B. Campbell, Ed.
Internet-Draft                                       Ping Identity Corp.
Intended status: Standards Track                            C. Mortimore
Expires: August 1, 7, 2011                                   Salesforce.com
                                                        January 28,
                                                        February 3, 2011

       SAML 2.0 Bearer Assertion Grant Type Profile for OAuth 2.0


   This specification defines the use of a SAML 2.0 bearer Bearer Assertion as
   means for requesting an OAuth 2.0 access token.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
   2.  SAML Assertion Access Token Request  . . . . . . . . . . . . .  4
     2.1.  Client Requests Access Token . . . . . . . . . . . . . . .  4
     2.2.  Assertion Format and Processing Requirements . . . . . . .  5
     2.3.  Error Response . . . . . . . . . . . . . . . . . . . . . .  7
     2.4.  Example (non-normative)  . . . . . . . . . . . . . . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Parameter Registration Request . . . . . . . . . . . . . .  9
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 10
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

1.  Introduction

   The Security Assertion Markup Language (SAML) 2.0
   [OASIS.saml-core-2.0-os] is an XML-based framework that allows for
   identity and security information to be shared across security
   domains.  The SAML specification, while primarily targeted at
   providing cross domain web browser single sign-on, was also designed
   to be modular and extensible to facilitate use in other contexts.

   The Assertion, an XML security token, is a fundamental construct of
   SAML that is often adopted for use in other protocols and
   specifications.  An Assertion is generally issued by an identity
   provider and consumed by a service provider who relies on its content
   to identify the Assertion's subject for security related purposes.

   The OAuth 2.0 Authorization Protocol [I-D.ietf.oauth-v2] provides a
   method for making authenticated HTTP requests to a resource using an
   access token.  Access tokens are issued to third-party clients by an
   authorization server (AS) with the (sometimes implicit) approval of
   the resource owner.  In OAuth, an authorization grant is an abstract
   term used to describe intermediate credentials that represent the
   resource owner authorization.  An authorization grant is used by the
   client to obtain an access token.

   Several authorization grant types are defined to support a wide range
   of client types and user experiences.  OAuth also allows for the
   definition of new extension grant types in companion specifications (such as this one) to support additional clients
   or to provide a bridge between OAuth and other trust frameworks.

   This specification defines an extension grant type that profiles the
   use of a SAML 2.0 bearer Bearer Assertion in requesting an OAuth 2.0 access
   token.  The format and processing rules for the SAML Assertion
   defined in this specification are intentionally similar, though not
   identical, to those in the Web Browser SSO Profile defined in
   [OASIS.saml-profiles-2.0-os] reusing, to the extent reasonable,
   concepts and patterns from that well-established profile.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

2.  SAML Assertion Access Token Request

   A SAML Assertion can be used to request an access token when a client
   wishes to utilize an existing trust relationship, expressed though
   the semantics of (and digital signature calculated over) the SAML
   Assertion, without a direct user approval step at the authorization

   The process by which the client obtains the SAML Assertion, prior to
   exchanging it with the authorization server, is out of scope.

        +--------+                                  +---------------+
        |        |                                  |               |
        |        |>--(A)-- SAML 2.0 Assertion ----->| Authorization |
        | Client |                                  |     Server    |
        |        |<--(B)---- Access Token ---------<|               |
        |        |                                  |               |
        +--------+                                  +---------------+

                 Figure 1: Assertion Access Token Request

   The request/response flow illustrated in Figure 1 includes the
   following steps:

   (A)  The client sends an access token request to the authorization
        server with the appropriate OAuth
        http://oauth.net/grant_type/assertion/saml/2.0/bearer grant_type
        and includes a SAML 2.0 Assertion.

   (B)  The authorization server validates the Assertion per the
        processing rules defined in this specification and issues an
        access token.

2.1.  Client Requests Access Token

   The client includes the Assertion in the access token request, the
   core details of which are defined in OAuth [I-D.ietf.oauth-v2], by
   specifying "http://oauth.net/grant_type/assertion/saml/2.0/bearer" as
   the absolute URI value of the "grant_type" parameter and by adding
   the following parameter:

         REQUIRED.  The value of the assertion parameter MUST contain a
         single SAML 2.0 Assertion.  When used with the
         "http://oauth.net/grant_type/assertion/saml/2.0/bearer"; grant
         type, the assertion MUST be a SAML 2.0 Assertion.  The SAML
         Assertion XML data MUST be encoded using base64url, where the
         encoding adheres to the definition in Section 5 of RFC4648
         [RFC4648] and where the padding bits are set to zero.  To to
         avoid the need for subsequent encoding steps (by "application/
         x-www-form-urlencoded" [W3C.REC-html401-19991224], for
         example), the base64url encoded data SHOULD NOT be line wrapped
         and pad characters ("=") SHOULD NOT be included.

         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value is defined by the
         authorization server.  If the value contains multiple space-
         delimited strings, their order does not matter, and each string
         adds an additional access range to the requested scope.

   Authorization servers SHOULD issue access tokens with a limited
   lifetime and require clients to refresh them by requesting a new
   access token using the same assertion, if it is still valid, or with
   a new assertion.  The authorization server SHOULD NOT issue a refresh

2.2.  Assertion Format and Processing Requirements

   Prior to issuing an access token response as described in
   [I-D.ietf.oauth-v2], the authorization server MUST validate the
   Assertion according to the criteria below.  If present, the
   authorization server MUST also validate the client credentials.
   Application of additional restrictions and policy are at the
   discretion of the authorization server.

   o  The Assertion's <Issuer> element MUST contain a unique identifier
      for the entity that issued the Assertion; the Format attribute
      MUST be omitted or have a value of

   o  The Assertion MUST contain a <Subject> element.  The subject MAY
      identify the resource owner for whom the access token is being

   o  The <Subject> element MUST contain at least one
      <SubjectConfirmation> element that allows the authorization server
      to confirm it as a bearer Bearer Assertion.  Conditions for bearer
      subject confirmation are described below.

      *  The <SubjectConfirmation> MUST have a Method attribute with a
         value of "urn:oasis:names:tc:SAML:2.0:cm:bearer" and MUST
         contain a <SubjectConfirmationData> element.

      *  The <SubjectConfirmationData> element MUST have a Recipient
         attribute with a value indicating the token endpoint URL of the
         authorization server.  The authorization server MUST verify
         that the value of the Recipient attribute matches the token
         endpoint URL (or an acceptable alias) to which the Assertion
         was delivered.

      *  The <SubjectConfirmationData> element MUST have a NotOnOrAfter
         attribute that limits the window during which the Assertion can
         be confirmed.  The authorization server MUST verify that the
         NotOnOrAfter instant has not passed, subject to allowable clock
         skew between systems.  The authorization server MAY ensure that
         Bearer Assertions are not replayed, by maintaining the set of
         used ID values for the length of time for which the Assertion
         would be considered valid based on the NotOnOrAfter attribute
         in the <SubjectConfirmationData>.  The authorization server MAY
         reject assertions with a NotOnOrAfter instant that is
         unreasonably far in the future.

      *  The <SubjectConfirmationData> element MAY also contain an
         Address attribute limiting the client address from which the
         Assertion can be delivered.  Verification of the Address is at
         the discretion of the authorization server.

   o  If the Assertion issuer authenticated the subject, the Assertion
      SHOULD contain a single <AuthnStatement> representing that
      authentication event.

   o  If the Assertion was issued with the intention that the client act
      autonomously on behalf of the subject, an <AuthnStatement> SHOULD
      NOT be included.  The client SHOULD be identified in the <NameID>
      or similar element the <SubjectConfirmation> element or by other
      available means like [OASIS.saml-deleg-cs].

   o  Other statements, in particular, <AttributeStatement> elements MAY
      be included in the Assertion.

   o  The Assertion MUST contain an <AudienceRestriction> element with
      an <Audience> element containing a URI reference that identifies
      the authorization server, or the service provider SAML entity of
      its controlling domain, as an intended audience.  The
      authorization server MUST verify that it is an intended audience
      for the Assertion.

   o  The Assertion MUST be digitally signed by the issuer and the
      authorization server MUST verify the signature.

   o  Encrypted elements MAY appear in place of their plain text
      counterparts as defined in [OASIS.saml-core-2.0-os].

   o  The authorization server MUST verify that the Assertion is valid
      in all other respects per [OASIS.saml-core-2.0-os] such as (but
      not limited to) evaluating all content within the Conditions
      element including the NotOnOrAfter and NotBefore attributes,
      rejecting unknown condition types, etc.

2.3.  Error Response

   If the Assertion is not valid, or its subject confirmation
   requirements cannot be met, the the authorization server MUST
   construct an error response as defined in [I-D.ietf.oauth-v2].  The
   value of the error parameter MUST be the "invalid_grant" error code.
   The authorization server MAY include additional information regarding
   the reasons the Assertion was considered invalid using the
   error_description or error_uri parameters.

   For example:

   HTTP/1.1 400 Bad Request
   Content-Type: application/json
   Cache-Control: no-store

     "error_description":"Audience validation failed"

2.4.  Example (non-normative)

   Though non-normative, the following examples illustrate what a
   conforming Assertion and access token request would look like.

   Below is an example SAML 2.0 Assertion (whitespace formatting is for
   display purposes only):

   <Assertion IssueInstant="2010-10-01T20:07:34.619Z"
    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
     [...omitted for brevity...]
     <AuthnStatement AuthnInstant="2010-10-01T20:07:34.371Z">

                   Figure 2: Example SAML 2.0 Assertion

   To present the Assertion shown in the previous example as part of an
   access token request, for example, the client might make the
   following HTTPS request (line breaks are for display purposes only):

   POST /token.oauth2 HTTP/1.1
   Host: authz.example.net
   Content-Type: application/x-www-form-urlencoded

   [...omitted for brevity...]V0aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-

                         Figure 3: Example Request

3.  Security Considerations

   No additional considerations beyond those described within the OAuth
   2.0 Protocol Framework [I-D.ietf.oauth-v2] and in the Security and
   Privacy Considerations for the OASIS Security Assertion Markup
   Language (SAML) V2.0 [OASIS.saml-sec-consider-2.0-os].

4.  IANA Considerations

4.1.  Parameter Registration Request

   The following is the parameter registration request, as defined in
   The OAuth Parameters Registry of The OAuth 2.0 Authorization Protocol
   [I-D.ietf.oauth-v2], for the "assertion" parameter:

   o  Parameter name: assertion

   o  Parameter usage location: token request

   o  Change controller: IETF

   o  Specification document(s): draft-ietf-oauth-saml2-bearer

Appendix A.  Contributors

   The following people contributed wording and concepts to this
   document: Paul Madsen, Patrick Harding, Peter Motyka, Eran Hammer-
   Lahav, Peter Saint-Andre, Ian Barnett, Eric Fazendin, Torsten
   Lodderstedt, Scott Cantor and David Waite

Appendix B.  Document History

   [[ to be removed by RFC editor before publication as an RFC ]]


   o  Added scope parameter with text copied from draft-ietf-oauth-v2-12
      (the reorg of draft-ietf-oauth-v2-12 made it so scope wasn't
      really inherited by this spec anymore)

   o  Change definition of the assertion parameter to be more generally
      applicable per the suggestion near the end of

   o  Editorial changes based on feedback


   o  Update spec name when referencing draft-ietf-oauth-v2 (The OAuth
      2.0 Protocol Framework -> The OAuth 2.0 Authorization Protocol)

   o  Update wording in Introduction to talk about extension grant types
      rather than the assertion grant type which is a term no longer
      used in OAuth 2.0

   o  Updated to reference draft-ietf-oauth-v2-12 and denote as work in

   o  Update Parameter Registration Request to use similar terms as
      draft-ietf-oauth-v2-12 and remove Related information part

   o  Add some text giving discretion to AS on rejecting assertions with
      unreasonably long validity window.


   o  Added Parameter Registration Request for "assertion" to IANA

   o  Changed document name to draft-ietf-oauth-saml2-bearer in
      anticipation of becoming a OAUTH WG item.

   o  Attempt to move the entire definition of the 'assertion' parameter
      into this draft (it will no longer be defined in OAuth 2 Protocol

   o  Updated to reference draft-ietf-oauth-v2-11 and reflect changes
      from -10 to -11.

   o  Updated examples.

   o  Relaxed processing rules to allow for more than one
      SubjectConfirmation element.

   o  Removed the 'MUST NOT contain a NotBefore attribute' on

   o  Relaxed wording that ties the subject of the Assertion to the
      resource owner.

   o  Added some wording about identifying the client when the subject
      hasn't directly authenticated including an informative reference
      to SAML V2.0 Condition for Delegation Restriction.

   o  Added a few examples to the language about verifying that the
      Assertion is valid in all other respects.

   o  Added some wording to the introduction about the similarities to
      Web SSO in the format and processing rules

   o  Changed the grant_type (was assertion_type) URI from
      http://oauth.net/assertion_type/saml/2.0/bearer to

   o  Changed title to include "Grant Type" in it.

   o  Editorial updates based on feedback from the WG and others
      (including capitalization of Assertion when referring to SAML).


   o  Initial I-D

5.  References

5.1.  Normative References

              Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
              OAuth 2.0 Authorization Protocol",
              ID draft-ietf-oauth-v2-12 (work in progress), Dec 2010.

              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

5.2.  Informative References

              Cantor, S., Ed., "SAML V2.0 Condition for Delegation
              Restriction", Nov 2009.

              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Maler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard OASIS.saml-profiles-2.0-os, March 2005.

              Hirsch, F., Philpott, R., and E. Maler, "Security and
              Privacy Considerations for the OASIS Security Markup
              Language (SAML) V2.0", OASIS Standard saml-sec-consider-
              2.0-os, March 2005.

              Hors, A., Raggett, D., and I. Jacobs, I., and D. Raggett, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,

Authors' Addresses

   Brian Campbell (editor)
   Ping Identity Corp.

   Email: brian.d.campbell@gmail.com
   Chuck Mortimore

   Email: cmortimore@salesforce.com