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OAuth Working Group                                             G. Pujol
Internet-Draft                                                        DS
Intended status: Standards Track                      September 10, 2018
Expires: March 14, 2019


                    OAuth 2.0 Token Revocation List
                       draft-gpujol-oauth-atrl-01

Abstract

   This document defines a format and a standardised uri for a Token
   Revocation List.  An OAuth 2.0 authorization server can use those to
   expose a current list of revoked access tokens identifiers that it
   previously issued, intended for use by OAuth 2.0 resource servers.

Status of This Memo

   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
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   This Internet-Draft will expire on March 14, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Token Revocation List . . . . . . . . . . . . . . . . . .   4
     2.2.  Symbols and abbreviated terms . . . . . . . . . . . . . .   4
   3.  Authorization server metadata . . . . . . . . . . . . . . . .   4
   4.  Token Revocation List Format  . . . . . . . . . . . . . . . .   4
   5.  Token Revocation List Request . . . . . . . . . . . . . . . .   6
   6.  Token Revocation List Response  . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  OAuth 2.0 Authorization Server Metadata . . . . . . . . .   7
       8.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .   7
   9.  Revision History  . . . . . . . . . . . . . . . . . . . . . .   7
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   OAuth 2.0 Token Revocation [RFC7009] defines a way for OAuth 2.0
   clients to revoke access tokens and refresh tokens issued by an OAuth
   2.0 authorization server.  While refresh token are typically only
   used by the authorization server itself, access tokens on the other
   hand are consumed by OAuth 2.0 resource servers; those are logically
   separated from the authorization server, and must learn about the
   revocation status of the access tokens they receive, if the local
   security requirements mandates it.

   Some deployments of OAuth 2.0 (or derived protocols) use signed JWT
   as access tokens.  Those access tokens are self-sufficient for
   resource server to validate that the access token is currently not
   expired, but provide no means for the resource server to obtain the
   revocation status of the token.

   OAuth 2.0 Token Introspection [RFC7662] defines a way for resource
   servers to obtain the metadata attached to a given access token.  For
   performance reasons, this metadata may be put in cache by the
   resource server, instead of calling the introspection endpoint
   synchronously every time the same access token is received.  This
   voids the possibility for the resource server to be informed about a
   revocation of an access token that occurs after the first call to the
   introspection endpoint.

   This specification defines a Token Revocation List (TRL), a document
   exposed by the OAuth 2.0 authorization server, containing a list of
   revoked access tokens identifiers.  Resource servers can periodically



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   retrieve that list to obtain the revocation status of access tokens.
   By doing so, they can either flag currently cached introspected token
   metadata as revoked, or avoid unnecessary calls to the introspection
   endpoint for unknown tokens that are already expired when they are
   received by the resource server.

   This allows better performance for the authorization server and lower
   response times for resource requests [RFC6750], since:

   for resource server  this allow caching the token introspection
      response until the expiration date of the access token.  The TRL
      can be retrieved asynchronously to actual resource requests, so
      the round trip to the authorization server does not add up to the
      resource server response time.

   for the authorization server  this avoids unnecessary calls to the
      introspection endpoint

   This is especially important in scenarios where the authorization
   server issues relatively long lived access tokens, and the
   authorization server and resource servers are loosely coupled (e.g.
   User Managed Access [UMA]), and the introspection endpoint is heavily
   used.

   Note that using short-lived access tokens should be the preferred way
   to protect sensitive resources, rather than relying on the Token
   Revocation List.  Issuing a TRL does not provide any assurance that
   resource servers will use it, nor does provide real time access token
   revocation; depending on their configuration, resource servers might
   take a few seconds or minutes to obtain a fresh TRL after any given
   token is expired.  During that time, access tokens may still be
   considered as active by resource servers.  This hardly avoidable
   delay may however be better than not checking the revocation status
   at all.

1.1.  Notational Conventions

   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.  Terminology

   For the purposes of this specification, the following terms and
   definitions in addition to what is defined in OAuth 2.0 Framework
   [RFC6749], Authorization server [RFC6749], Resource Server [RFC6749],
   Access Token [RFC6749], and JSON Web Encryption [RFC7519] apply.




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2.1.  Token Revocation List

   JWT [RFC7519] that holds a list of access tokens identifiers issued
   by a given authorization server, that are revoked but not expired at
   the time it is issued.

2.2.  Symbols and abbreviated terms

   The following abbreviations are common to this specification.

   JWT  JSON Web Token

   URI  Uniform Resource Identifier

   URL  Uniform Resource Locator

   TRL  Token Revocation List

3.  Authorization server metadata

   Authorization servers can have metadata describing their
   configuration.  The following authorization server metadata value is
   used by this specification and are registered in the IANA "OAuth
   Authorization Server Metadata" registry established in Section 7.1 of
   OAuth 2.0 Authorization Server Metadata [RFC8414]:

   token_revocation_list_uri  OPTIONAL.  URL [RFC3986] of an
      authorization server, where resource servers can retrieve the
      Token Revocation List.  This url MUST use the https scheme.  The
      referenced document contains a JWT whose payload contains the list
      of currently revoked access tokens.  The exact contents of this
      JWT is defined in the present specification.

   If an authorization server advertises the presence of its token
   revocation list, resource servers SHOULD use it as their preferred
   way to obtain the revocation status of access tokens, rather that
   using the token introspection endpoint.

4.  Token Revocation List Format

   A Token Revocation List (TRL) is a JWT.  The JWT payload MUST contain
   the following claims:

   iss  REQUIRED.  MUST be the authorization server issuer

   iat  REQUIRED. time at which the TRL was generated, as defined in the





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   exp  REQUIRED. as defined in [RFC7519], time at which the TRL SHOULD
      be considered as expired by resource servers.

   rev_token_ids  REQUIRED. a JSON list whose items are the identifiers
      of the revoked and non-expired access tokens at the time the TRL
      was generated.

   The JWT payload MAY contain other claims.  Claims not known or not
   understood by resource servers MUST be ignored.

   Token identifiers in the "rev_token_ids" MUST be unique token
   identifiers, allowing resource server to unambiguously identify
   revoked access tokens.  For access tokens in JWT format, those
   identifiers MUST match "jti" values of issued tokens.  For access
   tokens in CWT format, this must match the "cid" values.  For access
   tokens in other formats, "rev_token_ids" claim MAY include any kind
   of token identifier, depending on the format of access tokens issued
   by the authorisation server, as long as both resource servers and
   authorization server have beforehand agreement on the identifier.

   To allow resource servers to learn about the revocation status of a
   token, the resource server must be able to obtain this unique token
   identifier.  This can be done either by reading the jti or cti claim
   from access tokens that are in JWT or CWT formats, respectively, or
   by reading the jti attribute as returned by the authorization server
   introspection endpoint.

   The TRL exposed by an OAuth authorization server at any given time
   MUST always have an expiration date (exp) in the future and SHOULD
   have an expiration date reasonably far away in the future.  The exact
   frequency for an authorization server to generate a new TRL and the
   lifetime of generated TRLs is deployment specific and is out of scope
   of this specification.  Some authorization servers MAY choose to
   generate a new TRL every time an access token is revoked, while
   others MAY generate a new TRL periodically at a fixed time period.
   Some MAY also choose to generate a new TRL only when an access token
   considered as security sensitive is revoked (e.g. bound to a scope
   that is internally flagged as sensitive).

   A TRL SHOULD contain only a list of token identifiers that are
   revoked and not expired at the time the TRL is generated.  A TRL MUST
   NOT contain a token identifier of a token that has not been revoked
   before the TRL is generated.  A TRL MAY contain some token
   identifiers that have been revoked but are already expired at the
   time the TRL is generated.

   A TRL MUST be signed.  That signature MUST use an asymmetric
   algorithm, and the JWT header MUST contain both the "alg" and the



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   "kid" claims as defined in RFC7515 [RFC7515]; the JWK referenced by
   this kid MUST be part of the JWKS [RFC7517] exposed by the
   authorization server on its jwks_uri.

   A TRL MUST NOT be encrypted.

5.  Token Revocation List Request

   An authorization server Token Revocation List MUST be queried using
   an HTTP "GET" request [RFC7230] at the URL defined in the
   authorization server metadata.

   The following is a non-normative example TRL request (with line
   breaks for display purposes only):

           GET /token_revocation_list HTTP/1.1
           Host: server.example.com

   Once a resource server obtains a TRL and that TRL reaches its
   expiration date, a resource server SHOULD obtain a new TRL from the
   authorization server.  A resource server MAY obtain a new TRL before
   the last TRL in its possession is expired.  The frequency at which
   the resource server obtains updated TRLs is out of the scope of this
   specification, and depends on the resource server security
   requirements.

6.  Token Revocation List Response

   An authorization server responds to a Token Revocation List request
   with the most recently available TRL and the Content-Type
   "application/jwt".

   The following is a non-normative example TRL response (with line
   breaks for display purposes only):

     200 OK
     Content-Type: application/jwt
     Content-Length: 542

     eyJhbGciOiJSUzI1NiIsImtpZCI6IlRSTF8yMDE4MDcxMCJ9.eydleHAnOiAxNTM
     xMzIzMDY4LjI3NzM2LCAnaWF0JzogMTUzMTMyMzAwOC4yNzczNiwgJ2lzcyc6ICd
     zZXJ2ZXIuZXhhbXBsZS5jb20nLCAncmV2X3Rva2VuX2lkcyc6IFsnMTIzNDU2Nzk
     wJywgJzIzNDU2Nzg5MDEnXX0.LdMV-QMRsXwHxI4ZfQvQEv5_wNe22VHBa5x6CIb
     G-3H-0R2nMnB_tNeA8nngNNo_vdDRj6Z25Bu6wlTQOM8VufPbUGyAM1Q3LLjPU8p
     cEnM79Z8LW305M09ILaumgg94HrFSTPyEnlIGkVFF_x2vYTf-FbYEFlz2he3WDat
     oPXXXh9gVlfTeinPwVtEZv-k740nUHVJjoSLSS7f_ZVmRnT_wUF_Wisx5YRtrkcu
     8bXJqEykswgYmrzxewCHsc03qEV3HwQPc15_MJBF8tQT9vLTwnYSdMXJh9J5uREz
     IEFqBQpQyIAtbqVT7eD9OMQOttWfB5LVtlnAVHRRdFVkuWA



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7.  Security Considerations

   The TRL is publicly exposed to anyone, not just authorized resource
   servers.  However the TRL does not contain security sensitive values,
   just identifiers of tokens, that are not sufficient by themselves to
   gain any unauthorized access to any protected resource.  An
   alternative would be to require some form of authentication of TRL
   clients (which in this case, are themselves resource servers).
   However, the added complexity, and performance impact (however small)
   of managing that authentication would curb usage of the TRL.

   The TRL MUST be signed by its issuer (the authorization server).  The
   resource servers MUST validate the signature before trusting the
   contents of the TRL.  This avoids misuse or denial of service when
   the party controlling the https (server-side) connection (which, in
   complex environments, may be different than the entity controlling
   the authorization servers) removes revoked tokens from the TRL, and/
   or issues a TRL containing non-revoked tokens.  To effectively
   protect against such attacks, the jwks_uri exposing the public keys
   must not be exposed by the same party as the the
   token_revocation_list_uri.

8.  IANA Considerations

8.1.  OAuth 2.0 Authorization Server Metadata

   This specification registers the following values in the IANA "OAuth
   2.0 Authorization Server Metadata" registry [IANA.OAuth.Parameters]
   established by [RFC8414].

8.1.1.  Registry Contents

   o  Metadata name: token_revocation_list_uri
   o  Metadata Description: The Token Revocation List Uri.
   o  Change controller: IESG
   o  Specification Document: Section 3 of [[ this specification ]]

9.  Revision History

   Note to the RFC Editor: Please remove this section from the final
   RFC.

   -01

   o  TRLs MUST be signed.

   o  Single TRL claim "rev_token_ids" for all kind of token identifiers




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   o  Token IDs can include "jti" from JWTs, "cti" from CWT, or other
      form of ids.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/info/rfc7009>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.



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   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

Author's Address

   Guillaume Pujol
   Digital Security
   France

   Email: guill.p.linux@gmail.com



































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