



Internet Engineering Task Force (IETF)                           D. Fett
Request for Comments: 9449                                      Authlete
Category: Standards Track                                    B. Campbell
ISSN: 2070-1721                                            Ping Identity
                                                              J. Bradley
                                                                  Yubico
                                                          T. Lodderstedt
                                                                 Tuconic
                                                                M. Jones
                                                  Self-Issued Consulting
                                                                D. Waite
                                                           Ping Identity
                                                          September 2023


           OAuth 2.0 Demonstrating Proof of Possession (DPoP)

Abstract

   This document describes a mechanism for sender-constraining OAuth 2.0
   tokens via a proof-of-possession mechanism on the application level.
   This mechanism allows for the detection of replay attacks with access
   and refresh tokens.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9449.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction
     1.1.  Conventions and Terminology
   2.  Objectives
   3.  Concept
   4.  DPoP Proof JWTs
     4.1.  The DPoP HTTP Header
     4.2.  DPoP Proof JWT Syntax
     4.3.  Checking DPoP Proofs
   5.  DPoP Access Token Request
     5.1.  Authorization Server Metadata
     5.2.  Client Registration Metadata
   6.  Public Key Confirmation
     6.1.  JWK Thumbprint Confirmation Method
     6.2.  JWK Thumbprint Confirmation Method in Token Introspection
   7.  Protected Resource Access
     7.1.  The DPoP Authentication Scheme
     7.2.  Compatibility with the Bearer Authentication Scheme
     7.3.  Client Considerations
   8.  Authorization Server-Provided Nonce
     8.1.  Nonce Syntax
     8.2.  Providing a New Nonce Value
   9.  Resource Server-Provided Nonce
   10. Authorization Code Binding to a DPoP Key
     10.1.  DPoP with Pushed Authorization Requests
   11. Security Considerations
     11.1.  DPoP Proof Replay
     11.2.  DPoP Proof Pre-generation
     11.3.  DPoP Nonce Downgrade
     11.4.  Untrusted Code in the Client Context
     11.5.  Signed JWT Swapping
     11.6.  Signature Algorithms
     11.7.  Request Integrity
     11.8.  Access Token and Public Key Binding
     11.9.  Authorization Code and Public Key Binding
     11.10. Hash Algorithm Agility
     11.11. Binding to Client Identity
   12. IANA Considerations
     12.1.  OAuth Access Token Types Registration
     12.2.  OAuth Extensions Error Registration
     12.3.  OAuth Parameters Registration
     12.4.  HTTP Authentication Schemes Registration
     12.5.  Media Type Registration
     12.6.  JWT Confirmation Methods Registration
     12.7.  JSON Web Token Claims Registration
       12.7.1.  "nonce" Registration Update
     12.8.  Hypertext Transfer Protocol (HTTP) Field Name Registration
     12.9.  OAuth Authorization Server Metadata Registration
     12.10. OAuth Dynamic Client Registration Metadata
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Demonstrating Proof of Possession (DPoP) is an application-level
   mechanism for sender-constraining OAuth [RFC6749] access and refresh
   tokens.  It enables a client to prove the possession of a public/
   private key pair by including a DPoP header in an HTTP request.  The
   value of the header is a JSON Web Token (JWT) [RFC7519] that enables
   the authorization server to bind issued tokens to the public part of
   a client's key pair.  Recipients of such tokens are then able to
   verify the binding of the token to the key pair that the client has
   demonstrated that it holds via the DPoP header, thereby providing
   some assurance that the client presenting the token also possesses
   the private key.  In other words, the legitimate presenter of the
   token is constrained to be the sender that holds and proves
   possession of the private part of the key pair.

   The mechanism specified herein can be used in cases where other
   methods of sender-constraining tokens that utilize elements of the
   underlying secure transport layer, such as [RFC8705] or
   [TOKEN-BINDING], are not available or desirable.  For example, due to
   a sub-par user experience of TLS client authentication in user agents
   and a lack of support for HTTP token binding, neither mechanism can
   be used if an OAuth client is an application that is dynamically
   downloaded and executed in a web browser (sometimes referred to as a
   "single-page application").  Additionally, applications that are
   installed and run directly on a user's device are well positioned to
   benefit from DPoP-bound tokens that guard against the misuse of
   tokens by a compromised or malicious resource.  Such applications
   often have dedicated protected storage for cryptographic keys.

   DPoP can be used to sender-constrain access tokens regardless of the
   client authentication method employed, but DPoP itself is not used
   for client authentication.  DPoP can also be used to sender-constrain
   refresh tokens issued to public clients (those without authentication
   credentials associated with the client_id).

1.1.  Conventions and Terminology

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

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response",
   "client", "public client", and "confidential client" defined by "The
   OAuth 2.0 Authorization Framework" [RFC6749].

   The terms "request", "response", "header field", and "target URI" are
   imported from [RFC9110].

   The terms "JOSE" and "JOSE Header" are imported from [RFC7515].

   This document contains non-normative examples of partial and complete
   HTTP messages.  Some examples use a single trailing backslash to
   indicate line wrapping for long values, as per [RFC8792].  The
   character and leading spaces on wrapped lines are not part of the
   value.

2.  Objectives

   The primary aim of DPoP is to prevent unauthorized or illegitimate
   parties from using leaked or stolen access tokens, by binding a token
   to a public key upon issuance and requiring that the client proves
   possession of the corresponding private key when using the token.
   This constrains the legitimate sender of the token to only the party
   with access to the private key and gives the server receiving the
   token added assurances that the sender is legitimately authorized to
   use it.

   Access tokens that are sender-constrained via DPoP thus stand in
   contrast to the typical bearer token, which can be used by any party
   in possession of such a token.  Although protections generally exist
   to prevent unintended disclosure of bearer tokens, unforeseen vectors
   for leakage have occurred due to vulnerabilities and implementation
   issues in other layers in the protocol or software stack (see, e.g.,
   Compression Ratio Info-leak Made Easy (CRIME) [CRIME], Browser
   Reconnaissance and Exfiltration via Adaptive Compression of Hypertext
   (BREACH) [BREACH], Heartbleed [Heartbleed], and the Cloudflare parser
   bug [Cloudbleed]).  There have also been numerous published token
   theft attacks on OAuth implementations themselves ([GitHub.Tokens] is
   just one high-profile example).  DPoP provides a general defense in
   depth against the impact of unanticipated token leakage.  DPoP is
   not, however, a substitute for a secure transport and MUST always be
   used in conjunction with HTTPS.

   The very nature of the typical OAuth protocol interaction
   necessitates that the client discloses the access token to the
   protected resources that it accesses.  The attacker model in
   [SECURITY-TOPICS] describes cases where a protected resource might be
   counterfeit, malicious, or compromised and plays received tokens
   against other protected resources to gain unauthorized access.
   Audience-restricted access tokens (e.g., using the JWT [RFC7519] aud
   claim) can prevent such misuse.  However, doing so in practice has
   proven to be prohibitively cumbersome for many deployments (despite
   extensions such as [RFC8707]).  Sender-constraining access tokens is
   a more robust and straightforward mechanism to prevent such token
   replay at a different endpoint, and DPoP is an accessible
   application-layer means of doing so.

   Due to the potential for cross-site scripting (XSS), browser-based
   OAuth clients bring to bear added considerations with respect to
   protecting tokens.  The most straightforward XSS-based attack is for
   an attacker to exfiltrate a token and use it themselves completely
   independent of the legitimate client.  A stolen access token is used
   for protected resource access, and a stolen refresh token is used for
   obtaining new access tokens.  If the private key is non-extractable
   (as is possible with [W3C.WebCryptoAPI]), DPoP renders exfiltrated
   tokens alone unusable.

   XSS vulnerabilities also allow an attacker to execute code in the
   context of the browser-based client application and maliciously use a
   token indirectly through the client.  That execution context has
   access to utilize the signing key; thus, it can produce DPoP proofs
   to use in conjunction with the token.  At this application layer,
   there is most likely no feasible defense against this threat except
   generally preventing XSS; therefore, it is considered out of scope
   for DPoP.

   Malicious XSS code executed in the context of the browser-based
   client application is also in a position to create DPoP proofs with
   timestamp values in the future and exfiltrate them in conjunction
   with a token.  These stolen artifacts can later be used independent
   of the client application to access protected resources.  To prevent
   this, servers can optionally require clients to include a server-
   chosen value into the proof that cannot be predicted by an attacker
   (nonce).  In the absence of the optional nonce, the impact of pre-
   computed DPoP proofs is limited somewhat by the proof being bound to
   an access token on protected resource access.  Because a proof
   covering an access token that does not yet exist cannot feasibly be
   created, access tokens obtained with an exfiltrated refresh token and
   pre-computed proofs will be unusable.

   Additional security considerations are discussed in Section 11.

3.  Concept

   The main data structure introduced by this specification is a DPoP
   proof JWT that is sent as a header in an HTTP request, as described
   in detail below.  A client uses a DPoP proof JWT to prove the
   possession of a private key corresponding to a certain public key.

   Roughly speaking, a DPoP proof is a signature over:

   *  some data of the HTTP request to which it is attached,

   *  a timestamp,

   *  a unique identifier,

   *  an optional server-provided nonce, and

   *  a hash of the associated access token when an access token is
      present within the request.

   +--------+                                          +---------------+
   |        |--(A)-- Token Request ------------------->|               |
   | Client |        (DPoP Proof)                      | Authorization |
   |        |                                          |     Server    |
   |        |<-(B)-- DPoP-Bound Access Token ----------|               |
   |        |        (token_type=DPoP)                 +---------------+
   |        |
   |        |
   |        |                                          +---------------+
   |        |--(C)-- DPoP-Bound Access Token --------->|               |
   |        |        (DPoP Proof)                      |    Resource   |
   |        |                                          |     Server    |
   |        |<-(D)-- Protected Resource ---------------|               |
   |        |                                          +---------------+
   +--------+

                         Figure 1: Basic DPoP Flow

   The basic steps of an OAuth flow with DPoP (without the optional
   nonce) are shown in Figure 1.

   A.  In the token request, the client sends an authorization grant
       (e.g., an authorization code, refresh token, etc.) to the
       authorization server in order to obtain an access token (and
       potentially a refresh token).  The client attaches a DPoP proof
       to the request in an HTTP header.

   B.  The authorization server binds (sender-constrains) the access
       token to the public key claimed by the client in the DPoP proof;
       that is, the access token cannot be used without proving
       possession of the respective private key.  If a refresh token is
       issued to a public client, it is also bound to the public key of
       the DPoP proof.

   C.  To use the access token, the client has to prove possession of
       the private key by, again, adding a header to the request that
       carries a DPoP proof for that request.  The resource server needs
       to receive information about the public key to which the access
       token is bound.  This information may be encoded directly into
       the access token (for JWT-structured access tokens) or provided
       via token introspection endpoint (not shown).  The resource
       server verifies that the public key to which the access token is
       bound matches the public key of the DPoP proof.  It also verifies
       that the access token hash in the DPoP proof matches the access
       token presented in the request.

   D.  The resource server refuses to serve the request if the signature
       check fails or if the data in the DPoP proof is wrong, e.g., the
       target URI does not match the URI claim in the DPoP proof JWT.
       The access token itself, of course, must also be valid in all
       other respects.

   The DPoP mechanism presented herein is not a client authentication
   method.  In fact, a primary use case of DPoP is for public clients
   (e.g., single-page applications and applications on a user's device)
   that do not use client authentication.  Nonetheless, DPoP is designed
   to be compatible with private_key_jwt and all other client
   authentication methods.

   DPoP does not directly ensure message integrity, but it relies on the
   TLS layer for that purpose.  See Section 11 for details.

4.  DPoP Proof JWTs

   DPoP introduces the concept of a DPoP proof, which is a JWT created
   by the client and sent with an HTTP request using the DPoP header
   field.  Each HTTP request requires a unique DPoP proof.

   A valid DPoP proof demonstrates to the server that the client holds
   the private key that was used to sign the DPoP proof JWT.  This
   enables authorization servers to bind issued tokens to the
   corresponding public key (as described in Section 5) and enables
   resource servers to verify the key-binding of tokens that it receives
   (see Section 7.1), which prevents said tokens from being used by any
   entity that does not have access to the private key.

   The DPoP proof demonstrates possession of a key and, by itself, is
   not an authentication or access control mechanism.  When presented in
   conjunction with a key-bound access token as described in
   Section 7.1, the DPoP proof provides additional assurance about the
   legitimacy of the client to present the access token.  However, a
   valid DPoP proof JWT is not sufficient alone to make access control
   decisions.

4.1.  The DPoP HTTP Header

   A DPoP proof is included in an HTTP request using the following
   request header field.

   DPoP:  A JWT that adheres to the structure and syntax of Section 4.2.

   Figure 2 shows an example DPoP HTTP header field.  The example uses
   "\" line wrapping per [RFC8792].

   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik\
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR\
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE\
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj\
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia\
    WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg\
    4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg

                       Figure 2: Example DPoP Header

   Note that per [RFC9110], header field names are case insensitive;
   thus, DPoP, DPOP, dpop, etc., are all valid and equivalent header
   field names.  However, case is significant in the header field value.

   The DPoP HTTP header field value uses the token68 syntax defined in
   Section 11.2 of [RFC9110] and is repeated below in Figure 3 for ease
   of reference.

   DPoP       = token68
   token68    = 1*( ALPHA / DIGIT /
                    "-" / "." / "_" / "~" / "+" / "/" ) *"="

                      Figure 3: DPoP Header Field ABNF

4.2.  DPoP Proof JWT Syntax

   A DPoP proof is a JWT [RFC7519] that is signed (using JSON Web
   Signature (JWS) [RFC7515]) with a private key chosen by the client
   (see below).  The JOSE Header of a DPoP JWT MUST contain at least the
   following parameters:

   typ:  A field with the value dpop+jwt, which explicitly types the
      DPoP proof JWT as recommended in Section 3.11 of [RFC8725].

   alg:  An identifier for a JWS asymmetric digital signature algorithm
      from [IANA.JOSE.ALGS].  It MUST NOT be none or an identifier for a
      symmetric algorithm (Message Authentication Code (MAC)).

   jwk:  Represents the public key chosen by the client in JSON Web Key
      (JWK) [RFC7517] format as defined in Section 4.1.3 of [RFC7515].
      It MUST NOT contain a private key.

   The payload of a DPoP proof MUST contain at least the following
   claims:

   jti:  Unique identifier for the DPoP proof JWT.  The value MUST be
      assigned such that there is a negligible probability that the same
      value will be assigned to any other DPoP proof used in the same
      context during the time window of validity.  Such uniqueness can
      be accomplished by encoding (base64url or any other suitable
      encoding) at least 96 bits of pseudorandom data or by using a
      version 4 Universally Unique Identifier (UUID) string according to
      [RFC4122].  The jti can be used by the server for replay detection
      and prevention; see Section 11.1.

   htm:  The value of the HTTP method (Section 9.1 of [RFC9110]) of the
      request to which the JWT is attached.

   htu:  The HTTP target URI (Section 7.1 of [RFC9110]) of the request
      to which the JWT is attached, without query and fragment parts.

   iat:  Creation timestamp of the JWT (Section 4.1.6 of [RFC7519]).

   When the DPoP proof is used in conjunction with the presentation of
   an access token in protected resource access (see Section 7), the
   DPoP proof MUST also contain the following claim:

   ath:  Hash of the access token.  The value MUST be the result of a
      base64url encoding (as defined in Section 2 of [RFC7515]) the
      SHA-256 [SHS] hash of the ASCII encoding of the associated access
      token's value.

   When the authentication server or resource server provides a DPoP-
   Nonce HTTP header in a response (see Sections 8 and 9), the DPoP
   proof MUST also contain the following claim:

   nonce:  A recent nonce provided via the DPoP-Nonce HTTP header.

   A DPoP proof MAY contain other JOSE Header Parameters or claims as
   defined by extension, profile, or deployment-specific requirements.

   Figure 4 is a conceptual example showing the decoded content of the
   DPoP proof in Figure 2.  The JSON of the JWT header and payload are
   shown, but the signature part is omitted.  As usual, line breaks and
   extra spaces are included for formatting and readability.

   {
     "typ":"dpop+jwt",
     "alg":"ES256",
     "jwk": {
       "kty":"EC",
       "x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
       "y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
       "crv":"P-256"
     }
   }
   .
   {
     "jti":"-BwC3ESc6acc2lTc",
     "htm":"POST",
     "htu":"https://server.example.com/token",
     "iat":1562262616
   }

               Figure 4: Example JWT Content of a DPoP Proof

   Of the HTTP request, only the HTTP method and URI are included in the
   DPoP JWT; therefore, only these two message parts are covered by the
   DPoP proof.  The idea is to sign just enough of the HTTP data to
   provide reasonable proof of possession with respect to the HTTP
   request.  This design approach of using only a minimal subset of the
   HTTP header data is to avoid the substantial difficulties inherent in
   attempting to normalize HTTP messages.  Nonetheless, DPoP proofs can
   be extended to contain other information of the HTTP request (see
   also Section 11.7).

4.3.  Checking DPoP Proofs

   To validate a DPoP proof, the receiving server MUST ensure the
   following:

   1.   There is not more than one DPoP HTTP request header field.
   2.   The DPoP HTTP request header field value is a single and well-
        formed JWT.
   3.   All required claims per Section 4.2 are contained in the JWT.
   4.   The typ JOSE Header Parameter has the value dpop+jwt.
   5.   The alg JOSE Header Parameter indicates a registered asymmetric
        digital signature algorithm [IANA.JOSE.ALGS], is not none, is
        supported by the application, and is acceptable per local
        policy.
   6.   The JWT signature verifies with the public key contained in the
        jwk JOSE Header Parameter.
   7.   The jwk JOSE Header Parameter does not contain a private key.
   8.   The htm claim matches the HTTP method of the current request.
   9.   The htu claim matches the HTTP URI value for the HTTP request in
        which the JWT was received, ignoring any query and fragment
        parts.
   10.  If the server provided a nonce value to the client, the nonce
        claim matches the server-provided nonce value.
   11.  The creation time of the JWT, as determined by either the iat
        claim or a server managed timestamp via the nonce claim, is
        within an acceptable window (see Section 11.1).
   12.  If presented to a protected resource in conjunction with an
        access token,
        *  ensure that the value of the ath claim equals the hash of
           that access token, and
        *  confirm that the public key to which the access token is
           bound matches the public key from the DPoP proof.

   To reduce the likelihood of false negatives, servers SHOULD employ
   syntax-based normalization (Section 6.2.2 of [RFC3986]) and scheme-
   based normalization (Section 6.2.3 of [RFC3986]) before comparing the
   htu claim.

   These checks may be performed in any order.

5.  DPoP Access Token Request

   To request an access token that is bound to a public key using DPoP,
   the client MUST provide a valid DPoP proof JWT in a DPoP header when
   making an access token request to the authorization server's token
   endpoint.  This is applicable for all access token requests
   regardless of grant type (e.g., the common authorization_code and
   refresh_token grant types and extension grants such as the JWT
   authorization grant [RFC7523]).  The HTTP request shown in Figure 5
   illustrates such an access token request using an authorization code
   grant with a DPoP proof JWT in the DPoP header.  Figure 5 uses "\"
   line wrapping per [RFC8792].

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik\
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR\
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE\
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj\
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia\
    WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg\
    4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg

   grant_type=authorization_code\
   &client_id=s6BhdRkqt\
   &code=SplxlOBeZQQYbYS6WxSbIA
   &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb\
   &code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-

    Figure 5: Token Request for a DPoP Sender-Constrained Token Using an
                             Authorization Code

   The DPoP HTTP header field MUST contain a valid DPoP proof JWT.  If
   the DPoP proof is invalid, the authorization server issues an error
   response per Section 5.2 of [RFC6749] with invalid_dpop_proof as the
   value of the error parameter.

   To sender-constrain the access token after checking the validity of
   the DPoP proof, the authorization server associates the issued access
   token with the public key from the DPoP proof, which can be
   accomplished as described in Section 6.  A token_type of DPoP MUST be
   included in the access token response to signal to the client that
   the access token was bound to its DPoP key and can be used as
   described in Section 7.1.  The example response shown in Figure 6
   illustrates such a response.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
    "access_token": "Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU",
    "token_type": "DPoP",
    "expires_in": 2677,
    "refresh_token": "Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g"
   }

                      Figure 6: Access Token Response

   The example response in Figure 6 includes a refresh token that the
   client can use to obtain a new access token when the previous one
   expires.  Refreshing an access token is a token request using the
   refresh_token grant type made to the authorization server's token
   endpoint.  As with all access token requests, the client makes it a
   DPoP request by including a DPoP proof, as shown in Figure 7.
   Figure 7 uses "\" line wrapping per [RFC8792].

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik\
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR\
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE\
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj\
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia\
    WF0IjoxNTYyMjY1Mjk2fQ.pAqut2IRDm_De6PR93SYmGBPXpwrAk90e8cP2hjiaG5Qs\
    GSuKDYW7_X620BxqhvYC8ynrrvZLTk41mSRroapUA

   grant_type=refresh_token\
   &client_id=s6BhdRkqt\
   &refresh_token=Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g

    Figure 7: Token Request for a DPoP-Bound Token Using a Refresh Token

   When an authorization server supporting DPoP issues a refresh token
   to a public client that presents a valid DPoP proof at the token
   endpoint, the refresh token MUST be bound to the respective public
   key.  The binding MUST be validated when the refresh token is later
   presented to get new access tokens.  As a result, such a client MUST
   present a DPoP proof for the same key that was used to obtain the
   refresh token each time that refresh token is used to obtain a new
   access token.  The implementation details of the binding of the
   refresh token are at the discretion of the authorization server.
   Since the authorization server both produces and validates its
   refresh tokens, there is no interoperability consideration in the
   specific details of the binding.

   An authorization server MAY elect to issue access tokens that are not
   DPoP bound, which is signaled to the client with a value of Bearer in
   the token_type parameter of the access token response per [RFC6750].
   For a public client that is also issued a refresh token, this has the
   effect of DPoP-binding the refresh token alone, which can improve the
   security posture even when protected resources are not updated to
   support DPoP.

   If the access token response contains a different token_type value
   than DPoP, the access token protection provided by DPoP is not given.
   The client MUST discard the response in this case if this protection
   is deemed important for the security of the application; otherwise,
   the client may continue as in a regular OAuth interaction.

   Refresh tokens issued to confidential clients (those having
   established authentication credentials with the authorization server)
   are not bound to the DPoP proof public key because they are already
   sender-constrained with a different existing mechanism.  The OAuth
   2.0 Authorization Framework [RFC6749] already requires that an
   authorization server bind refresh tokens to the client to which they
   were issued and that confidential clients authenticate to the
   authorization server when presenting a refresh token.  As a result,
   such refresh tokens are sender-constrained by way of the client
   identifier and the associated authentication requirement.  This
   existing sender-constraining mechanism is more flexible (e.g., it
   allows credential rotation for the client without invalidating
   refresh tokens) than binding directly to a particular public key.

5.1.  Authorization Server Metadata

   This document introduces the following authorization server metadata
   [RFC8414] parameter to signal support for DPoP in general and the
   specific JWS alg values the authorization server supports for DPoP
   proof JWTs.

   dpop_signing_alg_values_supported:  A JSON array containing a list of
      the JWS alg values (from the [IANA.JOSE.ALGS] registry) supported
      by the authorization server for DPoP proof JWTs.

5.2.  Client Registration Metadata

   The Dynamic Client Registration Protocol [RFC7591] defines an API for
   dynamically registering OAuth 2.0 client metadata with authorization
   servers.  The metadata defined by [RFC7591], and registered
   extensions to it, also imply a general data model for clients that is
   useful for authorization server implementations even when the Dynamic
   Client Registration Protocol isn't in play.  Such implementations
   will typically have some sort of user interface available for
   managing client configuration.

   This document introduces the following client registration metadata
   [RFC7591] parameter to indicate that the client always uses DPoP when
   requesting tokens from the authorization server.

   dpop_bound_access_tokens:  A boolean value specifying whether the
      client always uses DPoP for token requests.  If omitted, the
      default value is false.

   If the value is true, the authorization server MUST reject token
   requests from the client that do not contain the DPoP header.

6.  Public Key Confirmation

   Resource servers MUST be able to reliably identify whether an access
   token is DPoP-bound and ascertain sufficient information to verify
   the binding to the public key of the DPoP proof (see Section 7.1).
   Such a binding is accomplished by associating the public key with the
   token in a way that can be accessed by the protected resource, such
   as embedding the JWK hash in the issued access token directly, using
   the syntax described in Section 6.1, or through token introspection
   as described in Section 6.2.  Other methods of associating a public
   key with an access token are possible per an agreement by the
   authorization server and the protected resource; however, they are
   beyond the scope of this specification.

   Resource servers supporting DPoP MUST ensure that the public key from
   the DPoP proof matches the one bound to the access token.

6.1.  JWK Thumbprint Confirmation Method

   When access tokens are represented as JWTs [RFC7519], the public key
   information is represented using the jkt confirmation method member
   defined herein.  To convey the hash of a public key in a JWT, this
   specification introduces the following JWT Confirmation Method
   [RFC7800] member for use under the cnf claim.

   jkt:  JWK SHA-256 Thumbprint confirmation method.  The value of the
      jkt member MUST be the base64url encoding (as defined in
      [RFC7515]) of the JWK SHA-256 Thumbprint (according to [RFC7638])
      of the DPoP public key (in JWK format) to which the access token
      is bound.

   The following example JWT in Figure 8 with a decoded JWT payload
   shown in Figure 9 contains a cnf claim with the jkt JWK Thumbprint
   confirmation method member.  The jkt value in these examples is the
   hash of the public key from the DPoP proofs in the examples shown in
   Section 5.  The example uses "\" line wrapping per [RFC8792].

   eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWIiOiJzb21lb25lQGV4YW1\
   wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJuYmYiOjE\
   1NjIyNjI2MTEsImV4cCI6MTU2MjI2NjIxNiwiY25mIjp7ImprdCI6IjBaY09DT1JaTll\
   5LURXcHFxMzBqWnlKR0hUTjBkMkhnbEJWM3VpZ3VBNEkifX0.3Tyo8VTcn6u_PboUmAO\
   YUY1kfAavomW_YwYMkmRNizLJoQzWy2fCo79Zi5yObpIzjWb5xW4OGld7ESZrh0fsrA

       Figure 8: JWT Containing a JWK SHA-256 Thumbprint Confirmation

   {
     "sub":"someone@example.com",
     "iss":"https://server.example.com",
     "nbf":1562262611,
     "exp":1562266216,
     "cnf":
     {
       "jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
     }
   }

    Figure 9: JWT Claims Set with a JWK SHA-256 Thumbprint Confirmation

6.2.  JWK Thumbprint Confirmation Method in Token Introspection

   "OAuth 2.0 Token Introspection" [RFC7662] defines a method for a
   protected resource to query an authorization server about the active
   state of an access token.  The protected resource also determines
   metainformation about the token.

   For a DPoP-bound access token, the hash of the public key to which
   the token is bound is conveyed to the protected resource as
   metainformation in a token introspection response.  The hash is
   conveyed using the same cnf content with jkt member structure as the
   JWK Thumbprint confirmation method, described in Section 6.1, as a
   top-level member of the introspection response JSON.  Note that the
   resource server does not send a DPoP proof with the introspection
   request, and the authorization server does not validate an access
   token's DPoP binding at the introspection endpoint.  Rather, the
   resource server uses the data of the introspection response to
   validate the access token binding itself locally.

   If the token_type member is included in the introspection response,
   it MUST contain the value DPoP.

   The example introspection request in Figure 10 and corresponding
   response in Figure 11 illustrate an introspection exchange for the
   example DPoP-bound access token that was issued in Figure 6.

   POST /as/introspect.oauth2 HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded
   Authorization: Basic cnM6cnM6TWt1LTZnX2xDektJZHo0ZnNON2tZY3lhK1Rp

   token=Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU

                  Figure 10: Example Introspection Request

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
     "active": true,
     "sub": "someone@example.com",
     "iss": "https://server.example.com",
     "nbf": 1562262611,
     "exp": 1562266216,
     "cnf":
     {
       "jkt": "0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
     }
   }

     Figure 11: Example Introspection Response for a DPoP-Bound Access
                                   Token

7.  Protected Resource Access

   Requests to DPoP-protected resources MUST include both a DPoP proof
   as per Section 4 and the access token as described in Section 7.1.
   The DPoP proof MUST include the ath claim with a valid hash of the
   associated access token.

   Binding the token value to the proof in this way prevents a proof to
   be used with multiple different access token values across different
   requests.  For example, if a client holds tokens bound to two
   different resource owners, AT1 and AT2, and uses the same key when
   talking to the authorization server, it's possible that these tokens
   could be swapped.  Without the ath field to bind it, a captured
   signature applied to AT1 could be replayed with AT2 instead, changing
   the rights and access of the intended request.  This same
   substitution prevention remains for rotated access tokens within the
   same combination of client and resource owner -- a rotated token
   value would require the calculation of a new proof.  This binding
   additionally ensures that a proof intended for use with the access
   token is not usable without an access token, or vice-versa.

   The resource server is required to calculate the hash of the token
   value presented and verify that it is the same as the hash value in
   the ath field as described in Section 4.3.  Since the ath field value
   is covered by the DPoP proof's signature, its inclusion binds the
   access token value to the holder of the key used to generate the
   signature.

   Note that the ath field alone does not prevent replay of the DPoP
   proof or provide binding to the request in which the proof is
   presented, and it is still important to check the time window of the
   proof as well as the included message parameters, such as htm and
   htu.

7.1.  The DPoP Authentication Scheme

   A DPoP-bound access token is sent using the Authorization request
   header field per Section 11.6.2 of [RFC9110] with an authentication
   scheme of DPoP.  The syntax of the Authorization header field for the
   DPoP scheme uses the token68 syntax defined in Section 11.2 of
   [RFC9110] for credentials and is repeated below for ease of
   reference.  The ABNF notation syntax for DPoP authentication scheme
   credentials is as follows:

   token68    = 1*( ALPHA / DIGIT /
                    "-" / "." / "_" / "~" / "+" / "/" ) *"="

   credentials = "DPoP" 1*SP token68

                 Figure 12: DPoP Authentication Scheme ABNF

   For such an access token, a resource server MUST check that a DPoP
   proof was also received in the DPoP header field of the HTTP request,
   check the DPoP proof according to the rules in Section 4.3, and check
   that the public key of the DPoP proof matches the public key to which
   the access token is bound per Section 6.

   The resource server MUST NOT grant access to the resource unless all
   checks are successful.

   Figure 13 shows an example request to a protected resource with a
   DPoP-bound access token in the Authorization header and the DPoP
   proof in the DPoP header.  The example uses "\" line wrapping per
   [RFC8792].  Figure 14 shows the decoded content of that DPoP proof.
   The JSON of the JWT header and payload are shown, but the signature
   part is omitted.  As usual, line breaks and indentation are included
   for formatting and readability.

   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: DPoP Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik\
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR\
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE\
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj\
    oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z\
    WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOCwiYXRoIjoiZlVIeU8ycjJaM0RaNTNF\
    c05yV0JiMHhXWG9hTnk1OUlpS0NBcWtzbVFFbyJ9.2oW9RP35yRqzhrtNP86L-Ey71E\
    OptxRimPPToA1plemAgR6pxHF8y6-yqyVnmcw6Fy1dqd-jfxSYoMxhAJpLjA

                 Figure 13: DPoP-Protected Resource Request

   {
     "typ":"dpop+jwt",
     "alg":"ES256",
     "jwk": {
       "kty":"EC",
       "x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
       "y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
       "crv":"P-256"
     }
   }
   .
   {
     "jti":"e1j3V_bKic8-LAEB",
     "htm":"GET",
     "htu":"https://resource.example.org/protectedresource",
     "iat":1562262618,
     "ath":"fUHyO2r2Z3DZ53EsNrWBb0xWXoaNy59IiKCAqksmQEo"
   }

       Figure 14: Decoded Content of the DPoP Proof JWT in Figure 13

   Upon receipt of a request to a protected resource within the
   protection space requiring DPoP authentication, the server can
   respond with a challenge to the client to provide DPoP authentication
   information if the request does not include valid credentials or does
   not contain an access token sufficient for access.  Such a challenge
   is made using the 401 (Unauthorized) response status code ([RFC9110],
   Section 15.5.2) and the WWW-Authenticate header field ([RFC9110],
   Section 11.6.1).  The server MAY include the WWW-Authenticate header
   in response to other conditions as well.

   In such challenges:

   *  The scheme name is DPoP.
   *  The authentication parameter realm MAY be included to indicate the
      scope of protection in the manner described in [RFC9110],
      Section 11.5.
   *  A scope authentication parameter MAY be included as defined in
      [RFC6750], Section 3.
   *  An error parameter ([RFC6750], Section 3) SHOULD be included to
      indicate the reason why the request was declined, if the request
      included an access token but failed authentication.  The error
      parameter values described in [RFC6750], Section 3.1 are suitable,
      as are any appropriate values defined by extension.  The value
      use_dpop_nonce can be used as described in Section 9 to signal
      that a nonce is needed in the DPoP proof of a subsequent
      request(s).  Additionally, invalid_dpop_proof is used to indicate
      that the DPoP proof itself was deemed invalid based on the
      criteria of Section 4.3.
   *  An error_description parameter ([RFC6750], Section 3) MAY be
      included along with the error parameter to provide developers a
      human-readable explanation that is not meant to be displayed to
      end-users.
   *  An algs parameter SHOULD be included to signal to the client the
      JWS algorithms that are acceptable for the DPoP proof JWT.  The
      value of the parameter is a space-delimited list of JWS alg
      (Algorithm) header values ([RFC7515], Section 4.1.1).
   *  Additional authentication parameters MAY be used, and unknown
      parameters MUST be ignored by recipients.

   Figure 15 shows a response to a protected resource request without
   authentication.

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP algs="ES256 PS256"

    Figure 15: HTTP 401 Response to a Protected Resource Request without
                               Authentication

   Figure 16 shows a response to a protected resource request that was
   rejected due to the failed confirmation of the DPoP binding in the
   access token.  Figure 16 uses "\" line wrapping per [RFC8792].

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: DPoP error="invalid_token", \
      error_description="Invalid DPoP key binding", algs="ES256"

     Figure 16: HTTP 401 Response to a Protected Resource Request with
                              an Invalid Token

   Note that browser-based client applications using Cross-Origin
   Resource Sharing (CORS) [WHATWG.Fetch] only have access to CORS-
   safelisted response HTTP headers by default.  In order for the
   application to obtain and use the WWW-Authenticate HTTP response
   header value, the server needs to make it available to the
   application by including WWW-Authenticate in the Access-Control-
   Expose-Headers response header list value.

   This authentication scheme is for origin-server authentication only.
   Therefore, this authentication scheme MUST NOT be used with the
   Proxy-Authenticate or Proxy-Authorization header fields.

   Note that the syntax of the Authorization header field for this
   authentication scheme follows the usage of the Bearer scheme defined
   in Section 2.1 of [RFC6750].  While it is not the preferred
   credential syntax of [RFC9110], it is compatible with the general
   authentication framework therein and is used for consistency and
   familiarity with the Bearer scheme.

7.2.  Compatibility with the Bearer Authentication Scheme

   Protected resources simultaneously supporting both the DPoP and
   Bearer schemes need to update how the evaluation process is performed
   for bearer tokens to prevent downgraded usage of a DPoP-bound access
   token.  Specifically, such a protected resource MUST reject a DPoP-
   bound access token received as a bearer token per [RFC6750].

   Section 11.6.1 of [RFC9110] allows a protected resource to indicate
   support for multiple authentication schemes (i.e., Bearer and DPoP)
   with the WWW-Authenticate header field of a 401 (Unauthorized)
   response.

   A protected resource that supports only [RFC6750] and is unaware of
   DPoP would most presumably accept a DPoP-bound access token as a
   bearer token (JWT [RFC7519] says to ignore unrecognized claims,
   Introspection [RFC7662] says that other parameters might be present
   while placing no functional requirements on their presence, and
   [RFC6750] is effectively silent on the content of the access token
   since it relates to validity).  As such, a client can send a DPoP-
   bound access token using the Bearer scheme upon receipt of a WWW-
   Authenticate: Bearer challenge from a protected resource (or it can
   send a DPoP-bound access token if it has prior knowledge of the
   capabilities of the protected resource).  The effect of this likely
   simplifies the logistics of phased upgrades to protected resources in
   their support DPoP or prolonged deployments of protected resources
   with mixed token type support.

   If a protected resource supporting both Bearer and DPoP schemes
   elects to respond with multiple WWW-Authenticate challenges,
   attention should be paid to which challenge(s) should deliver the
   actual error information.  It is RECOMMENDED that the following rules
   be adhered to:

   *  If no authentication information has been included with the
      request, then the challenges SHOULD NOT include an error code or
      other error information, as per Section 3.1 of [RFC6750]
      (Figure 17).

   *  If the mechanism used to attempt authentication could be
      established unambiguously, then the corresponding challenge SHOULD
      be used to deliver error information (Figure 18).

   *  Otherwise, both Bearer and DPoP challenges MAY be used to deliver
      error information (Figure 19).

   The following examples use "\" line wrapping per [RFC8792].

   GET /protectedresource HTTP/1.1
   Host: resource.example.org

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer, DPoP algs="ES256 PS256"

    Figure 17: HTTP 401 Response to a Protected Resource Request without
                               Authentication

   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: Bearer INVALID_TOKEN

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer error="invalid_token", \
       error_description="Invalid token", DPoP algs="ES256 PS256"

     Figure 18: HTTP 401 Response to a Protected Resource Request with
                           Invalid Authentication

   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: Bearer Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
   Authorization: DPoP Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU

   HTTP/1.1 400 Bad Request
   WWW-Authenticate: Bearer error="invalid_request", \
    error_description="Multiple methods used to include access token", \
    DPoP algs="ES256 PS256", error="invalid_request", \
    error_description="Multiple methods used to include access token"

     Figure 19: HTTP 400 Response to a Protected Resource Request with
                          Ambiguous Authentication

7.3.  Client Considerations

   Authorization including a DPoP proof may not be idempotent (depending
   on server enforcement of jti, iat, and nonce claims).  Consequently,
   all previously idempotent requests for protected resources that were
   previously idempotent may no longer be idempotent.  It is RECOMMENDED
   that clients generate a unique DPoP proof, even when retrying
   idempotent requests in response to HTTP errors generally understood
   as transient.

   Clients that encounter frequent network errors may experience
   additional challenges when interacting with servers with stricter
   nonce validation implementations.

8.  Authorization Server-Provided Nonce

   This section specifies a mechanism using opaque nonces provided by
   the server that can be used to limit the lifetime of DPoP proofs.
   Without employing such a mechanism, a malicious party controlling the
   client (potentially including the end-user) can create DPoP proofs
   for use arbitrarily far in the future.

   Including a nonce value contributed by the authorization server in
   the DPoP proof MAY be used by authorization servers to limit the
   lifetime of DPoP proofs.  The server determines when to issue a new
   DPoP nonce challenge and if it is needed, thereby requiring the use
   of the nonce value in subsequent DPoP proofs.  The logic through
   which the server makes that determination is out of scope of this
   document.

   An authorization server MAY supply a nonce value to be included by
   the client in DPoP proofs sent.  In this case, the authorization
   server responds to requests that do not include a nonce with an HTTP
   400 (Bad Request) error response per Section 5.2 of [RFC6749] using
   use_dpop_nonce as the error code value.  The authorization server
   includes a DPoP-Nonce HTTP header in the response supplying a nonce
   value to be used when sending the subsequent request.  Nonce values
   MUST be unpredictable.  This same error code is used when supplying a
   new nonce value when there was a nonce mismatch.  The client will
   typically retry the request with the new nonce value supplied upon
   receiving a use_dpop_nonce error with an accompanying nonce value.

   For example, in response to a token request without a nonce when the
   authorization server requires one, the authorization server can
   respond with a DPoP-Nonce value such as the following to provide a
   nonce value to include in the DPoP proof:

    HTTP/1.1 400 Bad Request
    DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v

    {
     "error": "use_dpop_nonce",
     "error_description":
       "Authorization server requires nonce in DPoP proof"
    }

      Figure 20: HTTP 400 Response to a Token Request without a Nonce

   Other HTTP headers and JSON fields MAY also be included in the error
   response, but there MUST NOT be more than one DPoP-Nonce header.

   Upon receiving the nonce, the client is expected to retry its token
   request using a DPoP proof including the supplied nonce value in the
   nonce claim of the DPoP proof.  An example unencoded JWT payload of
   such a DPoP proof including a nonce is shown below.

    {
     "jti": "-BwC3ESc6acc2lTc",
     "htm": "POST",
     "htu": "https://server.example.com/token",
     "iat": 1562262616,
     "nonce": "eyJ7S_zG.eyJH0-Z.HX4w-7v"
    }

           Figure 21: DPoP Proof Payload including a Nonce Value

   The nonce is opaque to the client.

   If the nonce claim in the DPoP proof does not exactly match a nonce
   recently supplied by the authorization server to the client, the
   authorization server MUST reject the request.  The rejection response
   MAY include a DPoP-Nonce HTTP header providing a new nonce value to
   use for subsequent requests.

   The intent is that clients need to keep only one nonce value and
   servers need to keep a window of recent nonces.  That said, transient
   circumstances may arise in which the stored nonce values for the
   server and the client differ.  However, this situation is self-
   correcting.  With any rejection message, the server can send the
   client the nonce value it wants to use to the client, and the client
   can store that nonce value and retry the request with it.  Even if
   the client and/or server discard their stored nonce values, that
   situation is also self-correcting because new nonce values can be
   communicated when responding to or retrying failed requests.

   Note that browser-based client applications using CORS [WHATWG.Fetch]
   only have access to CORS-safelisted response HTTP headers by default.
   In order for the application to obtain and use the DPoP-Nonce HTTP
   response header value, the server needs to make it available to the
   application by including DPoP-Nonce in the Access-Control-Expose-
   Headers response header list value.

8.1.  Nonce Syntax

   The nonce syntax in ABNF as used by [RFC6749] (which is the same as
   the scope-token syntax) is shown below.

   nonce = 1*NQCHAR

                           Figure 22: Nonce ABNF

8.2.  Providing a New Nonce Value

   It is up to the authorization server when to supply a new nonce value
   for the client to use.  The client is expected to use the existing
   supplied nonce in DPoP proofs until the server supplies a new nonce
   value.

   The authorization server MAY supply the new nonce in the same way
   that the initial one was supplied: by using a DPoP-Nonce HTTP header
   in the response.  The DPoP-Nonce HTTP header field uses the nonce
   syntax defined in Section 8.1.  Each time this happens, it requires
   an extra protocol round trip.

   A more efficient manner of supplying a new nonce value is also
   defined by including a DPoP-Nonce HTTP header in the HTTP 200 (OK)
   response from the previous request.  The client MUST use the new
   nonce value supplied for the next token request and for all
   subsequent token requests until the authorization server supplies a
   new nonce.

   Responses that include the DPoP-Nonce HTTP header should be
   uncacheable (e.g., using Cache-Control: no-store in response to a GET
   request) to prevent the response from being used to serve a
   subsequent request and a stale nonce value from being used as a
   result.

   An example 200 OK response providing a new nonce value is shown
   below.

    HTTP/1.1 200 OK
    Cache-Control: no-store
    DPoP-Nonce: eyJ7S_zG.eyJbYu3.xQmBj-1

        Figure 23: HTTP 200 Response Providing the Next Nonce Value

9.  Resource Server-Provided Nonce

   Resource servers can also choose to provide a nonce value to be
   included in DPoP proofs sent to them.  They provide the nonce using
   the DPoP-Nonce header in the same way that authorization servers do
   as described in Sections 8 and 8.2.  The error signaling is performed
   as described in Section 7.1.  Resource servers use an HTTP 401
   (Unauthorized) error code with an accompanying WWW-Authenticate: DPoP
   value and DPoP-Nonce value to accomplish this.

   For example, in response to a resource request without a nonce when
   the resource server requires one, the resource server can respond
   with a DPoP-Nonce value such as the following to provide a nonce
   value to include in the DPoP proof.  The example below uses "\" line
   wrapping per [RFC8792].

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP error="use_dpop_nonce", \
      error_description="Resource server requires nonce in DPoP proof"
    DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v

     Figure 24: HTTP 401 Response to a Resource Request without a Nonce

   Note that the nonces provided by an authorization server and a
   resource server are different and should not be confused with one
   another since nonces will be only accepted by the server that issued
   them.  Likewise, should a client use multiple authorization servers
   and/or resource servers, a nonce issued by any of them should be used
   only at the issuing server.  Developers should also be careful to not
   confuse DPoP nonces with the OpenID Connect [OpenID.Core] ID Token
   nonce.

10.  Authorization Code Binding to a DPoP Key

   Binding the authorization code issued to the client's proof-of-
   possession key can enable end-to-end binding of the entire
   authorization flow.  This specification defines the dpop_jkt
   authorization request parameter for this purpose.  The value of the
   dpop_jkt authorization request parameter is the JWK Thumbprint
   [RFC7638] of the proof-of-possession public key using the SHA-256
   hash function, which is the same value as used for the jkt
   confirmation method defined in Section 6.1.

   When a token request is received, the authorization server computes
   the JWK Thumbprint of the proof-of-possession public key in the DPoP
   proof and verifies that it matches the dpop_jkt parameter value in
   the authorization request.  If they do not match, it MUST reject the
   request.

   An example authorization request using the dpop_jkt authorization
   request parameter is shown below and uses "\" line wrapping per
   [RFC8792].

   GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz\
       &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb\
       &code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM\
       &code_challenge_method=S256\
       &dpop_jkt=NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs HTTP/1.1
   Host: server.example.com

       Figure 25: Authorization Request Using the dpop_jkt Parameter

   Use of the dpop_jkt authorization request parameter is OPTIONAL.
   Note that the dpop_jkt authorization request parameter MAY also be
   used in combination with Proof Key for Code Exchange (PKCE)
   [RFC7636], which is recommended by [SECURITY-TOPICS] as a
   countermeasure to authorization code injection.  The dpop_jkt
   authorization request parameter only provides similar protections
   when a unique DPoP key is used for each authorization request.

10.1.  DPoP with Pushed Authorization Requests

   When Pushed Authorization Requests (PARs) [RFC9126] are used in
   conjunction with DPoP, there are two ways in which the DPoP key can
   be communicated in the PAR request:

   *  The dpop_jkt parameter can be used as described in Section 10 to
      bind the issued authorization code to a specific key.  In this
      case, dpop_jkt MUST be included alongside other authorization
      request parameters in the POST body of the PAR request.
   *  Alternatively, the DPoP header can be added to the PAR request.
      In this case, the authorization server MUST check the provided
      DPoP proof JWT as defined in Section 4.3.  It MUST further behave
      as if the contained public key's thumbprint was provided using
      dpop_jkt, i.e., reject the subsequent token request unless a DPoP
      proof for the same key is provided.  This can help to simplify the
      implementation of the client, as it can "blindly" attach the DPoP
      header to all requests to the authorization server regardless of
      the type of request.  Additionally, it provides a stronger
      binding, as the DPoP header contains a proof of possession of the
      private key.

   Both mechanisms MUST be supported by an authorization server that
   supports PAR and DPoP.  If both mechanisms are used at the same time,
   the authorization server MUST reject the request if the JWK
   Thumbprint in dpop_jkt does not match the public key in the DPoP
   header.

   Allowing both mechanisms ensures that clients using dpop_jkt do not
   need to distinguish between front-channel and pushed authorization
   requests, and at the same time, clients that only have one code path
   for protecting all calls to authorization server endpoints do not
   need to distinguish between requests to the PAR endpoint and the
   token endpoint.

11.  Security Considerations

   In DPoP, the prevention of token replay at a different endpoint (see
   Section 2) is achieved through authentication of the server per
   [RFC6125] and the binding of the DPoP proof to a certain URI and HTTP
   method.  However, DPoP has a somewhat different nature of protection
   than TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth
   Token Binding [TOKEN-BINDING] (see also Sections 11.1 and 11.7).
   TLS-based mechanisms can leverage a tight integration between the TLS
   layer and the application layer to achieve strong message integrity,
   authenticity, and replay protection.

11.1.  DPoP Proof Replay

   If an adversary is able to get hold of a DPoP proof JWT, the
   adversary could replay that token at the same endpoint (the HTTP
   endpoint and method are enforced via the respective claims in the
   JWTs).  To limit this, servers MUST only accept DPoP proofs for a
   limited time after their creation (preferably only for a relatively
   brief period on the order of seconds or minutes).

   In the context of the target URI, servers can store the jti value of
   each DPoP proof for the time window in which the respective DPoP
   proof JWT would be accepted to prevent multiple uses of the same DPoP
   proof.  HTTP requests to the same URI for which the jti value has
   been seen before would be declined.  When strictly enforced, such a
   single-use check provides a very strong protection against DPoP proof
   replay, but it may not always be feasible in practice, e.g., when
   multiple servers behind a single endpoint have no shared state.

   In order to guard against memory exhaustion attacks, a server that is
   tracking jti values should reject DPoP proof JWTs with unnecessarily
   large jti values or store only a hash thereof.

   Note: To accommodate for clock offsets, the server MAY accept DPoP
   proofs that carry an iat time in the reasonably near future (on the
   order of seconds or minutes).  Because clock skews between servers
   and clients may be large, servers MAY limit DPoP proof lifetimes by
   using server-provided nonce values containing the time at the server
   rather than comparing the client-supplied iat time to the time at the
   server.  Nonces created in this way yield the same result even in the
   face of arbitrarily large clock skews.

   Server-provided nonces are an effective means for further reducing
   the chances for successful DPoP proof replay.  Unlike cryptographic
   nonces, it is acceptable for clients to use the same nonce multiple
   times and for the server to accept the same nonce multiple times.  As
   long as the jti value is tracked and duplicates are rejected for the
   lifetime of the nonce, there is no additional risk of token replay.

11.2.  DPoP Proof Pre-generation

   An attacker in control of the client can pre-generate DPoP proofs for
   specific endpoints arbitrarily far into the future by choosing the
   iat value in the DPoP proof to be signed by the proof-of-possession
   key.  Note that one such attacker is the person who is the legitimate
   user of the client.  The user may pre-generate DPoP proofs to
   exfiltrate from the machine possessing the proof-of-possession key
   upon which they were generated and copy them to another machine that
   does not possess the key.  For instance, a bank employee might pre-
   generate DPoP proofs on a bank computer and then copy them to another
   machine for use in the future, thereby bypassing bank audit controls.
   When DPoP proofs can be pre-generated and exfiltrated, all that is
   actually being proved in DPoP protocol interactions is possession of
   a DPoP proof -- not of the proof-of-possession key.

   Use of server-provided nonce values that are not predictable by
   attackers can prevent this attack.  By providing new nonce values at
   times of its choosing, the server can limit the lifetime of DPoP
   proofs, preventing pre-generated DPoP proofs from being used.  When
   server-provided nonces are used, possession of the proof-of-
   possession key is being demonstrated -- not just possession of a DPoP
   proof.

   The ath claim limits the use of pre-generated DPoP proofs to the
   lifetime of the access token.  Deployments that do not utilize the
   nonce mechanism SHOULD NOT issue long-lived DPoP constrained access
   tokens, preferring instead to use short-lived access tokens and
   refresh tokens.  Whilst an attacker could pre-generate DPoP proofs to
   use the refresh token to obtain a new access token, they would be
   unable to realistically pre-generate DPoP proofs to use a newly
   issued access token.

11.3.  DPoP Nonce Downgrade

   A server MUST NOT accept any DPoP proofs without the nonce claim when
   a DPoP nonce has been provided to the client.

11.4.  Untrusted Code in the Client Context

   If an adversary is able to run code in the client's execution
   context, the security of DPoP is no longer guaranteed.  Common issues
   in web applications leading to the execution of untrusted code are
   XSS and remote code inclusion attacks.

   If the private key used for DPoP is stored in such a way that it
   cannot be exported, e.g., in a hardware or software security module,
   the adversary cannot exfiltrate the key and use it to create
   arbitrary DPoP proofs.  The adversary can, however, create new DPoP
   proofs as long as the client is online and uses these proofs
   (together with the respective tokens) either on the victim's device
   or on a device under the attacker's control to send arbitrary
   requests that will be accepted by servers.

   To send requests even when the client is offline, an adversary can
   try to pre-compute DPoP proofs using timestamps in the future and
   exfiltrate these together with the access or refresh token.

   An adversary might further try to associate tokens issued from the
   token endpoint with a key pair under the adversary's control.  One
   way to achieve this is to modify existing code, e.g., by replacing
   cryptographic APIs.  Another way is to launch a new authorization
   grant between the client and the authorization server in an iframe.
   This grant needs to be "silent", i.e., not require interaction with
   the user.  With code running in the client's origin, the adversary
   has access to the resulting authorization code and can use it to
   associate their own DPoP keys with the tokens returned from the token
   endpoint.  The adversary is then able to use the resulting tokens on
   their own device even if the client is offline.

   Therefore, protecting clients against the execution of untrusted code
   is extremely important even if DPoP is used.  Besides secure coding
   practices, Content Security Policy [W3C.CSP] can be used as a second
   layer of defense against XSS.

11.5.  Signed JWT Swapping

   Servers accepting signed DPoP proof JWTs MUST verify that the typ
   field is dpop+jwt in the headers of the JWTs to ensure that
   adversaries cannot use JWTs created for other purposes.

11.6.  Signature Algorithms

   Implementers MUST ensure that only asymmetric digital signature
   algorithms (such as ES256) that are deemed secure can be used for
   signing DPoP proofs.  In particular, the algorithm none MUST NOT be
   allowed.

11.7.  Request Integrity

   DPoP does not ensure the integrity of the payload or headers of
   requests.  The DPoP proof only contains claims for the HTTP URI and
   method, but not the message body or general request headers, for
   example.

   This is an intentional design decision intended to keep DPoP simple
   to use, but as described, it makes DPoP potentially susceptible to
   replay attacks where an attacker is able to modify message contents
   and headers.  In many setups, the message integrity and
   confidentiality provided by TLS is sufficient to provide a good level
   of protection.

   Note: While signatures covering other parts of requests are out of
   the scope of this specification, additional information to be signed
   can be added into DPoP proofs.

11.8.  Access Token and Public Key Binding

   The binding of the access token to the DPoP public key, as specified
   in Section 6, uses a cryptographic hash of the JWK representation of
   the public key.  It relies on the hash function having sufficient
   second-preimage resistance so as to make it computationally
   infeasible to find or create another key that produces to the same
   hash output value.  The SHA-256 hash function was used because it
   meets the aforementioned requirement while being widely available.

   Similarly, the binding of the DPoP proof to the access token uses a
   hash of that access token as the value of the ath claim in the DPoP
   proof (see Section 4.2).  This relies on the value of the hash being
   sufficiently unique so as to reliably identify the access token.  The
   collision resistance of SHA-256 meets that requirement.

11.9.  Authorization Code and Public Key Binding

   Cryptographic binding of the authorization code to the DPoP public
   key is specified in Section 10.  This binding prevents attacks in
   which the attacker captures the authorization code and creates a DPoP
   proof using a proof-of-possession key other than the one held by the
   client and redeems the authorization code using that DPoP proof.  By
   ensuring end to end that only the client's DPoP key can be used, this
   prevents captured authorization codes from being exfiltrated and used
   at locations other than the one to which the authorization code was
   issued.

   Authorization codes can, for instance, be harvested by attackers from
   places where the HTTP messages containing them are logged.  Even when
   efforts are made to make authorization codes one-time-use, in
   practice, there is often a time window during which attackers can
   replay them.  For instance, when authorization servers are
   implemented as scalable replicated services, some replicas may
   temporarily not yet have the information needed to prevent replay.
   DPoP binding of the authorization code solves these problems.

   If an authorization server does not (or cannot) strictly enforce the
   single-use limitation for authorization codes and an attacker can
   access the authorization code (and if PKCE is used, the
   code_verifier), the attacker can create a forged token request,
   binding the resulting token to an attacker-controlled key.  For
   example, using XSS, attackers might obtain access to the
   authorization code and PKCE parameters.  Use of the dpop_jkt
   parameter prevents this attack.

   The binding of the authorization code to the DPoP public key uses a
   JWK Thumbprint of the public key, just as the access token binding
   does.  The same JWK Thumbprint considerations apply.

11.10.  Hash Algorithm Agility

   The jkt confirmation method member, the ath JWT claim, and the
   dpop_jkt authorization request parameter defined herein all use the
   output of the SHA-256 hash function as their value.  The use of a
   single hash function by this specification was intentional and aimed
   at simplicity and avoidance of potential security and
   interoperability issues arising from common mistakes implementing and
   deploying parameterized algorithm agility schemes.  However, the use
   of a different hash function is not precluded if future circumstances
   change and make SHA-256 insufficient for the requirements of this
   specification.  Should that need arise, it is expected that a short
   specification will be produced that updates this one.  Using the
   output of an appropriate hash function as the value, that
   specification will likely define a new confirmation method member, a
   new JWT claim, and a new authorization request parameter.  These
   items will be used in place of, or alongside, their respective
   counterparts in the same message structures and flows of the larger
   protocol defined by this specification.

11.11.  Binding to Client Identity

   In cases where DPoP is used with client authentication, it is only
   bound to authentication by being coincident in the same TLS tunnel.
   Since the DPoP proof is not directly bound to the authentication
   cryptographically, it's possible that the authentication or the DPoP
   messages were copied into the tunnel.  While including the URI in the
   DPoP can partially mitigate some of this risk, modifying the
   authentication mechanism to provide cryptographic binding between
   authentication and DPoP could provide better protection.  However,
   providing additional binding with authentication through the
   modification of authentication mechanisms or other means is beyond
   the scope of this specification.

12.  IANA Considerations

12.1.  OAuth Access Token Types Registration

   IANA has registered the following access token type in the "OAuth
   Access Token Types" registry [IANA.OAuth.Params] established by
   [RFC6749].

   Name:  DPoP

   Additional Token Endpoint Response Parameters:  (none)

   HTTP Authentication Scheme(s):  DPoP

   Change Controller:  IETF

   Reference:  RFC 9449

12.2.  OAuth Extensions Error Registration

   IANA has registered the following error values in the "OAuth
   Extensions Error" registry [IANA.OAuth.Params] established by
   [RFC6749].

   Invalid DPoP proof:

      Name:  invalid_dpop_proof

      Usage Location:  token error response, resource access error
         response

      Protocol Extension:  Demonstrating Proof of Possession (DPoP)

      Change Controller:  IETF

      Reference:  RFC 9449

   Use DPoP nonce:

      Name:  use_dpop_nonce

      Usage Location:  token error response, resource access error
         response

      Protocol Extension:  Demonstrating Proof of Possession (DPoP)

      Change Controller:  IETF

      Reference:  RFC 9449

12.3.  OAuth Parameters Registration

   IANA has registered the following authorization request parameter in
   the "OAuth Parameters" registry [IANA.OAuth.Params] established by
   [RFC6749].

   Name:  dpop_jkt

   Parameter Usage Location:  authorization request

   Change Controller:  IETF

   Reference:  Section 10 of RFC 9449

12.4.  HTTP Authentication Schemes Registration

   IANA has registered the following scheme in the "HTTP Authentication
   Schemes" registry [IANA.HTTP.AuthSchemes] established by [RFC9110],
   Section 16.4.1.

   Authentication Scheme Name:  DPoP

   Reference:  Section 7.1 of RFC 9449

12.5.  Media Type Registration

   IANA has registered the application/dpop+jwt media type [RFC2046] in
   the "Media Types" registry [IANA.MediaTypes] in the manner described
   in [RFC6838], which is used to indicate that the content is a DPoP
   JWT.

   Type name:  application

   Subtype name:  dpop+jwt

   Required parameters:  n/a

   Optional parameters:  n/a

   Encoding considerations:  binary.  A DPoP JWT is a JWT; JWT values
      are encoded as a series of base64url-encoded values (some of which
      may be the empty string) separated by period ('.') characters.

   Security considerations:  See Section 11 of RFC 9449

   Interoperability considerations:  n/a

   Published specification:  RFC 9449

   Applications that use this media type:  Applications using RFC 9449
      for application-level proof of possession

   Fragment identifier considerations:  n/a

   Additional information:

      File extension(s):  n/a
      Macintosh file type code(s):  n/a

   Person & email address to contact for further information:  Michael
      B. Jones, michael_b_jones@hotmail.com

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  Michael B. Jones, michael_b_jones@hotmail.com

   Change controller:  IETF

12.6.  JWT Confirmation Methods Registration

   IANA has registered the following JWT cnf member value in the "JWT
   Confirmation Methods" registry [IANA.JWT] established by [RFC7800].

   Confirmation Method Value:  jkt

   Confirmation Method Description:  JWK SHA-256 Thumbprint

   Change Controller:  IETF

   Reference:  Section 6 of RFC 9449

12.7.  JSON Web Token Claims Registration

   IANA has registered the following Claims in the "JSON Web Token
   Claims" registry [IANA.JWT] established by [RFC7519].

   HTTP method:

      Claim Name:  htm

      Claim Description:  The HTTP method of the request

      Change Controller:  IETF

      Reference:  Section 4.2 of RFC 9449

   HTTP URI:

      Claim Name:  htu

      Claim Description:  The HTTP URI of the request (without query and
         fragment parts)

      Change Controller:  IETF

      Reference:  Section 4.2 of RFC 9449

   Access token hash:

      Claim Name:  ath

      Claim Description:  The base64url-encoded SHA-256 hash of the
         ASCII encoding of the associated access token's value

      Change Controller:  IETF

      Reference:  Section 4.2 of RFC 9449

12.7.1.  "nonce" Registration Update

   The Internet Security Glossary [RFC4949] provides a useful definition
   of nonce as a random or non-repeating value that is included in data
   exchanged by a protocol, usually for the purpose of guaranteeing
   liveness and thus detecting and protecting against replay attacks.

   However, the initial registration of the nonce claim by [OpenID.Core]
   used language that was contextually specific to that application,
   which was potentially limiting to its general applicability.

   Therefore, IANA has updated the entry for nonce in the "JSON Web
   Token Claims" registry [IANA.JWT] with an expanded definition to
   reflect that the claim can be used appropriately in other contexts
   and with the addition of this document as a reference, as follows.

   Claim Name:  nonce

   Claim Description:  Value used to associate a Client session with an
      ID Token (MAY also be used for nonce values in other applications
      of JWTs)

   Change Controller:  OpenID Foundation Artifact Binding Working Group,
      openid-specs-ab@lists.openid.net

   Specification Document(s):  Section 2 of [OpenID.Core] and RFC 9449

12.8.  Hypertext Transfer Protocol (HTTP) Field Name Registration

   IANA has registered the following HTTP header fields, as specified by
   this document, in the "Hypertext Transfer Protocol (HTTP) Field Name
   Registry" [IANA.HTTP.Fields] established by [RFC9110]:

   DPoP:

      Field Name:  DPoP

      Status:  permanent

      Reference:  RFC 9449

   DPoP-Nonce:

      Field Name:  DPoP-Nonce

      Status:  permanent

      Reference:  RFC 9449

12.9.  OAuth Authorization Server Metadata Registration

   IANA has registered the following value in the "OAuth Authorization
   Server Metadata" registry [IANA.OAuth.Params] established by
   [RFC8414].

   Metadata Name:  dpop_signing_alg_values_supported

   Metadata Description:  JSON array containing a list of the JWS
      algorithms supported for DPoP proof JWTs

   Change Controller:  IETF

   Reference:  Section 5.1 of RFC 9449

12.10.  OAuth Dynamic Client Registration Metadata

   IANA has registered the following value in the IANA "OAuth Dynamic
   Client Registration Metadata" registry [IANA.OAuth.Params]
   established by [RFC7591].

   Client Metadata Name:  dpop_bound_access_tokens

   Client Metadata Description:  Boolean value specifying whether the
      client always uses DPoP for token requests

   Change Controller:  IETF

   Reference:  Section 5.2 of RFC 9449

13.  References

13.1.  Normative References

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

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

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

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

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

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <https://www.rfc-editor.org/info/rfc7638>.

   [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,
              <https://www.rfc-editor.org/info/rfc7800>.

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

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.

13.2.  Informative References

   [BREACH]   CVE, "CVE-2013-3587", <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2013-3587>.

   [Cloudbleed]
              Graham-Cumming, J., "Incident report on memory leak caused
              by Cloudflare parser bug", February 2017,
              <https://blog.cloudflare.com/incident-report-on-memory-
              leak-caused-by-cloudflare-parser-bug/>.

   [CRIME]    CVE, "CVE-2012-4929", <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=cve-2012-4929>.

   [GitHub.Tokens]
              Hanley, M., "Security alert: Attack campaign involving
              stolen OAuth user tokens issued to two third-party
              integrators", April 2022, <https://github.blog/2022-04-15-
              security-alert-stolen-oauth-user-tokens/>.

   [Heartbleed]
              "CVE-2014-0160", <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=cve-2014-0160>.

   [IANA.HTTP.AuthSchemes]
              IANA, "Hypertext Transfer Protocol (HTTP) Authentication
              Scheme Registry",
              <https://www.iana.org/assignments/http-authschemes/>.

   [IANA.HTTP.Fields]
              IANA, "Hypertext Transfer Protocol (HTTP) Field Name
              Registry",
              <https://www.iana.org/assignments/http-fields/>.

   [IANA.JOSE.ALGS]
              IANA, "JSON Web Signature and Encryption Algorithms",
              <https://www.iana.org/assignments/jose/>.

   [IANA.JWT] IANA, "JSON Web Token Claims",
              <https://www.iana.org/assignments/jwt/>.

   [IANA.MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types/>.

   [IANA.OAuth.Params]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters/>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

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

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

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

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

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

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

   [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>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

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

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

   [SECURITY-TOPICS]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-23, 5 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-23>.

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

   [W3C.CSP]  West, M., "Content Security Policy Level 3", W3C Working
              Draft, July 2023, <https://www.w3.org/TR/CSP3/>.

   [W3C.WebCryptoAPI]
              Watson, M., "Web Cryptography API", W3C Recommendation,
              January 2017,
              <https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126>.

   [WHATWG.Fetch]
              WHATWG, "Fetch Living Standard", July 2023,
              <https://fetch.spec.whatwg.org/>.

Acknowledgements

   We would like to thank Brock Allen, Annabelle Backman, Dominick
   Baier, Spencer Balogh, Vittorio Bertocci, Jeff Corrigan, Domingos
   Creado, Philippe De Ryck, Andrii Deinega, William Denniss, Vladimir
   Dzhuvinov, Mike Engan, Nikos Fotiou, Mark Haine, Dick Hardt, Joseph
   Heenan, Bjorn Hjelm, Jacob Ideskog, Jared Jennings, Benjamin Kaduk,
   Pieter Kasselman, Neil Madden, Rohan Mahy, Karsten Meyer zu
   Selhausen, Nicolas Mora, Steinar Noem, Mark Nottingham, Rob Otto,
   Aaron Parecki, Michael Peck, Roberto Polli, Paul Querna, Justin
   Richer, Joseph Salowey, Rifaat Shekh-Yusef, Filip Skokan, Dmitry
   Telegin, Dave Tonge, Jim Willeke, and others for their valuable
   input, feedback, and general support of this work.

   This document originated from discussions at the 4th OAuth Security
   Workshop in Stuttgart, Germany.  We thank the organizers of this
   workshop (Ralf Küsters and Guido Schmitz).

Authors' Addresses

   Daniel Fett
   Authlete
   Email: mail@danielfett.de


   Brian Campbell
   Ping Identity
   Email: bcampbell@pingidentity.com


   John Bradley
   Yubico
   Email: ve7jtb@ve7jtb.com


   Torsten Lodderstedt
   Tuconic
   Email: torsten@lodderstedt.net


   Michael Jones
   Self-Issued Consulting
   Email: michael_b_jones@hotmail.com
   URI:   https://self-issued.info/


   David Waite
   Ping Identity
   Email: david@alkaline-solutions.com
