OAuth 2.0 Token Revocation

Abstract

This document proposes an additional endpoint for OAuth authorization servers, which allows clients to notify the authorization server that a previously obtained refresh or access token is no longer needed. This allows the authorization server to clean up security credentials. A revocation request will invalidate the actual token and, if applicable, other tokens based on the same authorization grant.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7009.

Copyright Notice

Copyright © 2013 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 (http://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Token Revocation  . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Revocation Request  . . . . . . . . . . . . . . . . . . .   4
     2.2.  Revocation Response . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Error Response  . . . . . . . . . . . . . . . . . . .   6
     2.3.  Cross-Origin Support  . . . . . . . . . . . . . . . . . .   6
   3.  Implementation Note . . . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  OAuth Extensions Error Registration . . . . . . . . . . .   8
       4.1.1.  The "unsupported_token_type" Error Value  . . . . . .   8
       4.1.2.  OAuth Token Type Hints Registry . . . . . . . . . . .   8
         4.1.2.1.  Registration Template . . . . . . . . . . . . . .   9
         4.1.2.2.  Initial Registry Contents . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11

1. Introduction

The OAuth 2.0 core specification [RFC6749] defines several ways for a client to obtain refresh and access tokens. This specification supplements the core specification with a mechanism to revoke both types of tokens. A token is a string representing an authorization grant issued by the resource owner to the client. A revocation request will invalidate the actual token and, if applicable, other tokens based on the same authorization grant and the authorization grant itself.

From an end-user's perspective, OAuth is often used to log into a certain site or application. This revocation mechanism allows a client to invalidate its tokens if the end-user logs out, changes identity, or uninstalls the respective application. Notifying the authorization server that the token is no longer needed allows the authorization server to clean up data associated with that token (e.g., session data) and the underlying authorization grant. This behavior prevents a situation in which there is still a valid authorization grant for a particular client of which the end-user is not aware. This way, token revocation prevents abuse of abandoned tokens and facilitates a better end-user experience since invalidated authorization grants will no longer turn up in a list of authorization grants the authorization server might present to the end-user.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

2. Token Revocation

Implementations MUST support the revocation of refresh tokens and SHOULD support the revocation of access tokens (see Implementation Note).

The client requests the revocation of a particular token by making an HTTP POST request to the token revocation endpoint URL. This URL MUST conform to the rules given in [RFC6749], Section 3.1. Clients MUST verify that the URL is an HTTPS URL.

The means to obtain the location of the revocation endpoint is out of the scope of this specification. For example, the client developer may consult the server's documentation or automatic discovery may be used. As this endpoint is handling security credentials, the endpoint location needs to be obtained from a trustworthy source.

Since requests to the token revocation endpoint result in the transmission of plaintext credentials in the HTTP request, URLs for token revocation endpoints MUST be HTTPS URLs. The authorization server MUST use Transport Layer Security (TLS) [RFC5246] in a version compliant with [RFC6749], Section 1.6. Implementations MAY also support additional transport-layer security mechanisms that meet their security requirements.

If the host of the token revocation endpoint can also be reached over HTTP, then the server SHOULD also offer a revocation service at the corresponding HTTP URI, but it MUST NOT publish this URI as a token revocation endpoint. This ensures that tokens accidentally sent over HTTP will be revoked.

2.1. Revocation Request

The client constructs the request by including the following parameters using the "application/x-www-form-urlencoded" format in the HTTP request entity-body:

   token   REQUIRED.  The token that the client wants to get revoked.
   
   token_type_hint  OPTIONAL.  A hint about the type of the token
           submitted for revocation.  Clients MAY pass this parameter in
           order to help the authorization server to optimize the token
           lookup.  If the server is unable to locate the token using
           the given hint, it MUST extend its search across all of its
           supported token types.  An authorization server MAY ignore
           this parameter, particularly if it is able to detect the
           token type automatically.  This specification defines two
           such values:

• access_token: An access token as defined in [RFC6749], Section 1.4

• refresh_token: A refresh token as defined in [RFC6749], Section 1.5

Specific implementations, profiles, and extensions of this specification MAY define other values for this parameter using the registry defined in Section 4.1.2.

The client also includes its authentication credentials as described in Section 2.3. of [RFC6749].

For example, a client may request the revocation of a refresh token with the following request:

     POST /revoke HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     
     token=45ghiukldjahdnhzdauz&token_type_hint=refresh_token

The authorization server first validates the client credentials (in case of a confidential client) and then verifies whether the token was issued to the client making the revocation request. If this validation fails, the request is refused and the client is informed of the error by the authorization server as described below.

In the next step, the authorization server invalidates the token. The invalidation takes place immediately, and the token cannot be used again after the revocation. In practice, there could be a propagation delay, for example, in which some servers know about the invalidation while others do not. Implementations should minimize that window, and clients must not try to use the token after receipt of an HTTP 200 response from the server.

Depending on the authorization server's revocation policy, the revocation of a particular token may cause the revocation of related tokens and the underlying authorization grant. If the particular token is a refresh token and the authorization server supports the revocation of access tokens, then the authorization server SHOULD also invalidate all access tokens based on the same authorization grant (see Implementation Note). If the token passed to the request is an access token, the server MAY revoke the respective refresh token as well.

Note: A client compliant with [RFC6749] must be prepared to handle unexpected token invalidation at any time. Independent of the revocation mechanism specified in this document, resource owners may revoke authorization grants, or the authorization server may invalidate tokens in order to mitigate security threats. Thus, having different server policies with respect to cascading the revocation of tokens should not pose interoperability problems.

2.2. Revocation Response

The authorization server responds with HTTP status code 200 if the token has been revoked successfully or if the client submitted an invalid token.

Note: invalid tokens do not cause an error response since the client cannot handle such an error in a reasonable way. Moreover, the purpose of the revocation request, invalidating the particular token, is already achieved.

The content of the response body is ignored by the client as all necessary information is conveyed in the response code.

An invalid token type hint value is ignored by the authorization server and does not influence the revocation response.

2.2.1. Error Response

The error presentation conforms to the definition in Section 5.2 of [RFC6749]. The following additional error code is defined for the token revocation endpoint:

   unsupported_token_type:  The authorization server does not support
           the revocation of the presented token type.  That is, the
           client tried to revoke an access token on a server not
           supporting this feature.

If the server responds with HTTP status code 503, the client must assume the token still exists and may retry after a reasonable delay. The server may include a "Retry-After" header in the response to indicate how long the service is expected to be unavailable to the requesting client.

2.3. Cross-Origin Support

The revocation endpoint MAY support Cross-Origin Resource Sharing (CORS) [W3C.WD-cors-20120403] if it is aimed at use in combination with user-agent-based applications.

In addition, for interoperability with legacy user-agents, it MAY also offer JSONP (Remote JSON - JSONP) [jsonp] by allowing GET requests with an additional parameter:

   callback  OPTIONAL.  The qualified name of a JavaScript function.

For example, a client may request the revocation of an access token with the following request (line breaks are for display purposes only):

     https://example.com/revoke?token=agabcdefddddafdd&
     callback=package.myCallback

Successful response:

     package.myCallback();

Error response:

     package.myCallback({"error":"unsupported_token_type"});

Clients should be aware that when relying on JSONP, a malicious revocation endpoint may attempt to inject malicious code into the client.

3. Implementation Note

OAuth 2.0 allows deployment flexibility with respect to the style of access tokens. The access tokens may be self-contained so that a resource server needs no further interaction with an authorization server issuing these tokens to perform an authorization decision of the client requesting access to a protected resource. A system design may, however, instead use access tokens that are handles referring to authorization data stored at the authorization server. This consequently requires a resource server to issue a request to the respective authorization server to retrieve the content of the access token every time a client presents an access token.

While these are not the only options, they illustrate the implications for revocation. In the latter case, the authorization server is able to revoke an access token previously issued to a client when the resource server relays a received access token. In the former case, some (currently non-standardized) backend interaction between the authorization server and the resource server may be used when immediate access token revocation is desired. Another design alternative is to issue short-lived access tokens, which can be refreshed at any time using the corresponding refresh tokens. This allows the authorization server to impose a limit on the time revoked when access tokens are in use.

Which approach of token revocation is chosen will depend on the overall system design and on the application service provider's risk analysis. The cost of revocation in terms of required state and communication overhead is ultimately the result of the desired security properties.

4. IANA Considerations

This specification registers an error value in the "OAuth Extensions Error Registry" and establishes the "OAuth Token Type Hints" registry.

4.1. OAuth Extensions Error Registration

This specification registers the following error value in the "OAuth Extensions Error Registry" defined in [RFC6749].

4.1.1. The "unsupported_token_type" Error Value

   Error name:  unsupported_token_type
   
   Error Usage Location:  Revocation endpoint error response
   
   Related Protocol Extension:  Token Revocation Endpoint
   
   Change controller:  IETF
   
   Specification document(s):  [RFC7009]

4.1.2. OAuth Token Type Hints Registry

This specification establishes the "OAuth Token Type Hints" registry. Possible values of the parameter "token_type_hint" (see Section 2.1) are registered with a Specification Required ([RFC5226]) after a two- week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published. Registration requests must be sent to the oauth-ext-review@ietf.org mailing list for review and comment, with an appropriate subject (e.g., "Request for parameter: example"). Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.

4.1.2.1. Registration Template

   Hint Value:  The additional value, which can be used to indicate a
      certain token type to the authorization server.
   
   Change controller:  For Standards Track RFCs, state "IETF".  For
      others, give the name of the responsible party.  Other details
      (e.g., postal address, email address, and home page URI) may also
      be included.
   
   Specification document(s):  Reference to the document(s) that
      specifies the type, preferably including a URI that can be used to
      retrieve a copy of the document(s).  An indication of the relevant
      sections may also be included but is not required.

4.1.2.2. Initial Registry Contents

The OAuth Token Type Hint registry's initial contents are as follows.

             +---------------+-------------------+-----------+
             |   Hint Value  | Change Controller | Reference |
             +---------------+-------------------+-----------+
             |  access_token |        IETF       | [RFC7009] |
             | refresh_token |        IETF       | [RFC7009] |
             +---------------+-------------------+-----------+

Table 1: OAuth Token Type Hints initial registry contents

5. Security Considerations

If the authorization server does not support access token revocation, access tokens will not be immediately invalidated when the corresponding refresh token is revoked. Deployments must take this into account when conducting their security risk analysis.

Cleaning up tokens using revocation contributes to overall security and privacy since it reduces the likelihood for abuse of abandoned tokens. This specification in general does not intend to provide countermeasures against token theft and abuse. For a discussion of respective threats and countermeasures, consult the security considerations given in Section 10 of the OAuth core specification [RFC6749] and the OAuth threat model document [RFC6819].

Malicious clients could attempt to use the new endpoint to launch denial-of-service attacks on the authorization server. Appropriate countermeasures, which should be in place for the token endpoint as well, MUST be applied to the revocation endpoint (see [RFC6819], Section 4.4.1.11). Specifically, invalid token type hints may misguide the authorization server and cause additional database lookups. Care MUST be taken to prevent malicious clients from exploiting this feature to launch denial-of-service attacks.

A malicious client may attempt to guess valid tokens on this endpoint by making revocation requests against potential token strings. According to this specification, a client's request must contain a valid client_id, in the case of a public client, or valid client credentials, in the case of a confidential client. The token being revoked must also belong to the requesting client. If an attacker is able to successfully guess a public client's client_id and one of their tokens, or a private client's credentials and one of their tokens, they could do much worse damage by using the token elsewhere than by revoking it. If they chose to revoke the token, the legitimate client will lose its authorization grant and will need to prompt the user again. No further damage is done and the guessed token is now worthless.

Since the revocation endpoint is handling security credentials, clients need to obtain its location from a trustworthy source only. Otherwise, an attacker could capture valid security tokens by utilizing a counterfeit revocation endpoint. Moreover, in order to detect counterfeit revocation endpoints, clients MUST authenticate the revocation endpoint (certificate validation, etc.).

6. Acknowledgements

We would like to thank Peter Mauritius, Amanda Anganes, Mark Wubben, Hannes Tschofenig, Michiel de Jong, Doug Foiles, Paul Madsen, George Fletcher, Sebastian Ebling, Christian Stuebner, Brian Campbell, Igor Faynberg, Lukas Rosenstock, and Justin Richer for their valuable feedback.

7. References

7.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   
   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.
   
   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework",
              RFC 6749, October 2012.

7.2. Informative References

   [RFC6819]  Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              January 2013.

[W3C.WD-cors-20120403]

              Kesteren, A., "Cross-Origin Resource Sharing", World Wide
              Web Consortium LastCall WD-cors-20120403, April 2012,
              <http://www.w3.org/TR/2012/WD-cors-20120403>.
   
   [jsonp]    Ippolito, B., "Remote JSON - JSONP", December 2005,
              <http://bob.pythonmac.org/archives/2005/12/05/
              remote-json-jsonp>.

Authors' Addresses

Torsten Lodderstedt (editor)
Deutsche Telekom AG

EMail:

          torsten@lodderstedt.net
   
   Stefanie Dronia

EMail:

          sdronia@gmx.de

Marius Scurtescu
Google

EMail:

          mscurtescu@google.com