| Internet-Draft | OAuth DPoP | March 2022 |
| Fett, et al. | Expires 2 September 2022 | [Page] |
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.¶
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DPoP (for Demonstrating Proof-of-Possession at the Application Layer)
is an application-level mechanism for
sender-constraining OAuth 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 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 can prove possession of the private part of the
key pair.¶
The mechanism described 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 [I-D.ietf-oauth-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 a Single Page Application (SPA) running in a web browser. Native applications installed and run on a user's device are another example well positioned to benefit from DPoP-bound tokens to guard against 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).¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This specification uses the terms "access token", "refresh token", "authorization server", "resource server", "authorization endpoint", "authorization request", "authorization response", "token endpoint", "grant type", "access token request", "access token response", and "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].¶
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 (CRIME, BREACH, Heartbleed, and the Cloudflare parser bug are some examples). There have also been numerous published token theft attacks on OAuth implementations themselves. 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 [I-D.ietf-oauth-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. Properly audience restricting access tokens can prevent such misuse, however, doing so in practice has proven to be prohibitively cumbersome for many deployments (even 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 for obtaining new access tokens. If the private key is non-extractable (as is possible with [W3C.WebCryptoAPI]), DPoP renders exfiltrated tokens alone unusable.¶
XXS 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 and thus 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 together 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.¶
The main data structure introduced by this specification is a DPoP proof JWT, described in detail below, which is sent as a header in an HTTP request. 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 ---------------| | | | +---------------+ +--------+
The basic steps of an OAuth flow with DPoP (without the optional nonce) are shown in Figure 1:¶
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 native applications) that do not use client authentication. Nonetheless, DPoP
is designed such that it is compatible with private_key_jwt and all
other client authentication methods.¶
DPoP does not directly ensure message integrity but relies on the TLS layer for that purpose. See Section 11 for details.¶
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 for 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.¶
A DPoP proof is included in an HTTP request using the following message header field.¶
DPoPFigure 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
DPoP header
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.¶
A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS, [RFC7515]) with a private key chosen by the client (see below). The header of a DPoP JWT contains at least the following parameters:¶
typ: type header, value dpop+jwt (REQUIRED).¶
alg: a digital signature algorithm identifier as per [RFC7518]
(REQUIRED). MUST NOT be none or an identifier for a symmetric
algorithm (MAC).¶
jwk: representing the public key chosen by the client, in JWK
format, as defined in Section 4.1.3 of [RFC7515] (REQUIRED).
MUST NOT contain the private key.¶
The payload of a DPoP proof contains at least the following claims:¶
jti: Unique identifier for the DPoP proof JWT (REQUIRED).
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 for the request to which the JWT is
attached, as defined in [RFC7231] (REQUIRED).¶
htu: The HTTP URI used for the request, without query and
fragment parts (REQUIRED).¶
iat: Time at which the JWT was created (REQUIRED).¶
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 (REQUIRED).
The value MUST be the result of a base64url encoding (with no padding) the SHA-256
hash of the ASCII encoding of the associated access token's value.¶
A DPoP proof MAY contain other headers 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 JOSE 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
}
DPoP proof
Of the HTTP content in the request, only the HTTP method and URI are included in the DPoP JWT, and therefore only these 2 headers of the request are covered by the DPoP proof and its signature. 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).¶
To check if a string that was received as part of an HTTP Request is a valid DPoP proof, the receiving server MUST ensure that¶
DPoP header in the request,¶
typ field in the header has the value dpop+jwt,¶
none, is supported by the
application, and is deemed secure,¶
jwk
header of the JWT,¶
htm claim matches the HTTP method value of the HTTP
request in which the JWT was received,¶
htu claim matches the HTTPS URI value for the HTTP
request in which the JWT was received, ignoring any query and
fragment parts,¶
nonce claim matches the server-provided nonce value,¶
jti value has not previously been received at the same resource
during that time period (see Section 11.1),¶
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.¶
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.¶
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 (including, for example,
the common authorization_code and refresh_token grant types but also extension grants
such as the JWT authorization grant [RFC7523]). The HTTPS request shown in
Figure 4 illustrates such an access
token request using an authorization code grant with a DPoP proof JWT
in the DPoP header (extra line breaks and whitespace for display purposes only).¶
POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded;charset=UTF-8 DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg 4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg grant_type=authorization_code &code=SplxlOBeZQQYbYS6WxSbIA &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb &code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-
The DPoP HTTP header 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 5 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"
}
The example response in Figure 5 includes a refresh token which 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 the Figure 6 example
(extra line breaks and whitespace for display purposes only).¶
POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded;charset=UTF-8 DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia WF0IjoxNTYyMjY1Mjk2fQ.pAqut2IRDm_De6PR93SYmGBPXpwrAk90e8cP2hjiaG5Qs GSuKDYW7_X620BxqhvYC8ynrrvZLTk41mSRroapUA grant_type=refresh_token &refresh_token=Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g
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. The server both produces and validates the refresh tokens that it issues so there is no interoperability consideration in the specific details of the binding.¶
An authorization server MAY elect to issue access tokens which 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 a client receives a different token_type value than DPoP in the response, 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 and MAY continue as in a regular OAuth interaction otherwise.¶
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 ID 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.¶
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_supportedalg values supported
by the authorization server for DPoP proof JWTs.¶
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_tokensfalse.¶
If true, the authorization server MUST reject token requests from this client that do not contain the DPoP header.¶
Resource servers MUST be able to reliably identify whether an access token is bound using DPoP and ascertain sufficient information about the public key to which the token is bound in order to verify the binding with respect to the presented 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 agreement by the authorization server and the protected resource, but are beyond the scope of this specification.¶
Resource servers supporting DPoP MUST ensure that the public key from the DPoP proof matches the public key to which the access token is bound.¶
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.¶
jktjkt 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"}
}
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"}
}
To make use of an access token that is bound to a public key
using DPoP, a client MUST prove possession of the corresponding
private key by providing a DPoP proof in the DPoP request header.
As such, protected resource requests with a DPoP-bound access token
necessarily 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.¶
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 Augmented Backus-Naur Form (ABNF) notation [RFC5234] 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 JOSE 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"
}
DPoP Proof JWT in [Figure 12]
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:¶
DPoP.¶
realm MAY be included to indicate the
scope of protection in the manner described in [RFC7235], Section 2.2.¶
scope authentication parameter MAY be included as defined in
[RFC6750], Section 3.¶
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.¶
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.¶
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).¶
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"
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 tokens.
Specifically, such a protected resource MUST reject an access
token received as a bearer token per [RFC6750], if that token is
determined to be DPoP-bound.¶
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 MAY 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.¶
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 is in control of when to require the use of a new nonce value in subsequent DPoP proofs.¶
Without employing such a mechanism, a malicious party controlling the client (including potentially the end user) can create DPoP proofs for use arbitrarily far in the future. This section specifies how server-provided nonces are used with DPoP.¶
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 not including 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.
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"
}
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:¶
{
"jti": "-BwC3ESc6acc2lTc",
"htm": "POST",
"htu": "https://server.example.com/token",
"iat": 1562262616,
"nonce": "eyJ7S_zG.eyJH0-Z.HX4w-7v"
}
The nonce syntax in ABNF as used by [RFC6749]
(which is the same as the scope-token syntax) is:¶
nonce = 1*NQCHAR
The nonce is opaque to the client.¶
If the nonce claim in the DPoP proof of a token request
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 both clients and servers need to keep only one nonce value for one another. That said, transient circumstances may arise in which the server's and client's stored nonce values differ. However, this situation is self-correcting; with any rejection message, the server can send the client the nonce value that the server wants it to use 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.¶
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
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 same way that authorization servers do.
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:¶
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
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 take care to not confuse DPoP nonces with the OpenID Connect [OpenID.Core] ID Token nonce.¶
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
JSON Web Key (JWK) Thumbprint [RFC7638] of the proof-of-possession public key
using the SHA-256 hash function -
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:¶
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
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 PKCE [RFC7636].¶
In DPoP, the prevention of token replay at a different endpoint (see Section 2) is achieved through the binding of the DPoP proof to a certain URI and HTTP method plus the optional server-provided nonce. DPoP, however, has a somewhat different nature of protection than TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth Token Binding [I-D.ietf-oauth-token-binding] (see also Section 11.1 and Section 11.7). TLS-based mechanisms can leverage a tight integration between the TLS layer and the application layer to achieve a very high level of message integrity with respect to the transport layer to which the token is bound and replay protection in general.¶
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
prevent this, servers MUST only accept DPoP proofs for a limited time
window after their iat time, preferably only for a relatively brief period.¶
Servers SHOULD 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 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.
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.¶
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.¶
A server MUST NOT accept any DPoP proofs without the nonce claim when a DPoP nonce has been provided to the client.¶
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.¶
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.¶
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.¶
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.¶
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.¶
This specification requests registration of the following access token type in the "OAuth Access Token Types" registry [IANA.OAuth.Params] established by [RFC6749].¶
This specification requests registration of the following error values in the "OAuth Extensions Error" registry [IANA.OAuth.Params] established by [RFC6749].¶
Invalid DPoP proof:¶
invalid_dpop_proof¶
Use DPoP nonce:¶
This specification requests registration of the following authorization request parameter in the "OAuth Parameters" registry [IANA.OAuth.Params] established by [RFC6749].¶
This specification requests registration of the following scheme in the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" [RFC7235][IANA.HTTP.AuthSchemes]:¶
DPoP¶
[[
Is a media type registration at [IANA.MediaTypes] necessary for application/dpop+jwt?
There is a +jwt structured syntax suffix registered already at [IANA.MediaType.StructuredSuffix]
by Section 7.2 of [RFC8417], which is maybe sufficient? A full-blown registration
of application/dpop+jwt seems like it'd be overkill.
The dpop+jwt is used in the JWS/JWT typ header for explicit typing of the JWT per
Section 3.11 of [RFC8725] but it is not used anywhere else (such as the Content-Type of HTTP messages).¶
Note that there does seem to be some precedence for [IANA.MediaTypes] registration with
application/at+jwt in [RFC9068],
application/oauth-authz-req+jwt in [RFC9101],
application/secevent+jwt in [RFC8417],
and regular old application/jwt in [RFC7519].
But precedence isn't always right.
]]¶
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].¶
This specification requests registration of the following Claims in the IANA "JSON Web Token Claims" registry [IANA.JWT] established by [RFC7519].¶
HTTP method:¶
htm¶
HTTP URI:¶
htu¶
Access token hash:¶
ath¶
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].¶
This specification requests registration of the following value in the IANA "OAuth Dynamic Client Registration Metadata" registry [IANA.OAuth.Parameters] established by [RFC7591].¶
dpop_bound_access_tokens¶
We would like to thank Annabelle Backman, Dominick Baier, Andrii Deinega, William Denniss, Vladimir Dzhuvinov, Mike Engan, Nikos Fotiou, Mark Haine, Dick Hardt, Joseph Heenan, Bjorn Hjelm, Jared Jennings, Benjamin Kaduk, Pieter Kasselman, Steinar Noem, Neil Madden, Karsten Meyer zu Selhausen, Rob Otto, Aaron Parecki, Michael Peck, Paul Querna, Justin Richer, Filip Skokan, Dmitry Telegin, Dave Tonge, Jim Willeke, Philippe De Ryck, and others (please let us know, if you've been mistakenly omitted) 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 Kusters, Guido Schmitz).¶
[[ To be removed from the final specification ]]¶
-06¶
-05¶
dpop_jkt parameter.¶
dpop_jkt mitigates it.¶
use_dpop_nonce error for missing and mismatched nonce values.¶
400 (Bad Request) errors to supply nonces and resource servers use 401 (Unauthorized) errors to do so.¶
ath and pre-generated proofs to the security considerations.¶
always_uses_dpop client registration metadata parameter.¶
-04¶
invalid_dpop_proof and use_dpop_nonce error codes.¶
realm from the examples, as they added no value.¶
token_type, it has to be DPoP.¶
WWW-Authenticate with a 401.¶
-03¶
ath) claim to the DPoP proof when used in conjunction with the presentation of an access token for protected resource access¶
-02¶
htm claim check¶
-01¶
invalid_dpop_proof error code for DPoP errors in token request¶
dpop_signing_alg_values_supported authorization server metadata¶
-00 [[ Working Group Draft ]]¶
-04¶
-03¶
htm, htu, and jkt rather than http_method, http_uri, and jkt#S256 respectively¶
-02¶
-01¶
-00¶