SAML 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants
draft-ietf-oauth-saml2-bearer-17

Abstract

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

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 15, 2014.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 1.2. Terminology
• 2. HTTP Parameter Bindings for Transporting Assertions
• 2.1. Using SAML Assertions as Authorization Grants
• 2.2. Using SAML Assertions for Client Authentication
• 3. Assertion Format and Processing Requirements
• 3.1. Authorization Grant Processing
• 3.2. Client Authentication Processing
• 4. Authorization Grant Example
• 5. Interoperability Considerations
• 6. Security Considerations
• 7. IANA Considerations
• 7.1. Sub-Namespace Registration of urn:ietf:params:oauth:grant-type:saml2-bearer
• 7.2. Sub-Namespace Registration of urn:ietf:params:oauth:client-assertion-type:saml2-bearer
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Document History
• Authors' Addresses

1. Introduction

The Security Assertion Markup Language (SAML) 2.0 [OASIS.saml-core-2.0-os] is an XML-based framework that allows 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 Framework [RFC6749] 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 to support additional clients or to provide a bridge between OAuth and other trust frameworks. Finally, OAuth allows the definition of additional authentication mechanisms to be used by clients when interacting with the authorization server.

The Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions] specification is an abstract extension to OAuth 2.0 that provides a general framework for the use of Assertions as client credentials and/or authorization grants with OAuth 2.0. This specification profiles the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions] specification to define an extension grant type that uses a SAML 2.0 Bearer Assertion to request an OAuth 2.0 access token as well as for use as client credentials. 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 the SAML Profiles [OASIS.saml-profiles-2.0-os] specification. This specification is reusing, to the extent reasonable, concepts and patterns from that well-established Profile.

This document defines how a SAML Assertion can be used to request an access token when a client wishes to utilize an existing trust relationship, expressed through the semantics of (and digital signature or keyed message digest calculated over) the SAML Assertion, without a direct user approval step at the authorization server. It also defines how a SAML Assertion can be used as a client authentication mechanism. The use of an Assertion for client authentication is orthogonal to and separable from using an Assertion as an authorization grant. They can be used either in combination or separately. Client assertion authentication is nothing more than an alternative way for a client to authenticate to the token endpoint and must be used in conjunction with some grant type to form a complete and meaningful protocol request. Assertion authorization grants may be used with or without client authentication or identification. Whether or not client authentication is needed in conjunction with an assertion authorization grant, as well as the supported types of client authentication, are policy decisions at the discretion of the authorization server.

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

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].

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

1.2. Terminology

All terms are as defined in The OAuth 2.0 Authorization Framework [RFC6749], the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions], and the Security Assertion Markup Language (SAML) 2.0 [OASIS.saml-core-2.0-os] specifications.

2. HTTP Parameter Bindings for Transporting Assertions

The Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions] specification defines generic HTTP parameters for transporting Assertions during interactions with a token endpoint. This section defines specific parameters and treatments of those parameters for use with SAML 2.0 Bearer Assertions.

2.1. Using SAML Assertions as Authorization Grants

To use a SAML Bearer Assertion as an authorization grant, use an access token request as defined in Section 4 of the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions] specification with the following specific parameter values and encodings.

The value of the grant_type parameter MUST be urn:ietf:params:oauth:grant-type:saml2-bearer.

The value of the assertion parameter MUST contain a single 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 RFC 4648 [RFC4648] and where the padding bits are set to zero. 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.

The scope parameter may be used, as defined in the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I-D.ietf-oauth-assertions] specification, to indicate the requested scope.

Authentication of the client is optional, as described in Section 3.2.1 of OAuth 2.0 [RFC6749] and consequently, the client_id is only needed when a form of client authentication that relies on the parameter is used.

The following non-normative example demonstrates an Access Token Request with an assertion as an authorization grant (with extra line breaks for display purposes only):

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

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer&
  assertion=PHNhbWxwOl...[omitted for brevity]...ZT4

2.2. Using SAML Assertions for Client Authentication

To use a SAML Bearer Assertion for client authentication, use the following parameter values and encodings.

The value of the client_assertion_type parameter MUST be urn:ietf:params:oauth:client-assertion-type:saml2-bearer.

The value of the client_assertion parameter MUST contain a single 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 RFC 4648 [RFC4648] and where the padding bits are set to zero. 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.

The following non-normative example demonstrates a client authenticating using an assertion during the presentation of an authorization code grant in an Access Token Request (with extra line breaks for display purposes only):

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

  grant_type=authorization_code&
  code=vAZEIHjQTHuGgaSvyW9hO0RpusLzkvTOww3trZBxZpo&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
  %3Aclient-assertion-type%3Asaml2-bearer&
  client_assertion=PHNhbW...[omitted for brevity]...ZT

3. Assertion Format and Processing Requirements

In order to issue an access token response as described in OAuth 2.0 [RFC6749] or to rely on an Assertion for client authentication, the authorization server MUST validate the Assertion according to the criteria below. Application of additional restrictions and policy are at the discretion of the authorization server.

1. The Assertion's <Issuer> element MUST contain a unique identifier for the entity that issued the Assertion. Issuer values SHOULD be compared using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986], unless otherwise specified by the application.
2. The Assertion MUST contain a <Conditions> element with an <AudienceRestriction> element with an <Audience> element that identifies the authorization server as an intended audience. Section 2.5.1.4 of Assertions and Protocols for the OASIS Security Assertion Markup Language [OASIS.saml-core-2.0-os] defines the <AudienceRestriction> and <Audience> elements and, in addition to the URI references discussed there, the token endpoint URL of the authorization server MAY be used as a URI that identifies the authorization server as an intended audience. Assertions that do not identify the Authorization Server as an intended audience MUST be rejected. Audience values SHOULD be compared using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986], unless otherwise specified by the application.
3. The Assertion MUST contain a <Subject> element. The subject MAY identify the resource owner for Additional information identifying the subject/principal of the transaction MAY be included in an <AttributeStatement>.
   1. When using an Assertion as an authorization grant, the Subject SHOULD identify an authorized accessor for whom the access token is being requested (typically the resource owner, or an authorized delegate).
   2. For client authentication, the Subject MUST be the client_id of the OAuth client.

4. The Assertion MUST have an expiry that limits the time window during which it can be used. The expiry can be expressed either as the NotOnOrAfter attribute of the <Conditions> element or as the NotOnOrAfter attribute of a suitable <SubjectConfirmationData> element.
5. The <Subject> element MUST contain at least one <SubjectConfirmation> element that allows the authorization server to confirm it as a Bearer Assertion. Such a <SubjectConfirmation> element MUST have a Method attribute with a value of urn:oasis:names:tc:SAML:2.0:cm:bearer. The <SubjectConfirmation> element MUST contain a <SubjectConfirmationData> element, unless the Assertion has a suitable NotOnOrAfter attribute on the <Conditions> element, in which case the <SubjectConfirmationData> element MAY be omitted. When present, the <SubjectConfirmationData> element MUST have a Recipient attribute with a value indicating the token endpoint URL of the authorization server (or an acceptable alias). 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 <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.
6. The authorization server MUST verify that the NotOnOrAfter instant has not passed, subject to allowable clock skew between systems. An invalid NotOnOrAfter instant on the <Conditions> element invalidates the entire Assertion. An invalid NotOnOrAfter instant on a <SubjectConfirmationData> element only invalidates the individual <SubjectConfirmation>. The authorization server MAY reject Assertions with a NotOnOrAfter instant that is unreasonably far in the future. 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 applicable NotOnOrAfter instant.
7. If the Assertion issuer authenticated the subject, the Assertion SHOULD contain a single <AuthnStatement> representing that authentication event.
8. If the Assertion was issued with the intention that the presenter act autonomously on behalf of the subject, an <AuthnStatement> SHOULD NOT be included. The presenter SHOULD be identified in the <NameID> or similar element in the <SubjectConfirmation> element, or by other available means like SAML V2.0 Condition for Delegation Restriction [OASIS.saml-deleg-cs].
9. Other statements, in particular <AttributeStatement> elements, MAY be included in the Assertion.
10. The Assertion MUST be digitally signed or have a keyed message digest applied by the issuer. The authorization server MUST reject assertions with an invalid signature or keyed message digest.
11. Encrypted elements MAY appear in place of their plain text counterparts as defined in [OASIS.saml-core-2.0-os].
12. 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.

3.1. Authorization Grant Processing

Assertion authorization grants may be used with or without client authentication or identification. Whether or not client authentication is needed in conjunction with an assertion authorization grant, as well as the supported types of client authentication, are policy decisions at the discretion of the authorization server. However, if client credentials are present in the request, the authorization server MUST validate them.

If the Assertion is not valid, or its subject confirmation requirements cannot be met, the authorization server MUST construct an error response as defined in OAuth 2.0 [RFC6749]. 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":"invalid_grant",
    "error_description":"Audience validation failed"
  }

3.2. Client Authentication Processing

If the client Assertion is not valid, or its subject confirmation requirements cannot be met, the authorization server MUST construct an error response as defined in OAuth 2.0 [RFC6749]. The value of the error parameter MUST be the invalid_client 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.

4. Authorization Grant Example

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

The example shows an assertion issued and signed by the SAML Identity Provider identified as https://saml-idp.example.com. The subject of the assertion is identified by email address as brian@example.com, who authenticated to the Identity Provider by means of a digital signature where the key was validated as part of an X.509 Public Key Infrastructure. The intended audience of the assertion is https://saml-sp.example.net, which is an identifier for a SAML Service Provider with which the authorization server identifies itself. The assertion is sent as part of an access token request to the authorization server's token endpoint at https://authz.example.net/token.oauth2.

  <Assertion IssueInstant="2010-10-01T20:07:34.619Z"
    ID="ef1xsbZxPV2oqjd7HTLRLIBlBb7"
    Version="2.0"
    xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
   <Issuer>https://saml-idp.example.com</Issuer>
   <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    [...omitted for brevity...]
   </ds:Signature>
   <Subject>
    <NameID
      Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
     brian@example.com
    </NameID>
    <SubjectConfirmation
      Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
     <SubjectConfirmationData
       NotOnOrAfter="2010-10-01T20:12:34.619Z"
       Recipient="https://authz.example.net/token.oauth2"/>
     </SubjectConfirmation>
    </Subject>
    <Conditions>
      <AudienceRestriction>
        <Audience>https://saml-sp.example.net</Audience>
      </AudienceRestriction>
    </Conditions>
    <AuthnStatement AuthnInstant="2010-10-01T20:07:34.371Z">
      <AuthnContext>
        <AuthnContextClassRef>
          urn:oasis:names:tc:SAML:2.0:ac:classes:X509
        </AuthnContextClassRef>
      </AuthnContext>
    </AuthnStatement>
  </Assertion>

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

  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-
  bearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ9IjIwMTEtMDU
  [...omitted for brevity...]aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-