OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP)

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", "request URI" are imported from [RFC7231].¶

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

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 (line breaks and extra whitespace for display purposes only).¶

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

Note that per [RFC7230] header field names are case-insensitive; so DPoP, DPOP, dpop, etc., are all valid and equivalent header field names. Case is significant in the header field value, however.¶

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: with value `dpop+jwt.¶
• alg: a digital signature algorithm identifier as per [RFC7518]. MUST NOT be none or an identifier for a symmetric algorithm (MAC).¶
• jwk: representing the public key chosen by the client, in JSON Web Key (JWK) [RFC7517] format, as defined in Section 4.1.3 of [RFC7515]. 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 UUID string according to [RFC4122]. The jti can be used by the server for replay detection and prevention, see Section 11.1.¶
• htm: The HTTP method of the request to which the JWT is attached, as defined in [RFC7231].¶
• htu: The HTTP request URI (Section 5.5 of [RFC7230]), 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, 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.¶

A DPoP proof MAY contain other JOSE header parameters or claims as defined by extension, profile, or deployment specific requirements.¶

Figure 3 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 whitespace 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, and therefore only these two message parts are covered by the DPoP proof. The idea is sign just enough of the HTTP data to provide reasonable proof-of-possession with respect to the HTTP request. But that it be a minimal subset of the HTTP data so as 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 that¶

1. that there is not more than one DPoP HTTP request header field,¶
2. the header field value is a 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 an asymmetric digital signature algorithm, is not none, is supported by the application, and is deemed secure,¶
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 HTTPS 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,¶

    1. ensure that the value of the ath claim equals the hash of that access token,¶
    2. confirm that the public key to which the access token is bound matches the public key from the DPoP proof.¶

Servers SHOULD employ Syntax-Based Normalization and Scheme-Based Normalization in accordance with Section 6.2.2. and Section 6.2.3. of [RFC3986] before comparing the htu claim.¶

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 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

Boolean value specifying whether the client always uses DPoP for token requests. If omitted, the default value is false.¶

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

6.1. JWK Thumbprint Confirmation Method

When access tokens are represented as JSON Web Tokens (JWT) [RFC7519], the public key information SHOULD be 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 7 with decoded JWT payload shown in Figure 8 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 in Section 5.¶

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

{
  "sub":"someone@example.com",
  "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 as well as to determine 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 9 and corresponding response in Figure 10 illustrate an introspection exchange for the example DPoP-bound access token that was issued in Figure 5.¶

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": "someone@example.com",
  "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 2 of [RFC7235] using an authentication scheme of DPoP. The syntax of the Authorization header field for the DPoP scheme uses the token68 syntax defined in Section 2.1 of [RFC7235] (repeated below for ease of reference) for credentials. 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 12 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. Following that is Figure 13, which 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 extra whitespace are included for formatting and readability in both examples.¶

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, if the request does not include valid credentials or does not contain an access token sufficient for access, the server can respond with a challenge to the client to provide DPoP authentication information. Such a challenge is made using the 401 (Unauthorized) response status code ([RFC7235], Section 3.1) and the WWW-Authenticate header field ([RFC7235], Section 4.1). 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 [RFC7235], Section 2.2.¶
• 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 Section 3.1 of [RFC6750] 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 subsequent request(s). And 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.¶

For example, in response to a protected resource request without authentication:¶

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

And in response to a protected resource request that was rejected because the confirmation of the DPoP binding in the access token failed:¶

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

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 not the preferred credential syntax of [RFC7235], it is compatible with the general authentication framework therein and was 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 evaluation of bearer tokens is performed 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 4.1 of [RFC7235] 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 as 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 if it has prior such knowledge about 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 even prolonged deployments of protected resources with mixed token type support.¶

8.1. 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. Of course, 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 being used to serve a subsequent request and a stale nonce value being used as a result.¶

An example 200 OK response providing a new nonce value is:¶

 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 (PAR, [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 above 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.¶

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).¶

To prevent multiple uses of the same DPoP proof servers can store, in the context of the request URI, the jti value of each DPoP proof for the time window in which the respective DPoP proof JWT would be accepted and decline HTTP requests to the same URI for which the jti value has been seen before. 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 choose to 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, yielding intended results even in the face of arbitrarily large clock skews.¶

Server-provided nonces are an effective means of preventing DPoP proof replay.¶

11.2. DPoP Proof Pre-Generation

An attacker in control of the client can pre-generate DPoP proofs for use 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 cross-site scripting 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 use 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 cross-site scripting.¶

11.5. Signed JWT Swapping

Servers accepting signed DPoP proof JWTs MUST check the typ field 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 that are deemed secure can be used for signing DPoP proofs. In particular, the algorithm none MUST NOT be allowed.¶

11.7. Message 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, for example, the message body or general request headers.¶

This is an intentional design decision intended to keep DPoP simple to use, but as described, 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.¶

Implementers that have stronger requirements on the integrity of messages are encouraged to either use TLS-based mechanisms or signed requests. TLS-based mechanisms are in particular OAuth Mutual TLS [RFC8705] and OAuth Token Binding [I-D.ietf-oauth-token-binding].¶

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, which is 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. If, in the future, JWK Thumbprints need to be computed using hash function(s) other than SHA-256, it is suggested that an additional related JWT confirmation method member be defined for that purpose, registered in the respective IANA registry, and used in place of the jkt confirmation method defined herein.¶

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. If, in the future, access token digests need be computed using hash function(s) other than SHA-256, it is suggested that an additional related JWT claim be defined for that purpose, registered in the respective IANA registry, and used in place of the ath claim defined herein.¶

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 that 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 that 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 cross-site scripting, 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.¶

12.1. OAuth Access Token Type Registration

This specification requests registration of the following access token type in the "OAuth Access Token Types" registry [IANA.OAuth.Params] established by [RFC6749].¶

• Type name: DPoP¶
• Additional Token Endpoint Response Parameters: (none)¶
• HTTP Authentication Scheme(s): DPoP¶
• Change controller: IESG¶
• Specification document(s): [[ this specification ]]¶

12.2. OAuth Extensions Error Registration

This specification requests registration of 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 error response¶
• Protocol Extension: Demonstrating Proof of Possession (DPoP)¶
• Change controller: IETF¶
• Specification document(s): [[ this specification ]]¶

Use DPoP nonce:¶

• Name: use_dpop_nonce¶
• Usage Location: token error response, resource error response¶
• Protocol Extension: Demonstrating Proof of Possession (DPoP)¶
• Change controller: IETF¶
• Specification document(s): [[ this specification ]]¶

12.3. OAuth Parameters Registration

This specification requests registration of 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: IESG¶
• Reference: [[ {#dpop_jkt} of this specification ]]¶

12.4. HTTP Authentication Scheme Registration

This specification requests registration of the following scheme in the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" [RFC7235][IANA.HTTP.AuthSchemes]:¶

• Authentication Scheme Name: DPoP¶
• Reference: [[ Section 7.1 of this specification ]]¶

12.5. Media Type Registration

This section registers the application/dpop+jwt media type [RFC2046] in the IANA "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 [[ this specification ]]¶
• Interoperability considerations: n/a¶
• Published specification: [[ this specification ]]¶
• Applications that use this media type: Applications using [[ this specification ]] 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, mbj@microsoft.com¶
• Intended usage: COMMON¶
• Restrictions on usage: none¶
• Author: Michael B. Jones, mbj@microsoft.com¶
• Change controller: IETF¶
• Provisional registration? No¶

12.6. JWT Confirmation Methods Registration

This specification requests registration of the following value in the IANA "JWT Confirmation Methods" registry [IANA.JWT] for JWT cnf member values established by [RFC7800].¶

• Confirmation Method Value: jkt¶
• Confirmation Method Description: JWK SHA-256 Thumbprint¶
• Change Controller: IESG¶
• Specification Document(s): [[ Section 6 of this specification ]]¶

12.7. JSON Web Token Claims Registration

This specification requests registration of the following Claims in the IANA "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: IESG¶
• Specification Document(s): [[ Section 4.2 of this specification ]]¶

HTTP URI:¶

• Claim Name: htu¶
• Claim Description: The HTTP URI of the request (without query and fragment parts)¶
• Change Controller: IESG¶
• Specification Document(s): [[ Section 4.2 of this specification ]]¶

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: IESG¶
• Specification Document(s): [[ Section 4.2 of this specification ]]¶

12.8. HTTP Message Header Field Names Registration

This document specifies the following HTTP header fields, registration of which is requested in the "Permanent Message Header Field Names" registry [IANA.Headers] defined in [RFC3864].¶

• Header Field Name: DPoP¶
• Applicable protocol: HTTP¶
• Status: standard¶
• Author/change Controller: IETF¶
• Specification Document(s): [[ this specification ]]¶

12.9. OAuth Authorization Server Metadata Registration

This specification requests registration of the following value in the IANA "OAuth Authorization Server Metadata" registry [IANA.OAuth.Parameters] 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: IESG¶
• Specification Document(s): [[ Section 5.1 of this specification ]]¶

12.10. OAuth Dynamic Client Registration Metadata

This specification requests registration of the following value in the IANA "OAuth Dynamic Client Registration Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591].¶

• Metadata Name: dpop_bound_access_tokens¶
• Metadata Description: Boolean value specifying whether the client always uses DPoP for token requests¶
• Change Controller: IESG¶
• Specification Document(s): [[ Section 5.2 of this specification ]]¶