these issues, clients SHOULD NOT use the implicit
   grant (response type token) or other response types issuing access
   tokens in the authorization response, unless access token injection
   in the authorization response is prevented and the aforementioned
   token leakage vectors are mitigated.

   Clients SHOULD instead use the response type code (i.e.,
   authorization code grant type) as specified in Section 2.1.1 or any
   other response type that causes the authorization server to issue
   access tokens in the token response, such as the code id_token
   response type.  This allows the authorization server to detect replay
   attempts by attackers and generally reduces the attack surface since
   access tokens are not exposed in URLs.  It also allows the
   authorization server to sender-constrain the issued tokens (see
   Section 2.2).

2.2.  Token Replay Prevention

2.2.1.  Access Tokens

   A sender-constrained access token scopes the applicability of an
   access token to a certain sender.  This sender is obliged to
   demonstrate knowledge of a certain secret as a prerequisite for the
   acceptance of that token at the recipient (e.g., a resource server).

   Authorization and resource servers SHOULD use mechanisms for sender-
   constraining access tokens, such as mutual TLS for OAuth 2.0
   [RFC8705] or OAuth 2.0 Demonstrating Proof of Possession (DPoP)
   [RFC9449] (see Section 4.10.1), to prevent misuse of stolen and
   leaked access tokens.

2.2.2.  Refresh Tokens

   Refresh tokens for public clients MUST be sender-constrained or use
   refresh token rotation as described in Section 4.14.  [RFC6749]
   already mandates that refresh tokens for confidential clients can
   only be used by the client for which they were issued.

2.3.  Access Token Privilege Restriction

   The privileges associated with an access token SHOULD be restricted
   to the minimum required for the particular application or use case.
   This prevents clients from exceeding the privileges authorized by the
   resource owner.  It also prevents users from exceeding their
   privileges authorized by the respective security policy.  Privilege
   restrictions also help to reduce the impact of access token leakage.

   In particular, access tokens SHOULD be audience-restricted to a
   specific resource server or, if that is not feasible, to a small set
   of resource servers.  To put this into effect, the authorization
   server associates the access token with certain resource servers, and
   every resource server is obliged to verify, for every request,
   whether the access token sent with that request was meant to be used
   for that particular resource server.  If it was not, the resource
   server MUST refuse to serve the respective request.  The aud claim as
   defined in [RFC9068] MAY be used to audience-restrict access tokens.
   Clients and authorization servers MAY utilize the parameters scope or
   resource as specified in [RFC6749] and [RFC8707], respectively, to
   determine the resource server they want to access.

   Additionally, access tokens SHOULD be restricted to certain resources
   and actions on resource servers or resources.  To put this into
   effect, the authorization server associates the access token with the
   respective resource and actions and every resource server is obliged
   to verify, for every request, whether the access token sent with that
   request was meant to be used for that particular action on the
   particular resource.  If not, the resource server must refuse to
   serve the respective request.  Clients and authorization servers MAY
   utilize the parameter scope as specified in [RFC6749] and
   authorization_details as specified in [RFC9396] to determine those
   resources and/or actions.

2.4.  Resource Owner Password Credentials Grant

   The resource owner password credentials grant [RFC6749] MUST NOT be
   used.  This grant type insecurely exposes the credentials of the
   resource owner to the client.  Even if the client is benign, usage of
   this grant results in an increased attack surface (i.e., credentials
   can leak in more places than just the authorization server) and in
   training users to enter their credentials in places other than the
   authorization server.

   Furthermore, the resource owner password credentials grant is not
   designed to work with two-factor authentication and authentication
   processes that require multiple user interaction steps.
   Authentication with cryptographic credentials (cf. WebCrypto
   [W3C.WebCrypto], WebAuthn [W3C.WebAuthn]) may be impossible to
   implement with this grant type, as it is usually bound to a specific
   web origin.

2.5.  Client Authentication

   Authorization servers SHOULD enforce client authentication if it is
   feasible, in the particular deployment, to establish a process for
   issuance/registration of credentials for clients and ensuring the
   confidentiality of those credentials.

   It is RECOMMENDED to use asymmetric cryptography for client
   authentication, such as mutual TLS for OAuth 2.0 [RFC8705] or signed
   JWTs ("Private Key JWT") in accordance with [RFC7521] and [RFC7523].
   The latter is defined in [OpenID.Core] as the client authentication
   method private_key_jwt).  When asymmetric cryptography for client
   authentication is used, authorization servers do not need to store
   sensitive symmetric keys, making these methods more robust against
   leakage of keys.

2.6.  Other Recommendations

   The use of OAuth Authorization Server Metadata [RFC8414] can help to
   improve the security of OAuth deployments:

   *  It ensures that security features and other new OAuth features can
      be enabled automatically by compliant software libraries.
   *  It reduces chances for misconfigurations -- for example,
      misconfigured endpoint URLs (that might belong to an attacker) or
      misconfigured security features.
   *  It can help to facilitate rotation of cryptographic keys and to
      ensure cryptographic agility.

   It is therefore RECOMMENDED that authorization servers publish OAuth
   Authorization Server Metadata according to [RFC8414] and that clients
   make use of this Authorization Server Metadata (when available) to
   configure themselves.

   Under the conditions described in Section 4.15.1, authorization
   servers SHOULD NOT allow clients to influence their client_id or any
   other claim that could cause confusion with a genuine resource owner.

   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
   terminated at an intermediary, refer to Section 4.13 for further
   security advice.

   Authorization responses MUST NOT be transmitted over unencrypted
   network connections.  To this end, authorization servers MUST NOT
   allow redirection URIs that use the http scheme except for native
   clients that use loopback interface redirection as described in
   Section 7.3 of [RFC8252].

   If the authorization response is sent with in-browser communication
   techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
   redirects, both the initiator and receiver of the in-browser message
   MUST be strictly verified as described in Section 4.17.

   To support browser-based clients, endpoints directly accessed by such
   clients including the Token Endpoint, Authorization Server Metadata
   Endpoint, jwks_uri Endpoint, and Dynamic Client Registration Endpoint
   MAY support the use of Cross-Origin Resource Sharing (CORS)
   [WHATWG.CORS].  However, CORS MUST NOT be supported at the
   authorization endpoint, as the client does not access this endpoint
   directly; instead, the client redirects the user agent to it.

3.  The Updated OAuth 2.0 Attacker Model

   In [RFC6819], a threat model is laid out that describes the threats
   against which OAuth deployments must be protected.  While doing so,
   [RFC6819] makes certain assumptions about attackers and their
   capabilities, i.e., it implicitly establishes an attacker model.  In
   the following, this attacker model is made explicit and is updated
   and expanded to account for the potentially dynamic relationships
   involving multiple parties (as described in Section 1), to include
   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
   resource owner (with a user agent) at an authorization server and the
   subsequent usage of the access token at a resource server is
   protected, as well as practically possible, at least against the
   following attackers.

   (A1)  Web attackers that can set up and operate an arbitrary number
         of network endpoints (besides the "honest" ones) including
         browsers and servers.  Web attackers may set up websites that
         are visited by the resource owner, operate their own user
         agents, and participate in the protocol.

         In particular, web attackers may operate OAuth clients that are
         registered at the authorization server, and they may operate
         their own authorization and resource servers that can be used
         (in parallel to the "honest" ones) by the resource owner and
         other resource owners.

         It must also be assumed that web attackers can lure the user to
         navigate their browser to arbitrary attacker-chosen URIs at any
         time.  In practice, this can be achieved in many ways, for
         example, by injecting malicious advertisements into
         advertisement networks or by sending legitimate-looking emails.

         Web attackers can use their own user credentials to create new
         messages as well as any secrets they learned previously.  For
         example, if a web attacker learns an authorization code of a
         user through a misconfigured redirection URI, the web attacker
         can then try to redeem that code for an access token.

         They cannot, however, read or manipulate messages that are not
         targeted towards them (e.g., sent to a URL of an authorization
         server not under control of an attacker).

   (A2)  Network attackers that additionally have full control over the
         network over which protocol participants communicate.  They can
         eavesdrop on, manipulate, and spoof messages, except when these
         are properly protected by cryptographic methods (e.g., TLS).
         Network attackers can also block arbitrary messages.

   While an example for a web attacker would be a customer of an
   internet service provider, network attackers could be the internet
   service provider itself, an attacker in a public (Wi-Fi) network
   using ARP spoofing, or a state-sponsored attacker with access to
   internet exchange points, for instance.

   The aforementioned attackers (A1) and (A2) conform to the attacker
   model that was used in formal analysis efforts for OAuth
   [arXiv.1601.01229].  This is a minimal attacker model.  Implementers
   MUST take into account all possible types of attackers in the
   environment of their OAuth implementations.  For example, in
   [arXiv.1901.11520], a very strong attacker model is used that
   includes attackers that have full control over the token endpoint.
   This models effects of a possible misconfiguration of endpoints in
   the ecosystem, which can be avoided by using authorization server
   metadata as described in Section 2.6.  Such an attacker is therefore
   not listed here.

   However, previous attacks on OAuth have shown that the following
   types of attackers are relevant in particular:

   (A3)  Attackers that can read, but not modify, the contents of the
         authorization response (i.e., the authorization response can
         leak to an attacker).

         Examples of such attacks include open redirector attacks and
         mix-up attacks (see Section 4.4), where the client is tricked
         into sending credentials to an attacker-controlled
         authorization server.

         Also, this includes attacks that take advantage of:

         *  insufficient checking of redirect URIs (see Section 4.1);
         *  problems existing on mobile operating systems, where
            different apps can register themselves on the same URI; and
         *  URLs stored/logged by browsers (history), proxy servers, and
            operating systems.

   (A4)  Attackers that can read, but not modify, the contents of the
         authorization request (i.e., the authorization request can
         leak, in the same manner as above, to an attacker).

   (A5)  Attackers that can acquire an access token issued by an
         authorization server.  For example, a resource server may be
         compromised by an attacker, an access token may be sent to an
         attacker-controlled resource server due to a misconfiguration,
         or social engineering may be used to get a resource owner to
         use an attacker-controlled resource server.  Also see
         Section 4.9.2.

   (A3), (A4), and (A5) typically occur together with either (A1) or
   (A2).  Attackers can collaborate to reach a common goal.

   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
   replay tokens or authorization codes obtained by any of the attacks
   described above at the client or resource server.

   This document focuses on threats resulting from Attackers (A1) to
   (A5).

4.  Attacks and Mitigations

   This section gives a detailed description of attacks on OAuth
   implementations, along with potential countermeasures.  Attacks and
   mitigations already covered in [RFC6819] are not listed here, except
   where new recommendations are made.

   This section further defines additional requirements (beyond those
   defined in Section 2) for certain cases and protocol options.

4.1.  Insufficient Redirection URI Validation

   Some authorization servers allow clients to register redirection URI
   patterns instead of complete redirection URIs.  The authorization
   servers then match the redirection URI parameter value at the
   authorization endpoint against the registered patterns at runtime.
   This approach allows clients to encode transaction state into
   additional redirect URI parameters or to register a single pattern
   for multiple redirection URIs.

   This approach turned out to be more complex to implement and more
   error-prone to manage than exact redirection URI matching.  Several
   successful attacks exploiting flaws in the pattern-matching
   implementation or concrete configurations have been observed in the
   wild (see, e.g., [research.rub2]).  Insufficient validation of the
   redirection URI effectively breaks client identification or
   authentication (depending on grant and client type) and allows the
   attacker to obtain an authorization code or access token, either

   *  by directly sending the user agent to a URI under the attacker's
      control, or
   *  by exposing the OAuth credentials to an attacker by utilizing an
      open redirector at the client in conjunction with the way user
      agents handle URL fragments.

   These attacks are shown in detail in the following subsections.

4.1.1.  Redirect URI Validation Attacks on Authorization Code Grant

   For a client using the grant type code, an attack may work as
   follows:

   Assume the redirection URL pattern https://*.somesite.example/* is
   registered for the client with the client ID s6BhdRkqt3.  The
   intention is to allow any subdomain of somesite.example to be a valid
   redirection URI for the client, for example,
   https://app1.somesite.example/redirect.  However, a naive
   implementation on the authorization server might interpret the
   wildcard * as "any character" and not "any character valid for a
   domain name".  The authorization server, therefore, might permit
   https://attacker.example/.somesite.example as a redirection URI,
   although attacker.example is a different domain potentially
   controlled by a malicious party.

   The attack can then be conducted as follows:

   To begin, the attacker needs to trick the user into opening a
   tampered URL in their browser that launches a page under the
   attacker's control, say, https://www.evil.example (see attacker A1 in
   Section 3).

   This URL initiates the following authorization request with the
   client ID of a legitimate client to the authorization endpoint (line
   breaks for display only):

   GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
        &redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example
        HTTP/1.1
   Host: server.somesite.example

   The authorization server validates the redirection URI and compares
   it to the registered redirection URL patterns for the client
   s6BhdRkqt3.  The authorization request is processed and presented to
   the user.

   If the user does not see the redirection URI or does not recognize
   the attack, the code is issued and immediately sent to the attacker's
   domain.  If an automatic approval of the authorization is enabled
   (which is not recommended for public clients according to [RFC6749]),
   the attack can be performed even without user interaction.

   If the attacker impersonates a public client, the attacker can
   exchange the code for tokens at the respective token endpoint.

   This attack will not work as easily for confidential clients, since
   the code exchange requires authentication with the legitimate
   client's secret.  However, the attacker can use the legitimate
   confidential client to redeem the code by performing an authorization
   code injection attack; see Section 4.5.

   It is important to note that redirection URI validation
   vulnerabilities can also exist if the authorization server handles
   wildcards properly.  For example, assume that the client registers
   the redirection URL pattern https://*.somesite.example/* and the
   authorization server interprets this as "allow redirection URIs
   pointing to any host residing in the domain somesite.example".  If an
   attacker manages to establish a host or subdomain in
   somesite.example, the attacker can impersonate the legitimate client.
   For example, this could be caused by a subdomain takeover attack
   [research.udel], where an outdated CNAME record (say, external-
   service.somesite.example) points to an external DNS name that no
   longer exists (say, customer-abc.service.example) and can be taken
   over by an attacker (e.g., by registering as customer-abc with the
   external service).

4.1.2.  Redirect URI Validation Attacks on Implicit Grant

   The attack described above works for the implicit grant as well.  If
   the attacker is able to send the authorization response to an
   attacker-controlled URI, the attacker will directly get access to the
   fragment carrying the access token.

   Additionally, implicit grants (and also other grants when using
   response_mode=fragment as defined in [OAuth.Responses]) can be
   subject to a further kind of attack.  The attack utilizes the fact
   that user agents reattach fragments to the destination URL of a
   redirect if the location header does not contain a fragment (see
   Section 17.11 of [RFC9110]).  The attack described here combines this
   behavior with the client as an open redirector (see Section 4.11.1)
   in order to obtain access tokens.  This allows circumvention even of
   very narrow redirection URI patterns, but not of strict URL matching.

   Assume the registered URL pattern for client s6BhdRkqt3 is
   https://client.somesite.example/cb?*, i.e., any parameter is allowed
   for redirects to https://client.somesite.example/cb.  Unfortunately,
   the client exposes an open redirector.  This endpoint supports a
   parameter redirect_to which takes a target URL and will send the
   browser to this URL using an HTTP Location header redirect 303.

   The attack can now be conducted as follows:

   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
   attacker's control, say, https://www.evil.example.

   Afterwards, the website initiates an authorization request that is
   very similar to the one in the attack on the code flow.  Different to
   above, it utilizes the open redirector by encoding
   redirect_to=https://attacker.example into the parameters of the
   redirection URI, and it uses the response type token (line breaks for
   display only):

   GET /authorize?response_type=token&state=9ad67f13
       &client_id=s6BhdRkqt3
       &redirect_uri=https%3A%2F%2Fclient.somesite.example
        %2Fcb%26redirect_to%253Dhttps%253A%252F
        %252Fattacker.example%252F HTTP/1.1
   Host: server.somesite.example

   Then, since the redirection URI matches the registered pattern, the
   authorization server permits the request and sends the resulting
   access token in a 303 redirect (some response parameters omitted for
   readability):

   HTTP/1.1 303 See Other
   Location: https://client.somesite.example/cb?
             redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb
             #access_token=2YotnFZFEjr1zCsicMWpAA&...

   At client.somesite.example, the request arrives at the open
   redirector.  The endpoint will read the redirect parameter and will
   issue an HTTP 303 Location header redirect to the URL
   https://attacker.example/.

   HTTP/1.1 303 See Other
   Location: https://attacker.example/

   Since the redirector at client.somesite.example does not include a
   fragment in the Location header, the user agent will reattach the
   original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&amp;... to the
   URL and will navigate to the following URL:

   https://attacker.example/#access_token=2YotnFZFEjr1z...

   The attacker's page at attacker.example can then access the fragment
   and obtain the access token.

4.1.3.  Countermeasures

   The complexity of implementing and managing pattern matching
   correctly obviously causes security issues.  This document therefore
   advises simplifying the required logic and configuration by using
   exact redirection URI matching.  This means the authorization server
   MUST ensure that the two URIs are equal; see Section 6.2.1 of
   [RFC3986], Simple String Comparison, for details.  The only exception
   is native apps using a localhost URI: In this case, the authorization
   server MUST allow variable port numbers as described in Section 7.3
   of [RFC8252].

   Additional recommendations:

   *  Web servers on which redirection URIs are hosted MUST NOT expose
      open redirectors (see Section 4.11).
   *  Browsers reattach URL fragments to Location redirection URLs only
      if the URL in the Location header does not already contain a
      fragment.  Therefore, servers MAY prevent browsers from
      reattaching fragments to redirection URLs by attaching an
      arbitrary fragment identifier, for example #_, to URLs in Location
      headers.
   *  Clients SHOULD use the authorization code response type instead of
      response types that cause access token issuance at the
      authorization endpoint.  This offers countermeasures against the
      reuse of leaked credentials through the exchange process with the
      authorization server and against token replay through sender-
      constraining of the access tokens.

   If the origin and integrity of the authorization request containing
   the redirection URI can be verified, for example, when using
   [RFC9101] or [RFC9126] with client authentication, the authorization
   server MAY trust the redirection URI without further checks.

4.2.  Credential Leakage via Referer Headers

   The contents of the authorization request URI or the authorization
   response URI can unintentionally be disclosed to attackers through
   the Referer HTTP header (see Section 10.1.3 of [RFC9110]), by leaking
   from either the authorization server's or the client's website,
   respectively.  Most importantly, authorization codes or state values
   can be disclosed in this way.  Although specified otherwise in
   Section 10.1.3 of [RFC9110], the same may happen to access tokens
   conveyed in URI fragments due to browser implementation issues, as
   illustrated by a (now fixed) issue in the Chromium project
   [bug.chromium].

4.2.1.  Leakage from the OAuth Client

   Leakage from the OAuth client requires that the client, as a result
   of a successful authorization request, renders a page that

   *  contains links to other pages under the attacker's control and a
      user clicks on such a link, or
   *  includes third-party content (advertisements in iframes, images,
      etc.), for example, if the page contains user-generated content
      (blog).

   As soon as the browser navigates to the attacker's page or loads the
   third-party content, the attacker receives the authorization response
   URL and can extract code or state (and potentially access_token).

4.2.2.  Leakage from the Authorization Server

   In a similar way, an attacker can learn state from the authorization
   request if the authorization endpoint at the authorization server
   contains links or third-party content as above.

4.2.3.  Consequences

   An attacker that learns a valid code or access token through a
   Referer header can perform the attacks as described in Sections
   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
   as described in Section 4.4.1.8 of [RFC6819].

4.2.4.  Countermeasures

   The page rendered as a result of the OAuth authorization response and
   the authorization endpoint SHOULD NOT include third-party resources
   or links to external sites.

   The following measures further reduce the chances of a successful
   attack:

   *  Suppress the Referer header by applying an appropriate Referrer
      Policy [W3C.webappsec-referrer-policy] to the document (either as
      part of the "referrer" meta attribute or by setting a Referrer-
      Policy header).  For example, the header Referrer-Policy: no-
      referrer in the response completely suppresses the Referer header
      in all requests originating from the resulting document.

   *  Use authorization code instead of response types causing access
      token issuance from the authorization endpoint.

   *  Bind the authorization code to a confidential client or PKCE
      challenge.  In this case, the attacker lacks the secret to request
      the code exchange.

   *  As described in Section 4.1.2 of [RFC6749], authorization codes
      MUST be invalidated by the authorization server after their first
      use at the token endpoint.  For example, if an authorization
      server invalidated the code after the legitimate client redeemed
      it, the attacker would fail to exchange this code later.

      This does not mitigate the attack if the attacker manages to
      exchange the code for a token before the legitimate client does
      so.  Therefore, [RFC6749] further recommends that, when an attempt
      is made to redeem a code twice, the authorization server SHOULD
      revoke all tokens issued previously based on that code.

   *  The state value SHOULD be invalidated by the client after its
      first use at the redirection endpoint.  If this is implemented,
      and an attacker receives a token through the Referer header from
      the client's website, the state was already used, invalidated by
      the client and cannot be used again by the attacker.  (This does
      not help if the state leaks from the authorization server's
      website, since then the state has not been used at the redirection
      endpoint at the client yet.)

   *  Use the form post response mode instead of a redirect for the
      authorization response (see [OAuth.Post]).

4.3.  Credential Leakage via Browser History

   Authorization codes and access tokens can end up in the browser's
   history of visited URLs, enabling the attacks described in the
   following.

4.3.1.  Authorization Code in Browser History

   When a browser navigates to client.example/
   redirection_endpoint?code=abcd as a result of a redirect from a
   provider's authorization endpoint, the URL including the
   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.

   Countermeasures:

   *  Authorization code replay prevention as described in
      Section 4.4.1.1 of [RFC6819], and Section 4.5.
   *  Use the form post response mode instead of redirect for the
      authorization response (see [OAuth.Post]).

4.3.2.  Access Token in Browser History

   An access token may end up in the browser history if a client or a
   website that already has a token deliberately navigates to a page
   like provider.com/get_user_profile?access_token=abcdef.  [RFC6750]
   discourages this practice and advises transferring tokens via a
   header, but in practice websites often pass access tokens in query
   parameters.

   In the case of implicit grant, a URL like client.example/
   redirection_endpoint#access_token=abcdef may also end up in the
   browser history as a result of a redirect from a provider's
   authorization endpoint.

   Countermeasures:

   *  Clients MUST NOT pass access tokens in a URI query parameter in
      the way described in Section 2.3 of [RFC6750].  The authorization
      code grant or alternative OAuth response modes like the form post
      response mode [OAuth.Post] can be used to this end.

4.4.  Mix-Up Attacks

   Mix-up attacks can occur in scenarios where an OAuth client interacts
   with two or more authorization servers and at least one authorization
   server is under the control of the attacker.  This can be the case,
   for example, if the attacker uses dynamic registration to register
   the client at their own authorization server or if an authorization
   server becomes compromised.

   The goal of the attack is to obtain an authorization code or an
   access token for an uncompromised authorization server.  This is
   achieved by tricking the client into sending those credentials to the
   compromised authorization server (the attacker) instead of using them
   at the respective endpoint of the uncompromised authorization/
   resource server.

4.4.1.  Attack Description

   The description here follows [arXiv.1601.01229], with variants of the
   attack outlined below.

   Preconditions: For this variant of the attack to work, it is assumed
   that

   *  the implicit or authorization code grant is used with multiple
      authorization servers of which one is considered "honest" (H-AS)
      and one is operated by the attacker (A-AS), and
   *  the client stores the authorization server chosen by the user in a
      session bound to the user's browser and uses the same redirection
      URI for each authorization server.

   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).
   URLs shown in the following example are shortened for presentation to
   include only parameters relevant to the attack.

   Attack on the authorization code grant:

   1.  The user selects to start the grant using A-AS (e.g., by clicking
       on a button on the client's website).

   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
       with a Location header containing the URL
       https://attacker.example/
       authorize?response_type=code&client_id=666RVZJTA.

   3.  When the user's browser navigates to the attacker's authorization
       endpoint, the attacker immediately redirects the browser to the
       authorization endpoint of H-AS.  In the authorization request,
       the attacker replaces the client ID of the client at A-AS with
       the client's ID at H-AS.  Therefore, the browser receives a
       redirection (303 See Other) with a Location header pointing to
       https://honest.as.example/
       authorize?response_type=code&client_id=7ZGZldHQ

   4.  The user authorizes the client to access their resources at H-AS.
       (Note that a vigilant user might at this point detect that they
       intended to use A-AS instead of H-AS.  The first attack variant
       listed does not have this limitation.)  H-AS issues a code and
       sends it (via the browser) back to the client.

   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.

   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.

   Variants:

   *  Mix-Up with Interception: This variant works only if the attacker
      can intercept and manipulate the first request/response pair from
      a user's browser to the client (in which the user selects a
      certain authorization server and is then redirected by the client
      to that authorization server), as in Attacker (A2) (see
      Section 3).  This capability can, for example, be the result of an
      attacker-in-the-middle attack on the user's connection to the
      client.  In the attack, the user starts the flow with H-AS.  The
      attacker intercepts this request and changes the user's selection
      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
      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).
   *  Per-AS Redirect URIs: If clients use different redirection URIs
      for different authorization servers, clients do not store the
      selected authorization server in the user's session, and
      authorization servers do not check the redirection URIs properly,
      attackers can mount an attack called "Cross Social-Network Request
      Forgery".  These attacks have been observed in practice.  Refer to
      [research.jcs_14] for details.
   *  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").

4.4.2.  Countermeasures

   When an OAuth client can only interact with one authorization server,
   a mix-up defense is not required.  In scenarios where an OAuth client
   interacts with two or more authorization servers, however, clients
   MUST prevent mix-up attacks.  Two different methods are discussed
   below.

   For both defenses, clients MUST store, for each authorization
   request, the issuer they sent the authorization request to and bind
   this information to the user agent.  The issuer serves, via the
   associated metadata, as an abstract identifier for the combination of
   the authorization endpoint and token endpoint that are to be used in
   the flow.  If an issuer identifier is not available (for example, if
   neither OAuth Authorization Server Metadata [RFC8414] nor OpenID
   Connect Discovery [OpenID.Discovery] is used), a different unique
   identifier for this tuple or the tuple itself can be used instead.
   For brevity of presentation, such a deployment-specific identifier
   will be subsumed under the issuer (or issuer identifier) in the
   following.

   It is important to note that just storing the authorization server
   URL is not sufficient to identify mix-up attacks.  An attacker might
   declare an uncompromised authorization server's authorization
   endpoint URL as "their" authorization server URL, but declare a token
   endpoint under their own control.

4.4.2.1.  Mix-Up Defense via Issuer Identification

   This defense requires that the authorization server sends its issuer
   identifier in the authorization response to the client.  When
   receiving the authorization response, the client MUST compare the
   received issuer identifier to the stored issuer identifier.  If there
   is a mismatch, the client MUST abort the interaction.

   There are different ways this issuer identifier can be transported to
   the client:

   *  The issuer information can be transported, for example, via a
      separate response parameter iss, defined in [RFC9207].
   *  When OpenID Connect is used and an ID Token is returned in the
      authorization response, the client can evaluate the iss claim in
      the ID Token.

   In both cases, the iss value MUST be evaluated according to
   [RFC9207].

   While this defense may require deploying new OAuth features to
   transport the issuer information, it is a robust and relatively
   simple defense against mix-up.

4.4.2.2.  Mix-Up Defense via Distinct Redirect URIs

   For this defense, clients MUST use a distinct redirection URI for
   each issuer they interact with.

   Clients MUST check that the authorization response was received from
   the correct issuer by comparing the distinct redirection URI for the
   issuer to the URI where the authorization response was received on.
   If there is a mismatch, the client MUST abort the flow.

   While this defense builds upon existing OAuth functionality, it
   cannot be used in scenarios where clients only register once for the
   use of many different issuers (as in some open banking schemes) and
   due to the tight integration with the client registration, it is
   harder to deploy automatically.

   Furthermore, an attacker might be able to circumvent the protection
   offered by this defense by registering a new client with the "honest"
   authorization server using the redirect URI that the client assigned
   to the attacker's authorization server.  The attacker could then run
   the attack as described above, replacing the client ID with the
   client ID of their newly created client.

   This defense SHOULD therefore only be used if other options are not
   available.

4.5.  Authorization Code Injection

   An attacker who has gained access to an authorization code contained
   in an authorization response (see Attacker (A3) in Section 3) can try
   to redeem the authorization code for an access token or otherwise
   make use of the authorization code.

   In the case that the authorization code was created for a public
   client, the attacker can send the authorization code to the token
   endpoint of the authorization server and thereby get an access token.
   This attack was described in Section 4.4.1.1 of [RFC6819].

   For confidential clients, or in some special situations, the attacker
   can execute an authorization code injection attack, as described in
   the following.

   In an authorization code injection attack, the attacker attempts to
   inject a stolen authorization code into the attacker's own session
   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 attacker at least limited access to the victim's resources.

   Besides circumventing the client authentication of confidential
   clients, other use cases for this attack include:

   *  The attacker wants to access certain functions in this particular
      client.  As an example, the attacker wants to impersonate their
      victim in a certain app or on a certain website.
   *  The authorization or resource servers are limited to certain
      networks that the attacker is unable to access directly.

   Except in these special cases, authorization code injection is
   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
   powerful attack, as described above.

4.5.1.  Attack Description

   The authorization code injection attack works as follows:

   1.  The attacker obtains an authorization code (see Attacker (A3) in
       Section 3).  For the rest of the attack, only the capabilities of
       a web attacker (A1) are required.
   2.  From the attacker's device, the attacker starts a regular OAuth
       authorization process with the legitimate client.
   3.  In the response of the authorization server to the legitimate
       client, the attacker replaces the newly created authorization
       code with the stolen authorization code.  Since this response is
       passing through the attacker's device, the attacker can use any
       tool that can intercept and manipulate the authorization response
       to this end.  The attacker does not need to control the network.
   4.  The legitimate client sends the code to the authorization
       server's token endpoint, along with the redirect_uri and the
       client's client ID and client secret (or other means of client
       authentication).
   5.  The authorization server checks the client secret, whether the
       code was issued to the particular client, and whether the actual
       redirection URI matches the redirect_uri parameter (see
       [RFC6749]).
   6.  All checks succeed and the authorization server issues access and
       other tokens to the client.  The attacker has now associated
       their session with the legitimate client with the victim's
       resources and/or identity.

4.5.2.  Discussion

   Obviously, the check-in step (Step 5) will fail if the code was
   issued to another client ID, e.g., a client set up by the attacker.
   The check will also fail if the authorization code was already
   redeemed by the legitimate user and was one-time use only.

   An attempt to inject a code obtained via a manipulated redirection
   URI should also be detected if the authorization server stored the
   complete redirection URI used in the authorization request and
   compares it with the redirect_uri parameter.

   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 described in Section 4.1.1, and if included ensure that
   |  their values are identical.

   In the attack scenario described in Section 4.5.1, the legitimate
   client would use the correct redirection URI it always uses for
   authorization requests.  But this URI would not match the tampered
   redirection 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
   that had been obtained from another instance of the same client on
   another device if certain conditions are fulfilled:

   *  the redirection URI itself contains a nonce or another kind of
      one-time use, secret data and
   *  the client has bound this data to this particular instance of the
      client.

   But, this approach conflicts with the idea of enforcing exact
   redirect URI matching at the authorization endpoint.  Moreover, it
   has been observed that providers very often ignore the redirect_uri
   check requirement at this stage, maybe because it doesn't seem to be
   security-critical from reading the specification.

   Other providers just pattern match the redirect_uri parameter against
   the registered redirection URI pattern.  This saves the authorization
   server from storing the link between the actual redirect URI and the
   respective authorization code for every transaction.  However, this
   kind of check obviously does not fulfill the intent of the
   specification, since the tampered redirection URI is not considered.
   So, any attempt to inject an authorization code obtained using the
   client_id of a legitimate client or by utilizing the legitimate
   client on another device will not be detected in the respective
   deployments.

   It is also assumed that the requirements defined in Section 4.1.3 of
   [RFC6749] increase client implementation complexity as clients need
   to store or reconstruct the correct redirection URI for the call to
   the token endpoint.

   Asymmetric methods for client authentication do not stop this attack,
   as the legitimate client authenticates at the token endpoint.

   This document therefore recommends instead binding every
   authorization code to a certain client instance on a certain device
   (or in a certain user agent) in the context of a certain transaction
   using one of the mechanisms described next.

4.5.3.  Countermeasures

   There are two good technical solutions to binding authorization codes
   to client instances, as follows.

4.5.3.1.  PKCE

   The PKCE mechanism specified in [RFC7636] can be used as a
   countermeasure (even though it was originally designed to secure
   native apps).  When the attacker attempts to inject an authorization
   code, the check of the code_verifier fails: the client uses its
   correct verifier, but the code is associated with a code_challenge
   that does not match this verifier.

   PKCE not only protects against the authorization code injection
   attack but also protects authorization codes created for public
   clients: PKCE ensures that an attacker cannot redeem a stolen
   authorization code at the token endpoint of the authorization server
   without knowledge of the code_verifier.

4.5.3.2.  Nonce

   OpenID Connect's existing nonce parameter can protect against
   authorization code injection attacks.  The nonce value is one-time
   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
   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
   endpoint.  If an attacker injects an authorization code in 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
   match, and the attack is detected.  The assumption is that an
   attacker cannot get hold of the user agent state on the victim's
   device (from which the attacker has stolen the respective
   authorization code).

   It is important to note that this countermeasure only works if the
   client properly checks the nonce parameter in the ID Token obtained
   from the token endpoint and does not use any issued token until this
   check has succeeded.  More precisely, a client protecting itself
   against code injection using the nonce parameter

   1.  MUST validate the nonce in the ID Token obtained from the token
       endpoint, even if another ID Token was obtained from the
       authorization response (e.g., response_type=code+id_token), and
   2.  MUST ensure that, unless and until that check succeeds, all
       tokens (ID Tokens and the access token) are disregarded and not
       used for any other purpose.

   It is important to note that nonce does not protect authorization
   codes of public clients, as an attacker does not need to execute an
   authorization code injection attack.  Instead, an attacker can
   directly call the token endpoint with the stolen authorization code.

4.5.3.3.  Other Solutions

   Other solutions like binding state to the code, sender-constraining
   the code using cryptographic means, or per-instance client
   credentials are conceivable, but lack support and bring new security
   requirements.

   PKCE is the most obvious solution for OAuth clients, as it is
   available at the time of writing, while nonce is appropriate for
   OpenID Connect clients.

4.5.4.  Limitations

   An attacker can circumvent the countermeasures described above if
   they can modify the nonce or code_challenge values that are used in
   the victim's authorization request.  The attacker can modify these
   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
   victim's session with the client begins after the attacker started
   their session with the client.)  If the attacker is then able to
   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
   used.

   This attack is complex and requires a close interaction between the
   attacker and the victim's session.  Nonetheless, measures to prevent
   attackers from reading the contents of the authorization response
   still need to be taken, as described in Sections 4.1, 4.2, 4.3, 4.4,
   and 4.11.

4.6.  Access Token Injection

   In an access token injection attack, the attacker attempts to inject
   a stolen access token into a legitimate client (that is not under the
   attacker's control).  This will typically happen if the attacker
   wants to utilize a leaked access token to impersonate a user in a
   certain client.

   To conduct the attack, the attacker starts an OAuth flow with the
   client using the implicit grant and modifies the authorization
   response by replacing the access token issued by the authorization
   server or directly making up an authorization server response
   including the leaked access token.  Since the response includes the
   state value generated by the client for this particular transaction,
   the client does not treat the response as a CSRF attack and uses the
   access token injected by the attacker.

4.6.1.  Countermeasures

   There is no way to detect such an injection attack in pure-OAuth
   flows since the token is issued without any binding to the
   transaction or the particular user agent.

   In OpenID Connect, the attack can be mitigated, as the authorization
   response additionally contains an ID Token containing the at_hash
   claim.  The attacker therefore needs to replace both the access token
   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
   attacker also cannot inject a leaked ID Token matching the stolen
   access token, as the nonce claim in the leaked ID Token will contain
   (with a very high probability) a different value than the one
   expected in the authorization response.

   Note that further protection, like sender-constrained access tokens,
   is still required to prevent attackers from using the access token at
   the resource endpoint directly.

   The recommendations in Section 2.1.2 follow from this.

4.7.  Cross-Site Request Forgery

   An attacker might attempt to inject a request to the redirection URI
   of 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
   variant of an attack known as Cross-Site Request Forgery (CSRF).

4.7.1.  Countermeasures

   The long-established countermeasure is that clients pass a random
   value, also known as a CSRF Token, in the state parameter that links
   the request to the redirection URI to the user agent session as
   described.  This countermeasure is described in detail in
   Section 5.3.5 of [RFC6819].  The same protection is provided by PKCE
   or the OpenID Connect nonce value.

   When using PKCE instead of state or nonce for CSRF protection, it is
   important to note that:

   *  Clients MUST ensure that the authorization server supports PKCE
      before using PKCE for CSRF protection.  If an authorization server
      does not support PKCE, state or nonce MUST be used for CSRF
      protection.

   *  If state is used for carrying application state, and the integrity
      of its contents is a concern, clients MUST protect state against
      tampering and swapping.  This can be achieved by binding the
      contents of state to the browser session and/or by signing/
      encrypting state values.  One example of this is discussed in the
      expired Internet-Draft [JWT-ENCODED-STATE].

   The authorization server therefore MUST provide a way to detect their
   support for PKCE.  Using Authorization Server Metadata according to
   [RFC8414] is RECOMMENDED, but authorization servers MAY instead
   provide a deployment-specific way to ensure or determine PKCE
   support.

   PKCE provides robust protection against CSRF attacks even in the
   presence of an attacker that can read the authorization response (see
   Attacker (A3) in Section 3).  When state is used or an ID Token is
   returned in the authorization response (e.g.,
   response_type=code+id_token), the attacker either learns the state
   value and can replay it into the forged authorization response, or
   can extract the nonce from the ID Token and use it in a new request
   to the authorization server to mint an ID Token with the same nonce.
   The new ID Token can then be used for the CSRF attack.

4.8.  PKCE Downgrade Attack

   An authorization server that supports PKCE but does not make its use
   mandatory for all flows can be susceptible to a PKCE downgrade
   attack.

   The first prerequisite for this attack is that there is an attacker-
   controllable flag in the authorization request that enables or
   disables PKCE for the particular flow.  The presence or absence of
   the code_challenge parameter lends itself for this purpose, i.e., the
   authorization server enables and enforces PKCE if this parameter is
   present in the authorization request, but it does not enforce PKCE if
   the parameter is missing.

   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
   prevention) or that the client is not checking state correctly.

   Roughly speaking, this attack is a variant of a CSRF attack.  The
   attacker achieves the same goal as in the attack described in
   Section 4.7: The attacker injects an authorization code (and with
   that, an access token) that is bound to the attacker's resources into
   a session between their victim and the client.

4.8.1.  Attack Description

   1.  The user has started an OAuth session using some client at an
       authorization server.  In the authorization request, the client
       has set the parameter code_challenge=hash(abc) as the PKCE code
       challenge (with the hash function and parameter encoding as
       defined in [RFC7636]).  The client is now waiting to receive the
       authorization response from the user's browser.
   2.  To conduct the attack, the attacker uses their own device to
       start an authorization flow with the targeted client.  The client
       now uses another PKCE code challenge, say,
       code_challenge=hash(xyz), in the authorization request.  The
       attacker intercepts the request and removes the entire
       code_challenge parameter from the request.  Since this step is
       performed on the attacker's device, the attacker has full access
       to the request contents, for example, using browser debug tools.
   3.  If the authorization server allows for flows without PKCE, it
       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
       response URL that contains the code for the attacker's session
       with the authorization server.
   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
       authorization server.  The client will send code_verifier=abc as
       the PKCE code verifier in the token request.
   6.  Since the authorization server sees that this code is not bound
       to any PKCE code challenge, it will not check the presence or
       contents of the code_verifier parameter.  It will issue an access
       token (which belongs to the attacker's resource) to the client
       under the user's control.

4.8.2.  Countermeasures

   Using state properly would prevent this attack.  However, practice
   has shown that many OAuth clients do not use or check state properly.

   Therefore, authorization servers MUST mitigate this attack.

   Note that from the view of the authorization server, in the attack
   described above, a code_verifier parameter is received at the token
   endpoint although no code_challenge parameter was present in the
   authorization request for the OAuth flow in which the authorization
   code was issued.

   This fact can be used to mitigate this attack.  [RFC7636] already
   mandates that

   *  an authorization server that supports PKCE MUST check whether a
      code challenge is contained in the authorization request and bind
      this information to the code that is issued; and
   *  when a code arrives at the token endpoint, and there was a
      code_challenge in the authorization request for which this code
      was issued, there must be a valid code_verifier in the token
      request.

   Beyond this, to prevent PKCE downgrade attacks, the authorization
   server MUST ensure that if there was no code_challenge in the
   authorization request, a request to the token endpoint containing a
   code_verifier is rejected.

   Authorization servers that mandate the use of PKCE (in general or for
   particular clients) implicitly implement this security measure.

4.9.  Access Token Leakage at the Resource Server

   Access tokens can leak from a resource server under certain
   circumstances.

4.9.1.  Access Token Phishing by Counterfeit Resource Server

   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
   servers (see Attackers (A1) and (A5) in Section 3).  If the client
   sends a valid access token to this counterfeit resource server, the
   attacker in turn may use that token to access other services on
   behalf of the resource owner.

   This attack assumes the client is not bound to one specific resource
   server (and its URL) at development time, but client instances are
   provided with the resource server URL at runtime.  This kind of late
   binding is typical in situations where the client uses a service
   implementing a standardized API (e.g., for email, calendaring,
   eHealth, or open banking) and where the client is configured by a
   user or administrator.

4.9.2.  Compromised Resource Server

   An attacker may compromise a resource server to gain access to the
   resources of the respective deployment.  Such a compromise may range
   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
   circumvented and all resources can be accessed.  The attacker would
   also be able to obtain other access tokens held on the compromised
   system that would potentially be valid to access other resource
   servers.

   Preventing server breaches by hardening and monitoring server systems
   is considered a standard operational procedure and, therefore, out of
   the scope of this document.  Section 4.9 focuses on the impact of
   OAuth-related breaches and the replaying of captured access tokens.

4.9.3.  Countermeasures

   The following measures should be taken into account by implementers
   in order to cope with access token replay by malicious actors:

   *  Sender-constrained access tokens, as described in Section 4.10.1,
      SHOULD be used to prevent the attacker from replaying the access
      tokens on other resource servers.  If an attacker has only partial
      access to the compromised system, like a read-only access to web
      server logs, sender-constrained access tokens may also prevent
      replay on the compromised system.
   *  Audience restriction as described in Section 4.10.2 SHOULD be used
      to prevent replay of captured access tokens on other resource
      servers.
   *  The resource server MUST treat access tokens like other sensitive
      secrets and not store or transfer them in plaintext.

   The first and second recommendations also apply to other scenarios
   where access tokens leak (see Attacker (A5) in Section 3).

4.10.  Misuse of Stolen Access Tokens

   Access tokens can be stolen by an attacker in various ways, for
   example, via the attacks described in Sections 4.1, 4.2, 4.3, 4.4,
   and 4.9.  Some of these attacks can be mitigated by specific security
   measures, as described in the respective sections.  However, in some
   cases, these measures are not sufficient or are not implemented
   correctly.  Authorization servers therefore SHOULD ensure that access
   tokens are sender-constrained and audience-restricted as described in
   the following.  Architecture and performance reasons may prevent the
   use of these measures in some deployments.

4.10.1.  Sender-Constrained Access Tokens

   As the name suggests, sender-constrained access tokens scope the
   applicability of an access token to a certain sender.  This sender is
   obliged to demonstrate knowledge of a certain secret as a
   prerequisite for the acceptance of that token at a resource server.

   A typical flow looks like this:

   1.  The authorization server associates data with the access token
       that binds this particular token to a certain client.  The
       binding can utilize the client's identity, but in most cases, the
       authorization server utilizes key material (or data derived from
       the key material) known to the client.
   2.  This key material must be distributed somehow.  Either the key
       material already exists before the authorization server creates
       the binding or the authorization server creates ephemeral keys.
       The way preexisting key material is distributed varies among the
       different approaches.  For example, X.509 certificates can be
       used, in which case the distribution happens explicitly during
       the enrollment process.  Or, the key material is created and
       distributed at the TLS layer, in which case it might
       automatically happen during the setup of a TLS connection.
   3.  The resource server must implement the actual proof-of-possession
       check.  This is typically done on the application level, often
       tied to specific material provided by the transport layer (e.g.,
       TLS).  The resource server must also ensure that a replay of the
       proof of possession is not possible.

   Two methods for sender-constrained access tokens using proof of
   possession have been defined by the OAuth working group and are in
   use in practice:

   *  "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
      Access Tokens" [RFC8705]: The approach specified in this) document
      allows the use of mutual TLS for both client authentication and
      sender-constrained access tokens.  For the purpose of sender-
      constrained access tokens, the client is identified towards the
      resource server by the fingerprint of its public key.  During the
      processing of an access token request, the authorization server
      obtains the client's public key from the TLS stack and associates
      its fingerprint with the respective access tokens.  The resource
      server in the same way obtains the public key from the TLS stack
      and compares its fingerprint with the fingerprint associated with
      the access token.
   *  "OAuth 2.0 Demonstrating Proof of Possession (DPoP)" [RFC9449]:
      DPoP outlines an application-level mechanism for sender-
      constraining access and refresh tokens.  It uses proof-of-
      possession based on a public/private key pair and application-
      level signing.  DPoP can be used with public clients and, in the
      case of confidential clients, can be combined with any client
      authentication method.

   Note that the security of sender-constrained tokens is undermined
   when an attacker gets access to the token and the key material.  This
   is, in particular, the case for corrupted client software and cross-
   site scripting attacks (when the client is running in the browser).
   If the key material is protected in a hardware or software security
   module or only indirectly accessible (like in a TLS stack), sender-
   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
   not available to the attacker.  This applies to access tokens as well
   as to refresh tokens (see Section 4.14).

4.10.2.  Audience-Restricted Access Tokens

   Audience restriction essentially restricts access tokens to a
   particular resource server.  The authorization server associates the
   access token with the particular resource server, and the resource
   server is then supposed to verify the intended audience.  If the
   access token fails the intended audience validation, the resource
   server refuses to serve the respective request.

   In general, audience restriction limits the impact of token leakage.
   In the case of a counterfeit resource server, it may (as described
   below) also prevent abuse of the phished access token at the
   legitimate resource server.

   The audience can be expressed using logical names or physical
   addresses (like URLs).  To prevent phishing, it is necessary to use
   the actual URL the client will send requests to.  In the phishing
   case, this URL will point to the counterfeit resource server.  If the
   attacker tries to use the access token at the legitimate resource
   server (which has a different URL), the resource server will detect
   the mismatch (wrong audience) and refuse to serve the request.

   In deployments where the authorization server knows the URLs of all
   resource servers, the authorization server may just refuse to issue
   access tokens for unknown resource server URLs.

   For this to work, the client needs to tell the authorization server
   the intended resource server.  The mechanism in [RFC8707] can be used
   for this or the information can be encoded in the scope value
   (Section 3.3 of [RFC6749]).

   Instead of the URL, it is also possible to utilize the fingerprint of
   the resource server's X.509 certificate as the audience value.  This
   variant would also allow detection of an attempt to spoof the
   legitimate resource server's URL by using a valid TLS certificate
   obtained from a different CA.  It might also be considered a privacy
   benefit to hide the resource server URL from the authorization
   server.

   Audience restriction may seem easier to use since it does not require
   any cryptography on the client side.  Still, since every access token
   is bound to a specific resource server, the client also needs to
   obtain a single resource server-specific access token when accessing
   several resource servers.  (Resource indicators, as specified in
   [RFC8707], can help to achieve this.)  [TOKEN-BINDING] had the same
   property since different token-binding IDs must be associated with
   the access token.  Using mutual TLS for OAuth 2.0 [RFC8705], on the
   other hand, allows a client to use the access token at multiple
   resource servers.

   It should be noted that audience restrictions -- or, generally
   speaking, an indication by the client to the authorization server
   where it wants to use the access token -- have additional benefits
   beyond the scope of token leakage prevention.  They allow the
   authorization server to create a different access token whose format
   and content are specifically minted for the respective server.  This
   has huge functional and privacy advantages in deployments using
   structured access tokens.

4.10.3.  Discussion: Preventing Leakage via Metadata

   An authorization server could provide the client with additional
   information about the locations where it is safe to use its access
   tokens.  This approach, and why it is not recommended, is discussed
   in the following.

   In the simplest form, this would require the authorization server to
   publish a list of its known resource servers, illustrated in the
   following example using a non-standard Authorization Server Metadata
   parameter resource_servers:

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "issuer":"https://server.somesite.example",
     "authorization_endpoint":
       "https://server.somesite.example/authorize",
     "resource_servers":[
       "email.somesite.example",
       "storage.somesite.example",
       "video.somesite.example"
     ]
     ...
   }

   The authorization server could also return the URL(s) an access token
   is good for in the token response, illustrated by the example and
   non-standard return parameter access_token_resource_server:

   HTTP/1.1 200 OK
   Content-Type: application/json;charset=UTF-8
   Cache-Control: no-store
   Pragma: no-cache

   {
     "access_token":"2YotnFZFEjr1zCsicMWpAA",
     "access_token_resource_server":
       "https://hostedresource.somesite.example/path1",
   ...
   }

   This mitigation strategy would rely on the client to enforce the
   security policy and to only send access tokens to legitimate
   destinations.  Results of OAuth-related security research (see, for
   example, [research.ubc] and [research.cmu]) indicate a large portion
   of client implementations do not or fail to properly implement
   security controls, like state checks.  So, relying on clients to
   prevent access token phishing is likely to fail as well.  Moreover,
   given the ratio of clients to authorization and resource servers, it
   is considered the more viable approach to move as much as possible
   security-related logic to those servers.  Clearly, the client has to
   contribute to the overall security.  However, there are alternative
   countermeasures, as described in Sections 4.10.1 and 4.10.2, that
   provide a better balance between the involved parties.

4.11.  Open Redirection

   The following attacks can occur when an authorization server or
   client has an open redirector.  Such endpoints are sometimes
   implemented, for example, to show a message before a user is then
   redirected to an external website, or to redirect users back to a URL
   they were intending to visit before being interrupted, e.g., by a
   login prompt.

4.11.1.  Client as Open Redirector

   Clients MUST NOT expose open redirectors.  Attackers may use open
   redirectors to produce URLs pointing to the client and utilize them
   to exfiltrate authorization codes and access tokens, as described in
   Section 4.1.2.  Another abuse case is to produce URLs that appear to
   point to the client.  This might trick users into trusting the URL
   and following it in their browser.  This can be abused for phishing.

   In order to prevent open redirection, clients should only redirect if
   the target URLs are allowed or if the origin and integrity of a
   request can be authenticated.  Countermeasures against open
   redirection are described by OWASP [owasp.redir].

4.11.2.  Authorization Server as Open Redirector

   Just as with clients, attackers could try to utilize a user's trust
   in the authorization server (and its URL in particular) for
   performing phishing attacks.  OAuth authorization servers regularly
   redirect users to other websites (the clients), but they must do so
   safely.

   Section 4.1.2.1 of [RFC6749] already prevents open redirects by
   stating that the authorization server MUST NOT automatically redirect
   the user agent in case of an invalid combination of client_id and
   redirect_uri.

   However, an attacker could also utilize a correctly registered
   redirection URI to perform phishing attacks.  The attacker could, for
   example, register a client via dynamic client registration [RFC7591]
   and execute one of the following attacks:

   1.  Intentionally send an erroneous authorization request, e.g., by
       using an invalid scope value, thus instructing the authorization
       server to redirect the user agent to its phishing site.
   2.  Intentionally send a valid authorization request with client_id
       and redirect_uri controlled by the attacker.  After the user
       authenticates, the authorization server prompts the user to
       provide consent to the request.  If the user notices an issue
       with the request and declines the request, the authorization
       server still redirects the user agent to the phishing site.  In
       this case, the user agent will be redirected to the phishing site
       regardless of the action taken by the user.
   3.  Intentionally send a valid silent authentication request
       (prompt=none) with client_id and redirect_uri controlled by the
       attacker.  In this case, the authorization server will
       automatically redirect the user agent to the phishing site.

   The authorization server MUST take precautions to prevent these
   threats.  The authorization server MUST always authenticate the user
   first and, with the exception of the silent authentication use case,
   prompt the user for credentials when needed, before redirecting the
   user.  Based on its risk assessment, the authorization server needs
   to decide whether or not it can trust the redirection URI.  It could
   take into account URI analytics done internally or through some
   external service to evaluate the credibility and trustworthiness of
   content behind the URI, and the source of the redirection URI and
   other client data.

   The authorization server SHOULD only automatically redirect the user
   agent if it trusts the redirection URI.  If the URI is not trusted,
   the authorization server MAY inform the user and rely on the user to
   make the correct decision.

4.12.  307 Redirect

   At the authorization endpoint, a typical protocol flow is that the
   authorization server prompts the user to enter their credentials in a
   form that is then submitted (using the HTTP POST method) back to the
   authorization server.  The authorization server checks the
   credentials and, if successful, redirects the user agent to the
   client's redirection endpoint.

   In [RFC6749], the HTTP status code 302 (Found) is used for this
   purpose, but "any other method available via the user-agent to
   accomplish this redirection is allowed".  When the status code 307 is
   used for redirection instead, the user agent will send the user's
   credentials via HTTP POST to the client.

   This discloses the sensitive credentials to the client.  If the
   client is malicious, it can use the credentials to impersonate the
   user at the authorization server.

   The behavior might be unexpected for developers but is defined in
   Section 15.4.8 of [RFC9110].  This status code (307) does not require
   the user agent to rewrite the POST request to a GET request and
   thereby drop the form data in the POST request body.

   In the HTTP standard [RFC9110], only the status code 303
   unambiguously enforces rewriting the HTTP POST request to an HTTP GET
   request.  For all other status codes, including the popular 302, user
   agents can opt not to rewrite POST to GET requests, thereby causing
   the user's credentials to be revealed to the client.  (In practice,
   however, most user agents will only show this behavior for 307
   redirects.)

   Authorization servers that redirect a request that potentially
   contains the user's credentials therefore MUST NOT use the HTTP 307
   status code for redirection.  If an HTTP redirection (and not, for
   example, JavaScript) is used for such a request, the authorization
   server SHOULD use HTTP status code 303 (See Other).

4.13.  TLS Terminating Reverse Proxies

   A common deployment architecture for HTTP applications is to hide the
   application server behind a reverse proxy that terminates the TLS
   connection and dispatches the incoming requests to the respective
   application server nodes.

   This section highlights some attack angles of this deployment
   architecture with relevance to OAuth and gives recommendations for
   security controls.

   In some situations, the reverse proxy needs to pass security-related
   data to the upstream application servers for further processing.
   Examples include the IP address of the request originator, token-
   binding IDs, and authenticated TLS client certificates.  This data is
   usually passed in HTTP headers added to the upstream request.  While
   the headers are often custom, application-specific headers,
   standardized header fields for client certificates and client
   certificate chains are defined in [RFC9440].

   If the reverse proxy passes through any header sent from the outside,
   an attacker could try to directly send the faked header values
   through the proxy to the application server in order to circumvent
   security controls that way.  For example, it is standard practice of
   reverse proxies to accept X-Forwarded-For headers and just add the
   origin of the inbound request (making it a list).  Depending on the
   logic performed in the application server, the attacker could simply
   add an allowed IP address to the header and render the protection
   useless.

   A reverse proxy MUST therefore sanitize any inbound requests to
   ensure the authenticity and integrity of all header values relevant
   for the security of the application servers.

   If an attacker were able to get access to the internal network
   between the proxy and application server, the attacker could also try
   to circumvent security controls in place.  Therefore, it is essential
   to ensure the authenticity of the communicating entities.
   Furthermore, the communication link between the reverse proxy and
   application server MUST be protected against eavesdropping,
   injection, and replay of messages.

4.14.  Refresh Token Protection

   Refresh tokens are a convenient and user-friendly way to obtain new
   access tokens.  They also add to the security of OAuth, since they
   allow the authorization server to issue access tokens with a short
   lifetime and reduced scope, thus reducing the potential impact of
   access token leakage.

4.14.1.  Discussion

   Refresh tokens are an attractive target for attackers because they
   represent the full scope of access granted to a certain client, and
   they are not further constrained to a specific resource.  If an
   attacker is able to exfiltrate and successfully replay a refresh
   token, the attacker will be able to mint access tokens and use them
   to access resource servers on behalf of the resource owner.

   [RFC6749] already provides robust baseline protection by requiring

   *  confidentiality of the refresh tokens in transit and storage,
   *  the transmission of refresh tokens over TLS-protected connections
      between authorization server and client,
   *  the authorization server to maintain and check the binding of a
      refresh token to a certain client and authentication of this
      client during token refresh, if possible, and
   *  that refresh tokens cannot be generated, modified, or guessed.

   [RFC6749] also lays the foundation for further (implementation-
   specific) security measures, such as refresh token expiration and
   revocation as well as refresh token rotation by defining respective
   error codes and response behaviors.

   This specification gives recommendations beyond the scope of
   [RFC6749] and clarifications.

4.14.2.  Recommendations

   Authorization servers MUST determine, based on a risk assessment,
   whether to issue refresh tokens to a certain client.  If the
   authorization server decides not to issue refresh tokens, the client
   MAY obtain a new access token by utilizing other grant types, such as
   the authorization code grant type.  In such a case, the authorization
   server may utilize cookies and persistent grants to optimize the user
   experience.

   If refresh tokens are issued, those refresh tokens MUST be bound to
   the scope and resource servers as consented by the resource owner.
   This is to prevent privilege escalation by the legitimate client and
   reduce the impact of refresh token leakage.

   For confidential clients, [RFC6749] already requires that refresh
   tokens can only be used by the client for which they were issued.

   Authorization servers MUST utilize one of these methods to detect
   refresh token replay by malicious actors for public clients:

   *  *Sender-constrained refresh tokens:* the authorization server
      cryptographically binds the refresh token to a certain client
      instance, e.g., by utilizing [RFC8705] or [RFC9449].

   *  *Refresh token rotation:* the authorization server issues a new
      refresh token with every access token refresh response.  The
      previous refresh token is invalidated, but information about the
      relationship is retained by the authorization server.  If a
      refresh token is compromised and subsequently used by both the
      attacker and the legitimate client, one of them will present an
      invalidated refresh token, which will inform the authorization
      server of the breach.  The authorization server cannot determine
      which party submitted the invalid refresh token, but it will
      revoke the active refresh token.  This stops the attack at the
      cost of forcing the legitimate client to obtain a fresh
      authorization grant.

      Implementation note: The grant to which a refresh token belongs
      may be encoded into the refresh token itself.  This can enable an
      authorization server to efficiently determine the grant to which a
      refresh token belongs, and by extension, all refresh tokens that
      need to be revoked.  Authorization servers MUST ensure the
      integrity of the refresh token value in this case, for example,
      using signatures.

   Authorization servers MAY revoke refresh tokens automatically in case
   of a security event, such as:

   *  password change or
   *  logout at the authorization server.

   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
   access tokens for some time.  The expiration time is at the
   discretion of the authorization server.  It might be a global value
   or determined based on the client policy or the grant associated with
   the refresh token (and its sensitivity).

4.15.  Client Impersonating Resource Owner

   Resource servers may make access control decisions based on the
   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.
   For example, [RFC9068] (JSON Web Token (JWT) Profile for OAuth 2.0
   Access Tokens) describes a data structure for access tokens
   containing a sub claim defined as follows:

   |  In cases of access tokens obtained through grants where a resource
   |  owner is involved, such as the authorization code grant, the value
   |  of "sub" SHOULD correspond to the subject identifier of the
   |  resource owner.  In cases of access tokens obtained through grants
   |  where no resource owner is involved, such as the client
   |  credentials grant, the value of "sub" SHOULD correspond to an
   |  identifier the authorization server uses to indicate the client
   |  application.

   If both options are possible, a resource server may mistake a
   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
   with the authorization server, a malicious client may set it to a
   value identifying a resource owner (e.g., a sub value if OpenID
   Connect is used).  If the resource server cannot properly distinguish
   between access tokens obtained with involvement of the resource owner
   and those without, the client may accidentally be able to access
   resources belonging to the resource owner.

   This attack potentially affects not only implementations using
   [RFC9068], but also similar, bespoke solutions.

4.15.1.  Countermeasures

   Authorization servers SHOULD NOT allow clients to influence their
   client_id or any other claim that could cause confusion with a
   genuine resource owner if a common namespace for client IDs and user
   identifiers exists, such as in the sub claim example from [RFC9068]
   shown in Section 4.15 above.  Where this cannot be avoided,
   authorization servers MUST provide other means for the resource
   server to distinguish between the two types of access tokens.

4.16.  Clickjacking

   As described in Section 4.4.1.9 of [RFC6819], the authorization
   request is susceptible to clickjacking attacks, also called user
   interface redressing.  In such an attack, an attacker embeds the
   authorization endpoint user interface in an innocuous context.  A
   user believing to interact with that context, for example, by
   clicking on buttons, inadvertently interacts with the authorization
   endpoint user interface instead.  The opposite can be achieved as
   well: A user believing to interact with the authorization endpoint
   might inadvertently type a password into an attacker-provided input
   field overlaid over the original user interface.  Clickjacking
   attacks can be designed such that users can hardly notice the attack,
   for example, using almost invisible iframes overlaid on top of other
   elements.

   An attacker can use this vector to obtain the user's authentication
   credentials, change the scope of access granted to the client, and
   potentially access the user's resources.

   Authorization servers MUST prevent clickjacking attacks.  Multiple
   countermeasures are described in [RFC6819], including the use of the
   X-Frame-Options HTTP response header field and frame-busting
   JavaScript.  In addition to those, authorization servers SHOULD also
   use Content Security Policy (CSP) level 2 [W3C.CSP-2] or greater.

   To be effective, CSP must be used on the authorization endpoint and,
   if applicable, other endpoints used to authenticate the user and
   authorize the client (e.g., the device authorization endpoint, login
   pages, error pages, etc.).  This prevents framing by unauthorized
   origins in user agents that support CSP.  The client MAY permit being
   framed by some other origin than the one used in its redirection
   endpoint.  For this reason, authorization servers SHOULD allow
   administrators to configure allowed origins for particular clients
   and/or for clients to register these dynamically.

   Using CSP allows authorization servers to specify multiple origins in
   a single response header field and to constrain these using flexible
   patterns (see [W3C.CSP-2] for details).  Level 2 of CSP provides a
   robust mechanism for protecting against clickjacking by using
   policies that restrict the origin of frames (by using frame-
   ancestors) together with those that restrict the sources of scripts
   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:

   HTTP/1.1 200 OK
   Content-Security-Policy: frame-ancestors https://ext.example.org:8000
   Content-Security-Policy: script-src 'self'
   X-Frame-Options: ALLOW-FROM https://ext.example.org:8000
   ...

   Because some user agents do not support [W3C.CSP-2], this technique
   SHOULD be combined with others, including those described in
   [RFC6819], unless such legacy user agents are explicitly unsupported
   by the authorization server.  Even in such cases, additional
   countermeasures SHOULD still be employed.

4.17.  Attacks on In-Browser Communication Flows

   If the authorization response is sent with in-browser communication
   techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
   redirects, messages may inadvertently be sent to malicious origins or
   injected from malicious origins.

4.17.1.  Examples

   The following non-normative pseudocode examples of attacks using in-
   browser communication are described in [research.rub].

4.17.1.1.  Insufficient Limitation of Receiver Origins

   When sending the authorization response or token response via
   postMessage, the authorization server sends the response to the
   wildcard origin "*" instead of the client's origin.  When the window
   to which the response is sent is controlled by an attacker, the
   attacker can read the response.

   window.opener.postMessage(
     {
       code: "ABC",
       state: "123"
     },
     "*" // any website in the opener window can receive the message
   )

4.17.1.2.  Insufficient URI Validation

   When sending the authorization response or token response via
   postMessage, the authorization server may not check the receiver
   origin against the redirection URI and instead, for example, may send
   the response to an origin provided by an attacker.  This is analogous
   to the attack described in Section 4.1.

   window.opener.postMessage(
     {
       code: "ABC",
       state: "123"
     },
     "https://attacker.example" // attacker-provided value
   )

4.17.1.3.  Injection after Insufficient Validation of Sender Origin

   A client that expects the authorization response or token response
   via postMessage may not validate the sender origin of the message.
   This may allow an attacker to inject an authorization response or
   token response into the client.

   In the case of a maliciously injected authorization response, the
   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
   well.

   In the case of a maliciously injected token response, sender-
   constrained access tokens as described in Section 4.10.1 may prevent
   the attack under some circumstances, but additional countermeasures
   as described in Section 4.17.2 are generally required.

4.17.2.  Recommendations

   When comparing client receiver origins against pre-registered
   origins, authorization servers MUST utilize exact string matching as
   described in Section 4.1.3.  Authorization servers MUST send
   postMessages to trusted client receiver origins, as shown in the
   following, non-normative example:

   window.opener.postMessage(
     {
       code: "ABC",
       state: "123"
     },
     "https://client.example" // use explicit client origin
   )

   Wildcard origins like "*" in postMessage MUST NOT be used, as
   attackers can use them to leak a victim's in-browser message to
   malicious origins.  Both measures contribute to the prevention of
   leakage of authorization codes and access tokens (see Section 4.1).

   Clients MUST prevent injection of in-browser messages on the client
   receiver endpoint.  Clients MUST utilize exact string matching to
   compare the initiator origin of an in-browser message with the
   authorization server origin, as shown in the following, non-normative
   example:

   window.addEventListener("message", (e) => {
     // validate exact authorization server origin
     if (e.origin === "https://honest.as.example") {
       // process e.data.code and e.data.state
     }
   })

   Since in-browser communication flows only apply a different
   communication technique (i.e., postMessage instead of HTTP redirect),
   all measures protecting the authorization response listed in
   Section 2.1 MUST be applied equally.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   Security considerations are described in Sections 2, 3, and 4.

7.  References

7.1.  Normative References

   [BCP195]   Best Current Practice 195,
              <https://www.rfc-editor.org/info/bcp195>.
              At the time of writing, this BCP comprises the following:

              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>.

              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
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

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

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,
              <https://www.rfc-editor.org/info/rfc6819>.

   [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/info/rfc7521>.

   [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/info/rfc7523>.

   [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/info/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/info/rfc8414>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

   [RFC9068]  Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
              Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
              2021, <https://www.rfc-editor.org/info/rfc9068>.

7.2.  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,
              DOI 10.48550/arXiv.1508.04324, 7 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,
              DOI 10.48550/arXiv.1601.01229, 6 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,
              DOI 10.48550/arXiv.1704.08539, 27 April 2017,
              <https://arxiv.org/abs/1704.08539/>.

   [arXiv.1901.11520]
              Fett, D., Hosseyni, P., and R. Küsters, "An Extensive
              Formal Security Analysis of the OpenID Financial-grade
              API", arXiv:1901.11520, DOI 10.48550/arXiv.1901.11520, 31
              January 2019, <https://arxiv.org/abs/1901.11520/>.

   [bug.chromium]
              "Referer header includes URL fragment when opening link
              using New Tab", Chromium Issue Tracker, Issue ID:
              40076763, <https://issues.chromium.org/issues/40076763>.

   [JWT-ENCODED-STATE]
              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", Work
              in Progress, Internet-Draft, draft-bradley-oauth-jwt-
              encoded-state-09, 4 November 2018,
              <https://datatracker.ietf.org/doc/html/draft-bradley-
              oauth-jwt-encoded-state-09>.

   [OAUTH-V2.1]
              Hardt, D., Parecki, A., and T. Lodderstedt, "The OAuth 2.1
              Authorization Framework", Work in Progress, Internet-
              Draft, draft-ietf-oauth-v2-1-12, 15 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              v2-1-12>.

   [OAuth.Post]
              Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
              Mode", The OpenID Foundation, 27 April 2015,
              <https://openid.net/specs/oauth-v2-form-post-response-
              mode-1_0.html>.

   [OAuth.Responses]
              de Medeiros, B., Ed., Scurtescu, M., Tarjan, P., and M.
              Jones, "OAuth 2.0 Multiple Response Type Encoding
              Practices", The OpenID Foundation, 25 February 2014,
              <https://openid.net/specs/oauth-v2-multiple-response-
              types-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", The OpenID Foundation, 15 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", The
              OpenID Foundation, 15 December 2023,
              <https://openid.net/specs/openid-connect-discovery-
              1_0.html>.

   [OpenID.JARM]
              Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
              Secured Authorization Response Mode for OAuth 2.0 (JARM)",
              The OpenID Foundation, 17 October 2018,
              <https://openid.net/specs/openid-financial-api-jarm.html>.

   [owasp.redir]
              OWASP Foundation, "Unvalidated Redirects and Forwards
              Cheat Sheet", OWASP Cheat Sheet Series,
              <https://cheatsheetseries.owasp.org/cheatsheets/
              Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.

   [research.cmu]
              Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
              Tague, "OAuth Demystified for Mobile Application
              Developers", CCS '14: Proceedings of the 2014 ACM SIGSAC
              Conference on Computer and Communications Security, pp.
              892-903, DOI 10.1145/2660267.2660323, November 2014,
              <https://www.microsoft.com/en-us/research/wp-
              content/uploads/2016/02/OAuthDemystified.pdf>.

   [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, DOI 10.3233/JCS-
              140503, 23 April 2014,
              <https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.

   [research.rub]
              Jannett, L., Mladenov, V., Mainka, C., and J. Schwenk,
              "DISTINCT: Identity Theft using In-Browser Communications
              in Dual-Window Single Sign-On", CCS '22: Proceedings of
              the 2022 ACM SIGSAC Conference on Computer and
              Communications Security, DOI 10.1145/3548606.3560692, 7
              November 2022,
              <https://dl.acm.org/doi/pdf/10.1145/3548606.3560692>.

   [research.rub2]
              Fries, C., "Security Analysis of Real-Life OpenID Connect
              Implementations", Master's thesis, Ruhr-Universität Bochum
              (RUB), 20 December 2020,
              <https://www.nds.rub.de/media/ei/arbeiten/2021/05/03/
              masterthesis.pdf>.

   [research.ubc]
              Sun, S.-T. and K. Beznosov, "The Devil is in the
              (Implementation) Details: An Empirical Analysis of OAuth
              SSO Systems", Proceedings of the 2012 ACM conference on
              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]
              Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
              to Us: Understanding the Security Threats of Dangling DNS
              Records", CCS '16: Proceedings of the 2016 ACM SIGSAC
              Conference on Computer and Communications Security, pp.
              1414-1425, DOI 10.1145/2976749.2978387, 24 October 2016,
              <https://dl.acm.org/doi/pdf/10.1145/2976749.2978387>.

   [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/info/rfc2119>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/info/rfc7591>.

   [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/info/rfc7636>.

   [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/info/rfc8174>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC9101]  Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
              Authorization Framework: JWT-Secured Authorization Request
              (JAR)", RFC 9101, DOI 10.17487/RFC9101, August 2021,
              <https://www.rfc-editor.org/info/rfc9101>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [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/info/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/info/rfc9207>.

   [RFC9396]  Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", RFC 9396,
              DOI 10.17487/RFC9396, May 2023,
              <https://www.rfc-editor.org/info/rfc9396>.

   [RFC9440]  Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
              <https://www.rfc-editor.org/info/rfc9440>.

   [RFC9449]  Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
              Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
              September 2023, <https://www.rfc-editor.org/info/rfc9449>.

   [TOKEN-BINDING]
              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", Work in Progress, Internet-
              Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              token-binding-08>.

   [W3C.CSP-2]
              West, M., Barth, A., and D. Veditz, "Content Security
              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.webappsec-referrer-policy]
              Eisinger, J. and E. Stark, "Referrer Policy", 26 January
              2017,
              <https://www.w3.org/TR/2017/CR-referrer-policy-20170126/>.
              Latest version available at
              <https://www.w3.org/TR/referrer-policy/>.

   [W3C.WebAuthn]
              Hodges, J., Jones, J.C., Jones, M.B., Kumar, A., and E.
              Lundberg, "Web Authentication: An API for accessing Public
              Key Credentials Level 2", W3C Recommendation, 8 April
              2021,
              <https://www.w3.org/TR/2021/REC-webauthn-2-20210408/>.
              Latest version available at
              <https://www.w3.org/TR/webauthn-2/>.

   [W3C.WebCrypto]
              Watson, M., Ed., "Web Cryptography API", W3C
              Recommendation, 26 January 2017,
              <https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>.
              Latest version available at
              <https://www.w3.org/TR/WebCryptoAPI/>.

   [WHATWG.CORS]
              WHATWG, "CORS protocol", Fetch: Living Standard,
              Section 3.2, 17 June 2024,
              <https://fetch.spec.whatwg.org/#http-cors-protocol>.

   [WHATWG.postmessage_api]
              WHATWG, "Cross-document messaging", HTML: Living Standard,
              Section 9.3, 19 August 2024,
              <https://html.spec.whatwg.org/multipage/web-
              messaging.html#web-messaging>.

Acknowledgements

   We would like to thank Brock Allen, Annabelle Richard Backman,
   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,
   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

   Torsten Lodderstedt
   SPRIND
   Email: torsten@lodderstedt.net


   John Bradley
   Yubico
   Email: ve7jtb@ve7jtb.com


   Andrey Labunets
   Independent Researcher
   Email: isciurus@gmail.com


   Daniel Fett
   Authlete
   Email: mail@danielfett.de
