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.¶

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

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 /
                 "-" / "." / "_" / "~" / "+" / "/" ) *"="

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
}

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.¶

5.1. 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.¶

6.1. 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

{
  "sub":"[email protected]",
  "iss":"https://server.example.com",
  "nbf":1562262611,
  "exp":1562266216,
  "cnf":
  {
    "jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
  }
}

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

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

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

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

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

{
  "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"
}

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 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"

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"

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"

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"

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.¶

8.1. 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

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

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.¶

11.1. 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.¶

12.1. 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, [email protected]¶

Intended usage:

COMMON¶

Restrictions on usage:

none¶

Author:

Michael B. Jones, [email protected]¶

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¶

12.8. 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¶