OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
draft-fett-oauth-dpop-01

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 to detect 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 October 4, 2019.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Conventions and Terminology
• 2. Main Objective
• 3. Concept
• 4. DPoP JWT Syntax
• 5. Token Request (Binding Tokens to a Public Key)
• 6. Resource Access (Proof of Possession for Access Tokens)
• 7. Refresh Token Usage (Proof of Possession for Refresh Tokens)
• 8. Public Key Confirmation
• 9. Acknowledgements
• 10. IANA Considerations
• 10.1. OAuth Access Token Type Registration
• 10.2. JWT Confirmation Methods Registration
• 10.3. JSON Web Signature and Encryption Type Values Registration
• 11. Security Considerations
• 11.1. Token Replay at the Same Authorization Server
• 11.2. Token Replay at the Same Resource Server Endpoint
• 11.3. Signed JWT Swapping
• 11.4. Comparison to mTLS and OAuth Token Binding
• 12. References
• 12.1. Normative References
• 12.2. 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 because it set up a counterfeit authorization server or resource server, the adversary is not able to replay the respective access token at another authorization or resource server.

Secondary objectives are discussed in Section 11.

3. Concept

+--------+                                          +---------------+
|        |--(A)-- Token Request ------------------->|               |
| Client |        (DPop-Binding/Proof)              | Authorization |
|        |                                          |     Server    |
|        |<-(B)-- PoP Access Token -----------------|               |
|        |        (token_type=Bearer+DPoP)          +---------------+
|        |        PoP Refresh Token for public clients
|        | 
|        |                                          +---------------+
|        |--(C)-- PoP Access Token ---------------->|               |
|        |        (DPoP-Proof)                      |    Resource   |
|        |                                          |     Server    |
|        |<-(D)-- Protected Resource ---------------|               |
|        |                                          +---------------+
+--------+

Figure 1: Basic DPoP Flow

The new elements introduced by this specification are shown in Figure 1:

• (A) In the Token Request, the client sends an authorization grant, e.g., an authorization code or a refresh token, to the authorization server in order to obtain an access token (and potentially a refresh token). The client proves the possession of a private key belonging to some public key by sending a request header containing a JWT that was signed using this private key. The corresponding public key is contained in the same request.
• (B) The AS binds (sender-constrains) the access token to the public key claimed by the client; 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 Bearer+DPoP. If a refresh token is issued to the client, it is sender-constrained in the same way if the client is a public client. Note: refresh tokens are automatically bound to the client_id of a confidential client, 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 adding a header to the request that, again, contains a JWT signed with this private key. The JWT contains the endpoint URL and the request method. 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 (request not shown).
• (D) The resource server refuses to serve the request if the signature check fails or the data in the JWT do not match, e.g., the request URI does not match the URI claim in the JWT.
• Steps (A) and (B) can be repeated using a refresh token to obtain fresh access tokens. In this case, the client sends a DPoP proof JWT as in step (C) above. The client can optionally proof the possession of a new private/public key pair to which the new tokens are then bound by the authorization server. Otherwise, the authorization server binds the new tokens to the previously used public key.

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

Note: DPoP does not directly ensure message integrity but relies on the TLS layer for that purpose.

4. DPoP JWT Syntax

DPoP uses so-called DPoP JWTs for binding public keys (DPoP Binding JWT) and proving knowledge about private keys (DPoP Proof JWT).

A DPoP JWT 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 the following fields:

• typ: type header, value dpop_binding+jwt for a DPoP Binding JWT or dpop_proof+jwt for a DPoP Proof JWT (REQUIRED).
• alg: a digital signature algorithm identifier as per [RFC7518] (REQUIRED). MUST NOT be none or an identifier for a symmetric algorithm (MAC).

The body of a DPoP JWT contains the following fields:

• jti: Unique identifier for this JWT chosen freshly when creating the JWT (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, in upper case ASCII characters, as defined in [RFC7231] (REQUIRED).
• http_uri: The HTTP URI used for the request, without query and fragment parts (REQUIRED).
• exp: Expiration time of the JWT (REQUIRED). See Security Considerations.
• cnf: Confirmation claim as per [RFC7800] containing a member dpop+jwk, representing the public key chosen by the client in JWK format (REQUIRED for DPoP Binding JWTs, OPTIONAL for DPoP Proof JWTs).

An example DPoP JWT is shown in Figure 2.

{
    "typ": "dpop_binding+jwt",
    "alg": "ES512",
}.{
    "jti": "HK2PmfnHKwXP",
    "http_method": "POST",
    "http_uri": "https://server.example.com/token",
    "exp": "..."
    "cnf":{
        "dpop+jwk": {
             "kty" : "EC",
             "kid" : "11",
             "crv" : "P-256",
             "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
             "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
        }
    }
}

Figure 2: Example JWT contents for DPoP-Binding header.

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 public key and prove the possession of the corresponding private key. 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-Binding: eyJhbGciOiJSU0ExXzUi ...

grant_type=authorization_code
&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
(remainder of JWK omitted for brevity)

Figure 3: Token Request for a DPoP bound token.

The HTTP header DPoP-Binding MUST contain a DPoP Binding JWT signed using the private key chosen by the client.

It is RECOMMENDED that clients reuse the same JWT if possible to improve the performance of the client, the data transfer (caching), and the authorization server.

If the authorization server receives a DPoP-Binding header in a token request, the authorization server MUST check that:

1. the header value is a well-formed JWT,
2. all required claims are contained in the JWT,
3. the typ field in the header has the correct value,
4. the algorithm in the header of the JWT designates a digital signature algorithm, is not none, is supported by the application, and is deemed secure,
5. the JWT is signed using the public key contained in the cnf claim of the JWT,
6. the http_method and http_uri claims match the respective values for the HTTP request in which the header was received,
7. the token has not expired, and
8. if replay protection is desired, that a JWT with the same jti value has not been received previously.

If these checks are successful, the authorization server MUST associate the access token with the public key. It then sets token_type to Bearer+DPoP in the token response. The client MAY use the value of the token_type parameter to determine whether the server supports the mechanisms specified in this document.

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. More precisely, the client MUST create a DPoP Proof JWT and sign it using the previously chosen private key. The signed JWT MUST then be sent in the DPoP-Proof request header.

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 8) it MUST check that:

1. a header DPoP-Proof was received in the HTTP request,
2. the header's value is a well-formed DPoP Proof JWT,
3. all required claims are contained in the JWT,
4. the algorithm in the header of the JWT designates a digital signature algorithm, is not none, is supported by the application, and is deemed secure,
5. the JWT is signed using the public key to which the access token was bound,
6. the typ field in the header has the correct value,
7. the http_method and http_uri claims match the respective values for the HTTP request in which the header was received,
8. the token has not expired, and
9. if replay protection is desired, that a JWT with the same jti value has not been received previously.

If any of these checks fails, the resource server MUST NOT grant access to the resource.

7. Refresh Token Usage (Proof of Possession for Refresh Tokens)

At the token endpoint, public clients using a refresh token MUST provide a proof of possession in the same way as for access tokens.

8. 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 dpop+jwk of the cnf claim, as shown in Figure 4.

{
    "iss": "https://server.example.com",
    "sub": "something@example.com",
    "exp": 1493726400,
    "nbf": 1493722800,
    "cnf":{
        "dpop+jwk": {
            "kty" : "EC",
            "kid" : "11",
            "crv" : "P-256",
            "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
            "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
        }
    }
}

Figure 4: 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.

9. Acknowledgements

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).

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

10.2. JWT Confirmation Methods Registration

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

• Confirmation Method Value: "dpop+jwk"
• Confirmation Method Description: JWK encoded public key for dpop proof token
• Change Controller: IESG
• Specification Document(s): [[ this specification ]]

10.3. JSON Web Signature and Encryption Type Values Registration

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

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

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

11. Security Considerations

The Prevention of Token Replay at a Different Endpoint is achieved through the binding of the DPoP JWT to a certain URI and HTTP method.

11.1. Token Replay at the Same Authorization Server

If an adversary is able to get hold of an DPoP-Binding JWT, it might replay it at the authorization server's token endpoint with the same or different payload. The issued access token is useless as long as the adversary does not get hold of a valid DPoP-Binding JWT for the corresponding resource server.

11.2. Token Replay at the Same Resource Server Endpoint

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, clients MUST limit the lifetime of the JWTs, preferably to 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 lifetime of the respective JWT and decline HTTP requests by clients if a jti value has been seen before.

11.3. Signed JWT Swapping

Servers accepting signed DPoP 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.

11.4. Comparison to mTLS and OAuth Token Binding

• mTLS stronger against intercepted connections

12. References

12.1. Normative References

12.2. Informative References

Appendix A. Document History

[[ To be removed from the final specification ]]

-00

• first draft

-01

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

Authors' Addresses