OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
draft-fett-oauth-dpop-02

Abstract

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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 9, 2020.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Conventions and Terminology
• 2. Main Objective
• 3. Concept
• 4. DPoP Proof JWTs
• 4.1. Syntax
• 4.2. Checking DPoP Proofs
• 5. Token Request (Binding Tokens to a Public Key)
• 6. Resource Access (Proof of Possession for Access Tokens)
• 7. Public Key Confirmation
• 8. Acknowledgements
• 9. Security Considerations
• 9.1. DPoP Proof Replay
• 9.2. Signed JWT Swapping
• 9.3. Signature Algorithms
• 9.4. Message Integrity
• 10. IANA Considerations
• 10.1. OAuth Access Token Type Registration
• 10.2. JSON Web Signature and Encryption Type Values Registration
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Appendix A. Document History
• Authors' Addresses

1. Introduction

[I-D.ietf-oauth-mtls] describes methods to bind (sender-constrain) access tokens using mutual Transport Layer Security (TLS) authentication with X.509 certificates.

[I-D.ietf-oauth-token-binding] provides mechanisms to sender-constrain access tokens using HTTP token binding.

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.

This document outlines an application-level sender-constraining for access tokens and refresh tokens that can be used if neither mTLS nor OAuth Token Binding are available. It uses proof-of-possession based on a public/private key pair and application-level signing.

DPoP can be used with public clients and, in case of confidential clients, can be combined with any client authentication method.

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 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].

2. Main Objective

Under the attacker model defined in [I-D.ietf-oauth-security-topics], the mechanism defined by this specification tries to ensure that token replay at a different endpoint is prevented.

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.

Secondary objectives are discussed in Section 9.

3. Concept

The main data structure introduced by this specification is a DPoP proof JWT, described in detail below. A client uses a DPoP proof JWT to prove the possession of a private key belonging to a certain public key. Roughly speaking, a DPoP proof is a signature over some data of the request to which it is attached to and a timestamp.

+--------+                                          +---------------+
|        |--(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 ---------------|               |
|        |                                          +---------------+
+--------+

Figure 1: Basic DPoP Flow

The basic steps of an OAuth flow with DPoP are shown in Figure 1:

• (A) In the Token Request, the client sends an authorization code to the authorization server in order to obtain an access token (and potentially a refresh token). The client attaches a DPoP proof to the request in an HTTP header.
• (B) The AS binds (sender-constrains) the access token to the public key claimed by the client in the DPoP proof; that is, the access token cannot be used without proving possession of the respective private key. This is signaled to the client by using the token_type value DPoP.
• If a refresh token is issued to a public client, it is sender-constrained in the same way. For confidential clients, refresh tokens are bound to the client_id, which is more flexible than binding it to a particular public key.
• (C) If the client wants to use the access token, it has to prove possession of the private key by, again, adding a header to the request that contains a DPoP proof. The resource server needs to receive information about which public key to check against. This information is either encoded directly into the access token (for JWT structured access tokens), or provided at the token introspection endpoint of the authorization server (not shown).
• (D) The resource server refuses to serve the request if the signature check fails or the data in the DPoP proof is wrong, e.g., the request URI does not match the URI claim in the DPoP proof JWT.
• When a refresh token that is sender-constrained using DPoP is used by the client, the client has to provide a DPoP proof just as in the case of a resource access. The new access token will be bound to the same public key.

The mechanism presented herein is not a client authentication method. In fact, a primary use case is public clients (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.

4. DPoP Proof JWTs

DPoP uses so-called DPoP proof JWTs for binding public keys and proving knowledge about private keys.

4.1. Syntax

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 this JWT chosen freshly when creating the DPoP proof (REQUIRED). SHOULD be used by the AS for replay detection and prevention. See Security Considerations.
• http_method: The HTTP method for the request to which the JWT is attached, as defined in [RFC7231] (REQUIRED).
• http_uri: The HTTP URI used for the request, without query and fragment parts (REQUIRED).
• iat: Time at which the JWT was created (REQUIRED).

An example DPoP proof is shown in Figure 2.

{
    "typ": "dpop+jwt",
    "alg": "ES256",
    "jwk": {
             "kty": "EC",
             "crv": "P-256",
             "x": "f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU",
             "y": "x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0"
    }
}.{
    "jti": "HK2PmfnHKwXP",
    "http_method": "POST",
    "http_uri": "https://server.example.com/token",
    "iat": 1555555555
}

Figure 2: Example JWT content for DPoP proof header.

Note: To keep DPoP simple to implement, only the HTTP method and URI are signed in DPoP proofs. Nonetheless, DPoP proofs can be extended to contain other information of the HTTP request (see also Section 9.4).

4.2. Checking DPoP Proofs

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

1. the string value is a well-formed JWT,
2. all required claims are contained in the JWT,
3. the typ field in the header has the value dpop+jwt,
4. the algorithm in the header of the JWT indicates an asymmetric digital signature algorithm, is not none, is supported by the application, and is deemed secure,
5. that the JWT is signed using the public key contained in the jwk header of the JWT,
6. the http_method claim matches the respective value for the HTTP request in which the JWT was received (case-insensitive),
7. the http_uri claims matches the respective value for the HTTP request in which the JWT was received, ignoring any query and fragment parts,
8. the token was issued within an acceptable timeframe (see Section 9.1), and
9. that, within a reasonable consideration of accuracy and resource utilization, a JWT with the same 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 http_uri claim.

5. Token Request (Binding Tokens to a Public Key)

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: eyJhbGciOiJSU0ExXzUi...

grant_type=authorization_code
&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

Figure 3: Token Request for a DPoP sender-constrained token.

The HTTP header DPoP MUST contain a valid DPoP proof.

The authorization server, after checking the validity of the token, MUST associate the access token issued at the token endpoint with the public key. It then sets token_type to DPoP in the token response.

A client typically cannot know whether a certain AS supports DPoP. It therefore SHOULD use the value of the token_type parameter returned from the AS to determine support for DPoP: If the token type returned is Bearer or another value, the AS does not support DPoP. If it is DPoP, DPoP is supported. Only then, the client needs to send the DPoP header in subsequent requests and use the token type DPoP in the Authorization header as described below.

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.

If a DPoP-bound refresh token is to be 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.

6. Resource Access (Proof of Possession for Access Tokens)

To make use of an access token that is token-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.

The DPoP-bound access token must be sent in the Authorization header with the prefix DPoP.

If a resource server detects that an access token that is to be used for resource access is bound to a public key using DPoP (via the methods described in Section 7) it MUST check that a header DPoP was received in the HTTP request, and check the header's contents according to the rules in Section 4.2.

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

GET /protectedresource HTTP/1.1
Host: resourceserver.example.com
Authorization: DPoP eyJhbGciOiJIUzI1...
DPoP: eyJhbGciOiJSU0ExXzUi...

Figure 4: Protected Resource Request with a DPoP sender-constrained access token.

7. Public Key Confirmation

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#S256 of the cnf claim, as shown in Figure 5.

The value in jkt#S256 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.

{
    "iss": "https://server.example.com",
    "sub": "something@example.com",
    "exp": 1503726400,
    "nbf": 1503722800,
    "cnf":{
        "jkt#S256": "oKIywvGUpTVTyxMQ3bwIIeQUudfr_CkLMjCE19ECD-U"
    }
}

Figure 5: Example access token body with 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#S256 claim in the access token or introspection response.

8. Acknowledgements

We would like to thank David Waite, Filip Skokan, Mike Engan, and Justin Richer for their valuable input and feedback.

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

9. Security Considerations

The Prevention of Token Replay at a Different Endpoint is achieved through the binding of the DPoP proof to a certain URI and HTTP method. However, DPoP does not achieve the same level of protection as, for example, OAuth Mutual TLS [I-D.ietf-oauth-mtls], as described in the following.

9.1. DPoP Proof Replay

If an adversary is able to get hold of a DPoP proof JWT, the adversary could replay that token later 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 brief period. Furthermore, the jti claim in each JWT MUST contain a unique (incrementing or randomly chosen) value, as proposed in [RFC7253]. Resource servers SHOULD store values at least for the time window in which the respective JWT is accepted and decline HTTP requests by clients if a jti value has been seen before.

Note: To acommodate for clock offsets, the server MAY accept DPoP proofs that carry an iat time in the near future (e.g., up to one second in the future).

9.2. 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 in the DPoP headers.

9.3. Signature Algorithms

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

9.4. Message Integrity

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 [I-D.ietf-oauth-mtls] 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.

10. IANA Considerations

10.1. OAuth Access Token Type Registration

This specification registers the following access token type in the OAuth Access Token Types registry defined in [RFC6749].

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

10.2. JSON Web Signature and Encryption Type Values Registration

This specification registers the dpop+jwt type value in the IANA JSON Web Signature and Encryption Type Values registry [RFC7515]:

• "typ" Header Parameter Value: "dpop+jwt"
• Abbreviation for MIME Type: None
• Change Controller: IETF
• Specification Document(s): [[ this specification ]]

11. References

11.1. Normative References

11.2. Informative References

Appendix A. Document History

[[ To be removed from the final specification ]]

-02

• added normalization rules for URIs
• removed distinction between proof and binding
• "jwk" header again used instead of "cnf" claim in DPoP proof
• renamed "Bearer-DPoP" token type to "DPoP"
• removed ability for key rotation
• added security considerations on request integrity
• explicit advice on extending DPoP proofs to sign other parts of the HTTP messages
• only use the jkt#S256 in ATs
• iat instead of exp in DPoP proof JWTs
• updated guidance on token_type evaluation

-01

• fixed inconsistencies
• moved binding and proof messages to headers instead of parameters
• extracted and unified definition of DPoP JWTs
• improved description

-00

• first draft

Authors' Addresses