| Internet-Draft | OAuth DPoP | May 2020 |
| Fett, et al. | Expires 2 November 2020 | [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.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 2 November 2020.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
This document outlines a relatively simple application-level mechanism for
sender-constraining OAuth access and refresh tokens. It enables a client to
demonstrate proof-of-possession of a public/private key pair by including
the DPoP header in an HTTP request. Using that header, an authorization
server is able to bind issued tokens to the public part of the 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 potentially stronger 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.¶
DPoP can be used with public clients to sender-constrain access tokens and refresh tokens. With confidential clients, DPoP can be used in conjunction with any client authentication method to sender-constrain access tokens.¶
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].¶
Under the attacker model defined in [I-D.ietf-oauth-security-topics], the mechanism defined by this specification aims to prevent token replay at a different endpoint.¶
More precisely, if an adversary is able to get hold of an access token or refresh token because it set up a counterfeit authorization server or resource server, the adversary is not able to replay the respective token at another authorization or resource server.¶
The main data structure introduced by this specification is a DPoP proof JWT, described in detail below, 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 a timestamp and some data of the HTTP request to which it is attached.¶
+--------+ +---------------+ | |--(A)-- Token Request ------------------->| | | Client | (DPoP Proof) | Authorization | | | | Server | | |<-(B)-- DPoP-bound Access Token ----------| | | | (token_type=DPoP) +---------------+ | | PoP Refresh Token for public clients | | | | +---------------+ | |--(C)-- DPoP-bound Access Token --------->| | | | (DPoP Proof) | Resource | | | | Server | | |<-(D)-- Protected Resource ---------------| | | | +---------------+ +--------+
The basic steps of an OAuth flow with DPoP are shown in Figure 1:¶
token_type value
DPoP.¶
client_id and associated authentication credentials,
which is a sender-constraining mechanism that is more flexible than binding
to a particular public key.¶
The 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) 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 9 for details.¶
DPoP introduces concept of a DPoP proof JWT, which is used for binding public
keys and proving knowledge about private keys. The DPoP proof JWT is sent with
an HTTP request using the DPoP header field.¶
A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS, [RFC7515]) using 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 [RFC7515] (REQUIRED)¶
The body 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 SHOULD be used by the server for replay
detection and prevention, see Section 9.1.¶
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).¶
Figure 2 shows the JSON header and payload of a DPoP proof JWT.¶
{
"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 header
Note: To keep DPoP simple to implement, only the HTTP method and URI are signed in DPoP proofs. 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 9.4).¶
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¶
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 (case-insensitive),¶
htu claims matches the HTTP URI value for the HTTP
request in which the JWT was received, ignoring any query and
fragment parts,¶
jti value has not been received
previously (see Section 9.1).¶
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 bind a token to a public key in the token request, the client MUST
provide a valid DPoP proof JWT in a DPoP header. The HTTPS request shown
in Figure 3 illustrates the protocol for this (with extra line breaks
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.¶
The authorization server, after checking the validity of the DPoP proof,
associates the access token issued at the token endpoint with the
public key. It then sets token_type to DPoP in the token
response, which signals to the client that the access token was bound to
its DPoP key and can used as described in Section 6.¶
If a refresh token is issued to a public client at the token endpoint and a valid DPoP proof is presented, the refresh token MUST be bound to the public key contained in the header of the DPoP proof JWT.¶
When a DPoP-bound refresh token is used at the token endpoint by a public client, the AS MUST ensure that the DPoP proof contains the same public key as the one the refresh token is bound to. The access token issued MUST be bound to the public key contained in the DPoP proof.¶
To make use of an access token that is bound to a public key
using DPoP, a client MUST prove the possession of the corresponding
private key by providing a DPoP proof in the DPoP request header.¶
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 Authorization 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 header was received in the HTTP request,
check the header's contents according to the rules in Section 4.2,
and check that the public key of the DPoP proof matches the public
key to which the access token is bound per Section 7.¶
The resource server MUST NOT grant access to the resource unless all checks are successful.¶
GET /protectedresource HTTP/1.1 Host: resource.example.org Authorization: DPoP eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWI iOiJzb21lb25lQGV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbX BsZS5jb20iLCJhdWQiOiJodHRwczovL3Jlc291cmNlLmV4YW1wbGUub3JnIiwibmJmI joxNTYyMjYyNjExLCJleHAiOjE1NjIyNjYyMTYsImNuZiI6eyJqa3QiOiIwWmNPQ09S Wk5ZeS1EV3BxcTMwalp5SkdIVE4wZDJIZ2xCVjN1aWd1QTRJIn19.vsFiVqHCyIkBYu 50c69bmPJsj8qYlsXfuC6nZcLl8YYRNOhqMuRXu6oSZHe2dGZY0ODNaGg1cg-kVigzY hF1MQ DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOH0.lNhmpAX1WwmpBvwhok4E74kWCiGB NdavjLAeevGy32H3dbF0Jbri69Nm2ukkwb-uyUI4AUg1JSskfWIyo4UCbQ
Upon receipt of a request for a URI of a protected resource within
the protection space requiring DPoP authorization, if the request does
not include valid credentials or or does not contain an access
token sufficient for access to the protected resource, the server
can reply with a challenge using the 401 (Unauthorized) 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 authorization.
Parameter values are described in Section 3.1 of [RFC6750].¶
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 realm="WallyWorld", 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 realm="WallyWorld", error="invalid_token", error_description="Invalid DPoP key binding", algs="ES256"
It MUST be ensured that resource servers can reliably identify whether a token is bound using DPoP and learn the public key to which the token is bound.¶
Access tokens that are represented as JSON Web Tokens (JWT) [RFC7519]
MUST contain information about the DPoP public key (in JWK format) in
the member jkt of the cnf claim, as shown in Figure 8.¶
The value in jkt MUST be the base64url encoding [RFC7515] of
the JWK SHA-256 Thumbprint (according to [RFC7638]) of the public
key to which the access token is bound.¶
{
"sub":"someone@example.com",
"iss":"https://server.example.com",
"aud":"https://resource.example.org",
"nbf":1562262611,
"exp":1562266216,
"cnf":{
"jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
}
}
cnf claim
When access token introspection is used, the same cnf claim as above
MUST be contained in the introspection response.¶
Resource servers MUST ensure that the fingerprint of the public key in
the DPoP proof JWT equals the value in the jkt claim in the access
token or introspection response.¶
This document introduces the following new authorization server metadata
[RFC8414] parameter to signal the 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¶
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. DPoP does not, however, achieve the same level of protection as TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth Token Binding [I-D.ietf-oauth-token-binding] (see also Section 9.1 and Section 9.4). 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 and replay protection. Therefore, it is RECOMMENDED to prefer TLS-based methods over DPoP if such methods are suitable for the scenario at hand.¶
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 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
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 near future (e.g., up to a few
seconds in the future).¶
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 signature of DPoP proofs only contains the HTTP URI and method, but not, for example, the message body or other request headers.¶
This is an intentional design decision 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 on (parts of) requests are out of the scope of this specification, signatures or information to be signed can be added into DPoP proofs.¶
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 scheme in the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" [RFC7235][IANA.HTTP.AuthSchemes]:¶
[[
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.StructuredSuffixs]
by Section 7.2 of [RFC8417], which is maybe sufficient? A fullblown 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 [I-D.ietf-oauth-access-token-jwt], [I-D.ietf-oauth-jwsreq], [RFC8417], and of course [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¶
This document specifies the following new HTTP header fields, registration of which is requested in the "Permanent Message Header Field Names" registry [IANA.Headers] defined in [RFC3864].¶
We would like to thank Filip Skokan, Mike Engan, Justin Richer, Michael Peck, Vladimir Dzhuvinov, Rob Otto, Dominick Baier, Jim Willeke, Annabelle Backman, Bjorn Hjelm, Steinar Noem, Aaron Parecki, Neil Madden, Paul Querna, Dick Hardt, Dave Tonge, Jared Jennings, Mark Haine 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 resulted 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 ]]¶
-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¶