Web Authorization Protocol                                T. Lodderstedt
Internet-Draft                                                   yes.com
Intended status: Best Current Practice                        J. Bradley
Expires: 29 January 2023                                          Yubico
                                                             A. Labunets
                                                  Independent Researcher
                                                                 D. Fett
                                                            28 July 2022

                OAuth 2.0 Security Best Current Practice


   This document describes best current security practice for OAuth 2.0.
   It updates and extends the OAuth 2.0 Security Threat Model 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 29 January 2023.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Conventions and Terminology . . . . . . . . . . . . . . .   5
   2.  Best Practices  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Protecting Redirect-Based Flows . . . . . . . . . . . . .   5
       2.1.1.  Authorization Code Grant  . . . . . . . . . . . . . .   6
       2.1.2.  Implicit Grant  . . . . . . . . . . . . . . . . . . .   7
     2.2.  Token Replay Prevention . . . . . . . . . . . . . . . . .   8
       2.2.1.  Access Tokens . . . . . . . . . . . . . . . . . . . .   8
       2.2.2.  Refresh Tokens  . . . . . . . . . . . . . . . . . . .   8
     2.3.  Access Token Privilege Restriction  . . . . . . . . . . .   8
     2.4.  Resource Owner Password Credentials Grant . . . . . . . .   9
     2.5.  Client Authentication . . . . . . . . . . . . . . . . . .   9
     2.6.  Other Recommendations . . . . . . . . . . . . . . . . . .   9
   3.  The Updated OAuth 2.0 Attacker Model  . . . . . . . . . . . .  10
   4.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .  12
     4.1.  Insufficient Redirect URI Validation  . . . . . . . . . .  12
       4.1.1.  Redirect URI Validation Attacks on Authorization Code
               Grant . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.1.2.  Redirect URI Validation Attacks on Implicit Grant . .  14
       4.1.3.  Countermeasures . . . . . . . . . . . . . . . . . . .  16
     4.2.  Credential Leakage via Referer Headers  . . . . . . . . .  16
       4.2.1.  Leakage from the OAuth Client . . . . . . . . . . . .  17
       4.2.2.  Leakage from the Authorization Server . . . . . . . .  17
       4.2.3.  Consequences  . . . . . . . . . . . . . . . . . . . .  17
       4.2.4.  Countermeasures . . . . . . . . . . . . . . . . . . .  17
     4.3.  Credential Leakage via Browser History  . . . . . . . . .  18
       4.3.1.  Authorization Code in Browser History . . . . . . . .  19
       4.3.2.  Access Token in Browser History . . . . . . . . . . .  19
     4.4.  Mix-Up Attacks  . . . . . . . . . . . . . . . . . . . . .  19
       4.4.1.  Attack Description  . . . . . . . . . . . . . . . . .  20
       4.4.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  22
     4.5.  Authorization Code Injection  . . . . . . . . . . . . . .  23
       4.5.1.  Attack Description  . . . . . . . . . . . . . . . . .  24
       4.5.2.  Discussion  . . . . . . . . . . . . . . . . . . . . .  25
       4.5.3.  Countermeasures . . . . . . . . . . . . . . . . . . .  26
       4.5.4.  Limitations . . . . . . . . . . . . . . . . . . . . .  28
     4.6.  Access Token Injection  . . . . . . . . . . . . . . . . .  28
       4.6.1.  Countermeasures . . . . . . . . . . . . . . . . . . .  28
     4.7.  Cross Site Request Forgery  . . . . . . . . . . . . . . .  29
       4.7.1.  Countermeasures . . . . . . . . . . . . . . . . . . .  29

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     4.8.  PKCE Downgrade Attack . . . . . . . . . . . . . . . . . .  30
       4.8.1.  Attack Description  . . . . . . . . . . . . . . . . .  30
       4.8.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  31
     4.9.  Access Token Leakage at the Resource Server . . . . . . .  32
       4.9.1.  Access Token Phishing by Counterfeit Resource
               Server  . . . . . . . . . . . . . . . . . . . . . . .  32
       4.9.2.  Compromised Resource Server . . . . . . . . . . . . .  37
     4.10. Open Redirection  . . . . . . . . . . . . . . . . . . . .  38
       4.10.1.  Client as Open Redirector  . . . . . . . . . . . . .  38
       4.10.2.  Authorization Server as Open Redirector  . . . . . .  38
     4.11. 307 Redirect  . . . . . . . . . . . . . . . . . . . . . .  39
     4.12. TLS Terminating Reverse Proxies . . . . . . . . . . . . .  40
     4.13. Refresh Token Protection  . . . . . . . . . . . . . . . .  41
       4.13.1.  Discussion . . . . . . . . . . . . . . . . . . . . .  41
       4.13.2.  Recommendations  . . . . . . . . . . . . . . . . . .  41
     4.14. Client Impersonating Resource Owner . . . . . . . . . . .  43
       4.14.1.  Countermeasures  . . . . . . . . . . . . . . . . . .  43
     4.15. Clickjacking  . . . . . . . . . . . . . . . . . . . . . .  43
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  45
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  45
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  45
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  46
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

1.  Introduction

   Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 ("OAuth"
   in the following) has gotten 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.  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.

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   *  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 client,
      authorization server, and resource servers.  The URLs of the AS
      and RS 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 talks to a legitimate server was
      based on TLS server authentication (see [RFC6819], Section 4.5.4).
      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
      e-mail 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 does not supplant the security advice given in
   [RFC6749], [RFC6750], and [RFC6819], but complements those documents.

   This document 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.  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 when

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1.1.  Structure

   The remainder of this document is organized as follows: The next
   section summarizes the most important best practices for every OAuth
   implementor.  Afterwards, the updated the OAuth attacker model is
   presented.  Subsequently, a detailed analysis of the threats and
   implementation issues that can be found in the wild today is given
   along with a discussion of potential countermeasures.

1.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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].

2.  Best Practices

   This section describes the set of security mechanisms and measures
   the OAuth working group considers best practices at the time of

2.1.  Protecting Redirect-Based Flows

   When comparing client redirect 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 AS MUST NOT expose URLs that forward the user's browser
   to arbitrary URIs obtained from a query parameter ("open
   redirector").  Open redirectors can enable exfiltration of
   authorization codes and access tokens, see Section 4.10.1.

   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 a malicious third
   party (see Section of [RFC6819] for details).  Clients that
   have ensured that the authorization server supports PKCE [RFC7636]

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   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],

   *  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 [JARM] responses), processing it as described
      in [RFC9207].

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

   An AS that redirects a request potentially containing user
   credentials MUST avoid forwarding these user credentials accidentally
   (see Section 4.11 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

   *  For confidential clients, the use of PKCE [RFC7636] is
      RECOMMENDED, as it provides a strong protection against misuse and
      injection of authorization codes as described in Section
      and, as a side-effect, prevents CSRF even in presence of strong
      attackers as described in Section 4.7.1.

   *  With additional precautions, described in Section,
      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.

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   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 [RFC7636] 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 AS to publish the element
   code_challenge_methods_supported in their AS metadata ([RFC8414])
   containing the supported PKCE challenge methods (which can be used by
   the client to detect PKCE support).  ASs MAY instead provide a
   deployment-specific way to ensure or determine PKCE support by the

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 Section 4.1, Section 4.2,
   Section 4.3, and Section 4.6.

   Moreover, no viable 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 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.

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   Clients SHOULD instead use the response type "code" (aka
   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 next

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 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 Demonstration of Proof of Possession (DPoP)
   [I-D.ietf-oauth-dpop] (see Section, 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 4.13.  [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 restricted to certain resource
   servers (audience restriction), preferably to a single resource
   server.  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 not, the resource server MUST refuse to serve

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   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 [I-D.ietf-oauth-rar] to
   determine those resources and/or actions.

2.4.  Resource Owner Password Credentials Grant

   The resource owner password credentials grant MUST NOT be used.  This
   grant type insecurely exposes the credentials of the resource owner
   to the client.  Even if the client is benign, this results in an
   increased attack surface (credentials can leak in more places than
   just the AS) and users are trained to enter their credentials in
   places other than the AS.

   Furthermore, adapting the resource owner password credentials grant
   to two-factor authentication, authentication with cryptographic
   credentials (cf.  WebCrypto [WebCrypto], WebAuthn [WebAuthn]), and
   authentication processes that require multiple steps can be hard or

2.5.  Client Authentication

   Authorization servers SHOULD use client authentication if possible.

   It is RECOMMENDED to use asymmetric (public-key based) methods for
   client authentication such as mTLS [RFC8705] or private_key_jwt
   [OpenID.Core].  When asymmetric methods for client authentication are
   used, authorization servers do not need to store sensitive symmetric
   keys, making these methods more robust against a number of attacks.

2.6.  Other Recommendations

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

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   *  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 ASs publish OAuth metadata according
   to [RFC8414] and that clients make use of this metadata to configure
   themselves when available.

   Authorization servers SHOULD NOT allow clients to influence their
   client_id or any other Claim if that can cause confusion with a
   genuine resource owner, as described in Section 4.14

   It is RECOMMENDED to use end-to-end TLS.  If TLS traffic needs to be
   terminated at an intermediary, refer to Section 4.12 for further
   security advice.

   Authorization responses MUST NOT be transmitted over unencrypted
   network connections.  To this end, AS MUST NOT allow redirect URIs
   that use the http scheme except for native clients that use Loopback
   Interface Redirection as described in [RFC8252], Section 7.3.

3.  The Updated OAuth 2.0 Attacker Model

   In [RFC6819], an attacker model is laid out that describes the
   capabilities of attackers against which OAuth deployments must be
   protected.  In the following, this attacker model is updated 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.

   OAuth MUST ensure that the authorization of the resource owner (RO)
   (with a user agent) at the authorization server (AS) and the
   subsequent usage of the access token at the resource server (RS) is
   protected at least against the following attackers:

   *  (A1) Web Attackers that can set up and operate an arbitrary number
      of network endpoints including browsers and servers (except for
      the concrete RO, AS, and RS).  Web attackers may set up web sites
      that are visited by the RO, operate their own user agents, and
      participate in the protocol.

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      Web attackers may, in particular, operate OAuth clients that are
      registered at AS, and operate their own authorization and resource
      servers that can be used (in parallel) by the RO and other
      resource owners.

      It must also be assumed that web attackers can lure the user to
      open 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 redirect 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 controlled by a non-
      attacker controlled AS).

   *  (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 (wifi) network using
   ARP spoofing, or a state-sponsored attacker with access to internet
   exchange points, for instance.

   These attackers 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 in which their OAuth
   implementations are expected to run.  Previous attacks on OAuth have
   shown that OAuth deployments SHOULD in particular consider the
   following, stronger attackers in addition to those listed above:

   *  (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 for such attacks include open redirector attacks,
      insufficient checking of redirect URIs (see Section 4.1), problems
      existing on mobile operating systems (where different apps can

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      register themselves on the same URI), mix-up attacks (see
      Section 4.4), where the client is tricked into sending credentials
      to a attacker-controlled AS, and the fact that URLs are often
      stored/logged by browsers (history), proxy servers, and operating

   *  (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 AS.  For
      example, a resource server can be compromised by an attacker, an
      access token may be sent to an attacker-controlled resource server
      due to a misconfiguration, or an RO is social-engineered into
      using a attacker-controlled RS.  See also 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 in this attacker model, an attacker (see A1) can be a RO or
   act as one.  For example, an attacker can use his own browser to
   replay tokens or authorization codes obtained by any of the attacks
   described above at the client or RS.

   This document focusses on threats resulting from these attackers.
   Attacks in an even stronger attacker model are discussed, for
   example, in [arXiv.1901.11520].

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.

4.1.  Insufficient Redirect URI Validation

   Some authorization servers allow clients to register redirect URI
   patterns instead of complete redirect URIs.  The authorization
   servers then match the redirect 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 redirect URIs.

   This approach turned out to be more complex to implement and more
   error prone to manage than exact redirect URI matching.  Several
   successful attacks exploiting flaws in the pattern matching

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   implementation or concrete configurations have been observed in the
   wild . Insufficient validation of the redirect 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 attackers
      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

   Assume the redirect 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
   redirect URI for the client, for example
   https://app1.somesite.example/redirect.  A naive implementation on
   the authorization server, however, 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 redirect URI,
   although attacker.example is a different domain potentially
   controlled by a malicious party.

   The attack can then be conducted as follows:

   First, the attacker needs to trick the user into opening a tampered
   URL in his 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
   Host: server.somesite.example

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

   If the user does not see the redirect 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 impersonated 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.  The attacker can, however, use the legitimate
   confidential client to redeem the code by performing an authorization
   code injection attack, see Section 4.5.

   Note: Vulnerabilities of this kind can also exist if the
   authorization server handles wildcards properly.  For example, assume
   that the client registers the redirect URL pattern
   https://*.somesite.example/* and the authorization server interprets
   this as "allow redirect URIs pointing to any host residing in the
   domain somesite.example".  If an attacker manages to establish a host
   or subdomain in somesite.example, he can impersonate the legitimate
   client.  This could be caused, for example, by a subdomain takeover
   attack [subdomaintakeover], where an outdated CNAME record (say,
   external-service.somesite.example) points to an external DNS name
   that does no longer exist (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 a URI
   under his control, he will directly get access to the fragment
   carrying the access token.

   Additionally, implicit clients can be subject to a further kind of
   attack.  It utilizes the fact that user agents re-attach fragments to
   the destination URL of a redirect if the location header does not
   contain a fragment (see [RFC7231], Section 9.5).  The attack
   described here combines this behavior with the client as an open
   redirector (see Section 4.10.1) in order to get access to access
   tokens.  This allows circumvention even of very narrow redirect URI
   patterns, but not strict URL matching.

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   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:

   First, and as above, the attacker needs to trick the user into
   opening a tampered URL in his 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
   redirect URI and it uses the response type "token" (line breaks for
   display only):

   GET /authorize?response_type=token&state=9ad67f13
        %252Fattacker.example%252F HTTP/1.1
   Host: server.somesite.example

   Now, since the redirect 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

   HTTP/1.1 303 See Other
   Location: https://client.somesite.example/cb?

   At example.com, 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 re-attach the
   original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the
   URL and will navigate to the following URL:

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   The attacker's page at attacker.example can now 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 to simplify the required logic and configuration by using
   exact redirect URI matching.  This means the authorization server
   MUST compare the two URIs using simple string comparison as defined
   in [RFC3986], Section 6.2.1.  The only exception are native apps
   using a localhost URI: In this case, the AS MUST allow variable port
   numbers as described in [RFC8252], Section 7.3.

   Additional recommendations:

   *  Servers on which callbacks are hosted MUST NOT expose open
      redirectors (see Section 4.10).

   *  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

   *  Clients SHOULD use the authorization code response type instead of
      response types causing access token issuance at the authorization
      endpoint.  This offers countermeasures against reuse of leaked
      credentials through the exchange process with the authorization
      server and token replay through sender-constraining of the access

   If the origin and integrity of the authorization request containing
   the redirect URI can be verified, for example when using [RFC9101] or
   [RFC9126] with client authentication, the authorization server MAY
   trust the redirect 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 [RFC7231], Section 5.5.2), by leaking
   either from the AS's or the client's web site, respectively.  Most
   importantly, authorization codes or state values can be disclosed in
   this way.  Although specified otherwise in [RFC7231], Section 5.5.2,

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   the same may happen to access tokens conveyed in URI fragments due to
   browser implementation issues, as illustrated by Chromium Issue
   168213 [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

   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 Section 4.1.1,
   Section 4.5, and Section 4.6.  If the attacker learns state, the CSRF
   protection achieved by using state is lost, resulting in CSRF attacks
   as described in [RFC6819], Section

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

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   *  Suppress the Referer header by applying an appropriate Referrer
      Policy [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 [RFC6749], Section 4.1.2, authorization codes MUST
      be invalidated by the AS after their first use at the token
      endpoint.  For example, if an AS invalidated the code after the
      legitimate client redeemed it, the attacker would fail exchanging
      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 AS 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 web site, 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 AS's web site, since then the
      state has not been used at the redirection endpoint at the client

   *  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

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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.


   *  Authorization code replay prevention as described in [RFC6819],
      Section, and Section 4.5.

   *  Use 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
   web site 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 to transfer tokens via a
   header, but in practice web sites often pass access tokens in query

   In case of the 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.


   *  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 is an attack on 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 his own authorization server or if an authorization
   server becomes compromised.

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   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, we assume that

   *  the implicit or authorization code grant are used with multiple AS
      of which one is considered "honest" (H-AS) and one is operated by
      the attacker (A-AS), and

   *  the client stores the AS chosen by the user in a session bound to
      the user's browser and uses the same redirection endpoint URI for
      each AS.

   In the following, we assume 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 only include
   parameters relevant for 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 at 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

   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

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   4.  The user authorizes the client to access her resources at H-AS.
       (Note that a vigilant user might at this point detect that she
       intended to use A-AS instead of H-AS.  The first attack variant
       listed below avoids this.)  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.


   *  *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 AS and is then redirected by the client to that
      AS), as in Attacker A2 (see Section 3).  This capability can, for
      example, be the result of a man-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
      Steps 2 and following above.

   *  *Implicit Grant*: In the implicit grant, the attacker receives an
      access token instead of the code; the rest of the attack works as

   *  *Per-AS Redirect URIs*: If clients use different redirect URIs for
      different ASs, do not store the selected AS in the user's session,
      and ASs do not check the redirect URIs properly, attackers can
      mount an attack called "Cross-Social Network Request Forgery".
      These attacks have been observed in practice.  Refer to
      [oauth_security_jcs_14] for details.

   *  *OpenID Connect*: There are variants that 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 [arXiv.1704.08539],
      Appendix A, and [arXiv.1508.04324v2], Section 6 ("Malicious
      Endpoints Attacks").

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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 in
   the following.

   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 metadata [RFC8414] nor OpenID Connect Discovery
   [OpenID.Discovery] are 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.

   Note: Just storing the authorization server URL is not sufficient to
   identify mix-up attacks.  An attacker might declare an uncompromised
   AS's authorization endpoint URL as "his" AS URL, but declare a token
   endpoint under his own control.  Mix-Up Defense via Issuer Identification

   This defense requires that the authorization server sends his 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

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   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.  Mix-Up Defense via Distinct Redirect URIs

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

   Clients MUST check that the authorization response was received from
   the correct issuer by comparing the distinct redirect 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"
   AS using the redirect URI that the client assigned to the attacker's
   AS.  The attacker could then run the attack as described above,
   replacing the client ID with the client ID of his newly created

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

4.5.  Authorization Code Injection

   An attacker that 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 of [RFC6819].

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

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   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 victum'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 his
      victim in a certain app or on a certain web site.

   *  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 was 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 own 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

   5.  The authorization server checks the client secret, whether the
       code was issued to the particular client, and whether the actual
       redirect URI matches the redirect_uri parameter (see [RFC6749]).

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   6.  All checks succeed and the authorization server issues access and
       other tokens to the client.  The attacker has now associated his
       session with the legitimate client with the victim's resources
       and/or identity.

4.5.2.  Discussion

   Obviously, the check in 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 redirect URI
   should also be detected if the authorization server stored the
   complete redirect URI used in the authorization request and compares
   it with the redirect_uri parameter.

   [RFC6749], Section 4.1.3, requires the AS 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 above, the legitimate
   client would use the correct redirect URI it always uses for
   authorization requests.  But this URI would not match the tampered
   redirect URI used by the attacker (otherwise, the redirect would not
   land at the attackers page).  So the authorization server would
   detect the attack and refuse to exchange the code.

   Note: 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

   *  the redirect URI itself needs to contain a nonce or another kind
      of one-time use, secret data and

   *  the client has bound this data to this particular instance of the

   But this approach conflicts with the idea to enforce 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 redirect URI pattern.  This saves the authorization
   server from storing the link between the actual redirect URI and the

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   respective authorization code for every transaction.  But this kind
   of check obviously does not fulfill the intent of the specification,
   since the tampered redirect 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 [RFC6749],
   Section 4.1.3, increase client implementation complexity as clients
   need to store or re-construct the correct redirect 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 to instead bind 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 achieve this goal, outlined
   in the following.  PKCE

   The PKCE mechanism specified in [RFC7636] can be used as a
   countermeasure.  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 is a deployed
   OAuth feature, although its originally intended use was solely
   focused on securing native apps, not the broader use recommended by
   this document.

   PKCE does not only protect against the autorization 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.

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Internet-Draft            oauth-security-topics                July 2022  Nonce

   OpenID Connect's existing nonce parameter can protect against
   authorization code injection attacks.  The nonce value is one-time
   use and created by the client.  The client is supposed to bind it to
   the user agent session and sends 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 injected an authorization code in the
   authorization response, the nonce value in the client session and the
   nonce value in the ID token 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, where 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 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.  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

   PKCE is the most obvious solution for OAuth clients as it is
   available today (originally intended for OAuth native apps) whereas
   nonce is appropriate for OpenID Connect clients.

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4.5.4.  Limitations

   An attacker can circumvent the countermeasures described above if he
   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 his own session
   in Step 2 of the attack above.  (This requires that the victim's
   session with the client begins after the attacker started his 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 Section 4.1, Section 4.2,
   Section 4.3, Section 4.4, and Section 4.10.

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 makes 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.

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   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 (with a
   very high probability) contain 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 redirect 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 traditional countermeasure is that clients pass a value in the
   state parameter that links the request to the redirect URI to the
   user agent session as described in detail in [RFC6819],
   Section 5.3.5.  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 AS 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 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 signed/encrypted
      state values as discussed in the now-expired draft

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   The AS therefore MUST provide a way to detect their support for PKCE.
   Using AS metadata according to [RFC8414] is RECOMMENDED, but AS MAY
   instead provide a deployment-specific way to ensure or determine PKCE

   PKCE provides robust protection against CSRF attacks even in presence
   of an 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

   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
   AS enables and enforces PKCE if this parameter is present in the
   authorization request, but does not enforce PKCE if the parameter is

   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 his victim and the client.

4.8.1.  Attack Description

   1.  The user has started an OAuth session using some client at an AS.
       In the authorization request, the client has set the parameter
       code_challenge=sha256(abc) as the PKCE code challenge.  The
       client is now waiting to receive the authorization response from
       the user's browse.

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   2.  To conduct the attack, the attacker uses his own device to start
       an authorization flow with the targeted client.  The client now
       uses another PKCE code challenge, say code_challenge=sha256(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 AS.

   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
       AS.  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 that 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, ASs MUST take precautions against this threat.

   Note that from the view of the AS, 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 AS 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

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   *  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

   Beyond this, to prevent PKCE downgrade attacks, the AS 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.

   Note: ASs 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

4.9.1.  Access Token Phishing by Counterfeit Resource Server

   An attacker may setup his 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 e-Mail, calendar, health,
   or banking) and where the client is configured by a user or
   administrator for a service that this user or company uses.  Countermeasures

   There are several potential mitigation strategies, which will be
   discussed in the following sections.  Metadata

   An authorization server could provide the client with additional
   information about the locations where it is safe to use its access

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

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


   The AS 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


   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 [oauth_security_ubc] and [oauth_security_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 entities.  Clearly, the
   client has to contribute to the overall security.  But there are
   alternative countermeasures, as described in the next sections, that
   provide a better balance between the involved parties.

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   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 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 identity, but in most cases the AS
       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 AS creates the binding or the
       AS creates ephemeral keys.  The way pre-existing 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

   3.  The RS must implement the actual proof of possession check.  This
       is typically done on the application level, often tied to
       specific material provided by transport layer (e.g., TLS).  The
       RS must also ensure that replay of the proof of possession is not

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

   *  *OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
      Access Tokens* ([RFC8705]): The approach as specified in this
      document allows the use of mutual TLS (mTLS) 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 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.

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   *  *DPoP* ([I-D.ietf-oauth-dpop]): DPoP (Demonstration of Proof-of-
      Possession at the Application Layer) outlines an application-level
      sender-constraining for access and refresh tokens that can be used
      in cases where neither mTLS nor OAuth Token Binding (see below)
      are available.  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 case of confidential clients, can be
      combined with any client authentication method.

   For reference, other approaches have been discussed as well but the
   relevant drafts are now expired:

   *  *OAuth Token Binding* ([I-D.ietf-oauth-token-binding]): In this
      approach, an access token is, via the token binding ID, bound to
      key material representing a long term association between a client
      and a certain TLS host.  Negotiation of the key material and proof
      of possession in the context of a TLS handshake is taken care of
      by the TLS stack.  The client needs to determine the token binding
      ID of the target resource server and pass this data to the access
      token request.  The authorization server then associates the
      access token with this ID.  The resource server checks on every
      invocation that the token binding ID of the active TLS connection
      and the token binding ID of associated with the access token
      match.  Since all crypto-related functions are covered by the TLS
      stack, this approach is very client developer friendly.  As a
      prerequisite, token binding as described in [RFC8473] (including
      federated token bindings) must be supported on all ends (client,
      authorization server, resource server).

   *  *Signed HTTP Requests* ([I-D.ietf-oauth-signed-http-request]):
      This approach utilizes [I-D.ietf-oauth-pop-key-distribution] and
      represents the elements of the signature in a JSON object.  The
      signature is built using JWS.  The mechanism has built-in support
      for signing of HTTP method, query parameters and headers.  It also
      incorporates a timestamp as basis for replay prevention.

   *  *JWT Pop Tokens* ([I-D.sakimura-oauth-jpop]): This draft describes
      different ways to constrain access token usage, namely TLS or
      request signing.  Note: Since the authors of this draft
      contributed the TLS-related proposal to [RFC8705], this document
      only considers the request signing part.  For request signing, the
      draft utilizes [I-D.ietf-oauth-pop-key-distribution] and
      [RFC7800].  The signature data is represented in a JWT and JWS is
      used for signing.  Replay prevention is provided by building the
      signature over a server-provided nonce, client-provided nonce and
      a nonce counter.

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   At the time of writing, OAuth Mutual TLS is the most widely
   implemented and the only standardized sender-constraining 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 a 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 4.13).  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 SHOULD verify the intended audience.  If the access token
   fails the intended audience validation, the resource server MUST
   refuse to serve the respective request.

   In general, audience restrictions limit 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.

   The client SHOULD tell the authorization server the intended resource
   server.  The proposed mechanism [RFC8707] could be used or by
   encoding the information in the scope value.

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   Instead of the URL, it is also possible to utilize the fingerprint of
   the resource server's X.509 certificate as audience value.  This
   variant would also allow to detect 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 crypto on the client side.  Still, since every access token is
   bound to a specific resource server, the client also needs to obtain
   a single RS-specific access token when accessing several resource
   servers.  (Resource indicators, as specified in [RFC8707], can help
   to achieve this.)  [I-D.ietf-oauth-token-binding] has the same
   property since different token binding IDs must be associated with
   the access token.  Using [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, has additional benefits beyond the
   scope of token leakage prevention.  It allows the authorization
   server to create a different access token whose format and content is
   specifically minted for the respective server.  This has huge
   functional and privacy advantages in deployments using structured
   access tokens.

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 of the respective server.

   If the attacker were able to gain full control, including shell
   access, 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.  This section focuses on the impact of
   OAuth-related breaches and the replaying of captured access tokens.

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

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   *  Sender-constrained access tokens, as described in
      Section, SHOULD be used to prevent the attacker from
      replaying the access tokens on other resource servers.  Depending
      on the severity of the penetration, sender-constrained access
      tokens will also prevent replay on the compromised system.

   *  Audience restriction as described in Section SHOULD be
      used to prevent replay of captured access tokens on other resource

   *  The resource server MUST treat access tokens like any other
      credentials.  It is considered good practice to not log them and
      not store them in plain text.

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

4.10.  Open Redirection

   The following attacks can occur when an AS or client has an open
   redirector.  An open redirector is an endpoint that forwards a user's
   browser to an arbitrary URI obtained from a query parameter.  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.

4.10.1.  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 follow 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 whitelisted or if the origin and integrity of a
   request can be authenticated.  Countermeasures against open
   redirection are described by OWASP [owasp_redir].

4.10.2.  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 web sites (the clients), but must do so in a
   safe way.

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   [RFC6749], Section, already prevents open redirects by
   stating that the AS 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
   redirect URI to perform phishing attacks.  The attacker could, for
   example, register a client via dynamic client registration [RFC7591]
   and intentionally send an erroneous authorization request, e.g., by
   using an invalid scope value, thus instructing the AS to redirect the
   user agent to its phishing site.

   The AS MUST take precautions to prevent this threat.  Based on its
   risk assessment, the AS needs to decide whether it can trust the
   redirect URI and SHOULD only automatically redirect the user agent if
   it trusts the redirect URI.  If the URI is not trusted, the AS MAY
   inform the user and rely on the user to make the correct decision.

4.11.  307 Redirect

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

   In [RFC6749], the HTTP status code 302 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 AS.

   The behavior might be unexpected for developers, but is defined in
   [RFC7231], Section 6.4.7.  This status code 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 [RFC7231], only the status code 303
   unambigiously 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 and therefore to
   reveal the user's credentials to the client.  (In practice, however,
   most user agents will only show this behaviour for 307 redirects.)

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   ASs 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 AS SHOULD use HTTP status
   code 303 (See Other).

4.12.  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 custom HTTP headers added to the upstream request.

   If the reverse proxy would pass 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 a whitelisted IP address to the header and
   render a IP whitelist 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 proxy and application server, the attacker could also try to
   circumvent security controls in place.  It is, therefore, essential
   to ensure the authenticity of the communicating entities.
   Furthermore, the communication link between reverse proxy and
   application server MUST be protected against eavesdropping,
   injection, and replay of messages.

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4.13.  Refresh Token Protection

   Refresh tokens are a convenient and user-friendly way to obtain new
   access tokens after the expiration of access tokens.  Refresh tokens
   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.

4.13.1.  Discussion

   Refresh tokens are an attractive target for attackers, since they
   represent the overall grant a resource owner delegated to a certain
   client.  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 a 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.

4.13.2.  Recommendations

   Authorization servers SHOULD 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 refresh access tokens 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

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   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 server 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 [I-D.ietf-oauth-dpop].

   *  *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

   *  logout at the authorization server

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   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).

4.14.  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.
   If both options are possible, depending on the details of the
   implementation, a client's identity may be mistaken 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 an end-user (e.g.,
   a sub value if OpenID Connect is used).  If the resource server
   cannot properly distinguish between access tokens issued to clients
   and access tokens issued to end-users, the client may then be able to
   access resource of the end-user.

4.14.1.  Countermeasures

   Authorization servers SHOULD NOT allow clients to influence their
   client_id or any other Claim if that can cause confusion with a
   genuine resource owner.  Where this cannot be avoided, authorization
   servers MUST provide other means for the resource server to
   distinguish between access tokens authorized by a resource owner from
   access tokens authorized by the client itself.

4.15.  Clickjacking

   As described in Section 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, 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.

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   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 [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 [CSP-2] for details).  Level 2 of this standard
   provides a robust mechanism for protecting against clickjacking by
   using policies that restrict the origin of frames (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 [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.

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5.  Acknowledgements

   We would like to thank Brock Allen, Annabelle Richard Backman,
   Dominick Baier, Vittorio Bertocci, Brian Campbell, William Dennis,
   George Fletscher, Dick Hardt, Joseph Heenan, Pedram Hosseyni, Phil
   Hunt, Jared Jennings, Michael B.  Jones, Konstantin Lapine, Neil
   Madden, Christian Mainka, Jim Manico, Nov Matake, Doug McDorman,
   Karsten Meyer zu Selhausen, Aaron Parecki, Michael Peck, Johan
   Peeters, Nat Sakimura, Guido Schmitz, Travis Spencer, Petteri
   Stenius, Tomek Stojecki, Tim Wuertele, David Waite and Hans Zandbelt
   for their valuable feedback.

6.  IANA Considerations

   This draft makes no requests to IANA.

7.  Security Considerations

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

8.  Normative References

   [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>.

              Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
              Connect Discovery 1.0 incorporating errata set 1", 8
              November 2014, <https://openid.net/specs/openid-connect-

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations",
              DOI 10.17487/RFC6819, RFC 6819, January 2013,

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, DOI 10.17487/RFC8252, RFC 8252, October 2017,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

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   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,

              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", 8 November 2014,

   [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,

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", DOI 10.17487/RFC8414,
              RFC 8414, June 2018,

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", February 2020,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content",
              DOI 10.17487/RFC7231, RFC 7231, June 2014,

9.  Informative References

              Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0", 6 January 2016,

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

              Sun, S.-T. and K. Beznosov, "The Devil is in the
              (Implementation) Details: An Empirical Analysis of OAuth
              SSO Systems", October 2012,

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              Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
              Tague, "OAuth Demystified for Mobile Application
              Developers", November 2014,

              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", Work in Progress,
              Internet-Draft, draft-ietf-oauth-signed-http-request-03,
              August 2016, <https://www.ietf.org/archive/id/draft-ietf-

              "OWASP Cheat Sheet Series - Unvalidated Redirects and

   [RFC9207]  Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0
              Authorization Server Issuer Identification", RFC 9207,
              DOI 10.17487/RFC9207, March 2022,

              "Referer header includes URL fragment when opening link
              using New Tab",

              Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
              Standard OpenID Connect: In-Depth Formal Security Analysis
              and Security Guidelines", 27 April 2017,

   [RFC8473]  Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
              J. Hodges, "Token Binding over HTTP", RFC 8473,
              DOI 10.17487/RFC8473, October 2018,

              Selhausen, K. M. Z. and D. Fett, "OAuth 2.0 Authorization
              Server Issuer Identification", Work in Progress, Internet-
              Draft, draft-ietf-oauth-iss-auth-resp-05, 11 January 2022,

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              Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", Work in Progress, Internet-
              Draft, draft-ietf-oauth-rar-12, May 2022,

              Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
              Mode", 27 April 2015, <http://openid.net/specs/oauth-v2-

              Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
              Maffeis, "Discovering concrete attacks on website
              authorization by formal analysis", 23 April 2014,

   [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,

   [RFC9126]  Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
              and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
              DOI 10.17487/RFC9126, RFC 9126, September 2021,

              Mladenov, V., Mainka, C., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", 7 January 2016,

              Bradley, J., Lodderstedt, D. 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, November 2018,

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              Bradley, J., Hunt, P., Jones, M. B., Tschofenig, H., and
              M. Meszaros, "OAuth 2.0 Proof-of-Possession: Authorization
              Server to Client Key Distribution", Work in Progress,
              Internet-Draft, draft-ietf-oauth-pop-key-distribution-07,
              27 March 2019, <https://www.ietf.org/archive/id/draft-

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

              Watson, M., "Web Cryptography API", 26 January 2017,

   [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,

              Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
              Authorization Framework: JWT Pop Token Usage", Work in
              Progress, Internet-Draft, draft-sakimura-oauth-jpop-05, 22
              July 2019, <https://www.ietf.org/archive/id/draft-

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients",
              DOI 10.17487/RFC7636, RFC 7636, September 2015,

              Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof-
              of-Possession at the Application Layer (DPoP)", Work in
              Progress, Internet-Draft, draft-ietf-oauth-dpop-10, 11
              July 2022, <https://www.ietf.org/archive/id/draft-ietf-

              Jones, M. B., 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,

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   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,

   [CSP-2]    West, M., Barth, A., and D. Veditz, "Content Security
              Policy Level 2", July 2015, <https://www.w3.org/TR/CSP2>.

   [JARM]     Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
              Secured Authorization Response Mode for OAuth 2.0 (JARM)",
              17 October 2018,

   [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>.

              Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
              to Us: Understanding the Security Threats of Dangling DNS
              Records", 24 October 2016,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", DOI 10.17487/RFC2119, BCP 14,
              RFC 2119, March 1997,

   [WebAuthn] Balfanz, D., Czeskis, A., Hodges, J., Jones, J.C., Jones,
              M.B., Kumar, A., Liao, A., Lindemann, R., and E. Lundberg,
              "Web Authentication: An API for accessing Public Key
              Credentials Level 1", 4 March 2019,

              Eisinger, J. and E. Stark, "Referrer Policy", 20 April
              2017, <https://w3c.github.io/webappsec-referrer-policy>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]


   *  Improved description of authorization code injection attacks and
      PKCE protection

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   *  Removed recommendation for MTLS in discussion (not reflected in
      actual Recommendations section)

   *  Reworded "placeholder" text in security considerations.

   *  Alphabetized list of names and fixed unicode problem

   *  Explained Clickjacking

   *  Explained Open Redirectors

   *  Clarified references to attacker model by including a link to
      Section 3

   *  Clarified description of "CSRF tokens" and reference to RFC6819

   *  Described that OIDC can prevent access token injection

   *  Updated references


   *  Changed affiliation of Andrey Labunets

   *  Editorial change to clarify the new recommendations for refresh


   *  Fix editorial and spelling issues.

   *  Change wording for disallowing HTTP redirect URIs.


   *  Make the use of metadata RECOMMENDED for both servers and clients

   *  Make announcing PKCE support in metadata the RECOMMENDED way
      (before: either metadata or deployment-specific way)

   *  AS also MUST NOT expose open redirectors.

   *  Mention that attackers can collaborate.

   *  Update recommendations regarding mix-up defense, building upon

   *  Improve description of mix-up attack.

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   *  Make HTTPS mandatory for most redirect URIs.


   *  Make MTLS a suggestion, not RECOMMENDED.

   *  Add important requirements when using nonce for code injection

   *  Highlight requirements for refresh token sender-constraining.

   *  Make PKCE a MUST for public clients.

   *  Describe PKCE Downgrade Attacks and countermeasures.

   *  Allow variable port numbers in localhost redirect URIs as in
      RFC8252, Section 7.3.


   *  Update reference to DPoP

   *  Fix reference to RFC8414

   *  Move to xml2rfcv3


   *  Added info about using CSP to prevent clickjacking

   *  Changes from WGLC feedback

   *  Editorial changes

   *  AS MUST announce PKCE support either in metadata or using
      deployment-specific ways (before: SHOULD)


   *  Discourage use of Resource Owner Password Credentials Grant

   *  Added text on client impersonating resource owner

   *  Recommend asymmetric methods for client authentication

   *  Encourage use of PKCE mode "S256"

   *  PKCE may replace state for CSRF protection

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   *  AS SHOULD publish PKCE support

   *  Cleaned up discussion on auth code injection

   *  AS MUST support PKCE


   *  Added updated attacker model


   *  Adapted section 2.1.2 to outcome of consensus call

   *  more text on refresh token inactivity and implementation note on
      refresh token replay detection via refresh token rotation


   *  incorporated feedback by Joseph Heenan

   *  changed occurrences of SHALL to MUST

   *  added text on lack of token/cert binding support tokens issued in
      the authorization response as justification to not recommend
      issuing tokens there at all

   *  added requirement to authenticate clients during code exchange
      (PKCE or client credential) to 2.1.1.

   *  added section on refresh tokens

   *  editorial enhancements to 2.1.2 based on feedback


   *  changed text to recommend not to use implicit but code

   *  added section on access token injection

   *  reworked sections 3.1 through 3.3 to be more specific on implicit
      grant issues


   *  added recommendations re implicit and token injection

   *  uppercased key words in Section 2 according to RFC 2119

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   *  incorporated findings of Doug McDorman

   *  added section on HTTP status codes for redirects

   *  added new section on access token privilege restriction based on
      comments from Johan Peeters


   *  reworked section 3.8.1

   *  incorporated Phil Hunt's feedback

   *  reworked section on mix-up

   *  extended section on code leakage via referrer header to also cover
      state leakage

   *  added Daniel Fett as author

   *  replaced text intended to inform WG discussion by recommendations
      to implementors

   *  modified example URLs to conform to RFC 2606


   *  Completed sections on code leakage via referrer header, attacks in
      browser, mix-up, and CSRF

   *  Reworked Code Injection Section

   *  Added reference to OpenID Connect spec

   *  removed refresh token leakage as respective considerations have
      been given in section 10.4 of RFC 6749

   *  first version on open redirection

   *  incorporated Christian Mainka's review feedback


   *  Restructured document for better readability

   *  Added best practices on Token Leakage prevention

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   *  Added section on Access Token Leakage at Resource Server

   *  incorporated Brian Campbell's findings


   *  Folded Mix up and Access Token leakage through a bad AS into new
      section for dynamic OAuth threats

   *  reworked dynamic OAuth section


   *  Added references to mitigation methods for token leakage

   *  Added reference to Token Binding for Authorization Code

   *  incorporated feedback of Phil Hunt

   *  fixed numbering issue in attack descriptions in section 2

   -00 (WG document)

   *  turned the ID into a WG document and a BCP

   *  Added federated app login as topic in Other Topics

Authors' Addresses

   Torsten Lodderstedt
   Email: torsten@lodderstedt.net

   John Bradley
   Email: ve7jtb@ve7jtb.com

   Andrey Labunets
   Independent Researcher
   Email: isciurus@gmail.com

   Daniel Fett

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   Email: mail@danielfett.de

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