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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.
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
This Internet-Draft will expire on January 24, 2015.
Copyright (c) 2014 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.
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.
Privacy Considerations
8.
IANA Considerations
8.1.
Sub-Namespace Registration of urn:ietf:params:oauth:grant-type:saml2-bearer
8.2.
Sub-Namespace Registration of urn:ietf:params:oauth:client-assertion-type:saml2-bearer
9.
References
9.1.
Normative References
9.2.
Informative References
Appendix A.
Acknowledgements
Appendix B.
Document History
§
Authors' Addresses
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The Security Assertion Markup Language (SAML) 2.0 [OASIS.saml‑core‑2.0‑os] (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) 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] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.) 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] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.) 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] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.) 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] (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,” March 2005.) 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.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
Unless otherwise noted, all the protocol parameter names and values are case sensitive.
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All terms are as defined in The OAuth 2.0 Authorization Framework [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.), the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I‑D.ietf‑oauth‑assertions] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.), and the Security Assertion Markup Language (SAML) 2.0 [OASIS.saml‑core‑2.0‑os] (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) specifications.
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The Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I‑D.ietf‑oauth‑assertions] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.) 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.
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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] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.) 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] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) 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] (Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.), 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] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.) specification, to indicate the requested scope.
Authentication of the client is optional, as described in Section 3.2.1 of OAuth 2.0 [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.) 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
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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] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) 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] (Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” December 1999.), 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
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In order to issue an access token response as described in OAuth 2.0 [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.) 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.
- A.
- For the authorization grant, the Subject typically identifies an authorized accessor for which the access token is being requested (i.e., the resource owner or an authorized delegate), but in some cases, may be a pseudonymous identifier or other value denoting an anonymous user.
- B.
- For client authentication, the Subject MUST be the client_id of the OAuth client.
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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 (including if its subject confirmation requirements cannot be met), the authorization server MUST construct an error response as defined in OAuth 2.0 [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.). 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" }
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If the client Assertion is not valid (including if its subject confirmation requirements cannot be met), the authorization server MUST construct an error response as defined in OAuth 2.0 [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.). 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.
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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.
Below is an example SAML 2.0 Assertion (whitespace formatting is for display purposes only):
<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>
Figure 1: 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 (with extra line breaks for display purposes only):
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-
Figure 2: Example Request |
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Agreement between system entities regarding identifiers, keys, and endpoints is required in order to achieve interoperable deployments of this profile. Specific items that require agreement are as follows: values for the issuer and audience identifiers, the location of the token endpoint, the key used to apply and verify the digital signature over the assertion, one-time use restrictions on assertions, maximum assertion lifetime allowed, and the specific subject and attribute requirements of the assertion. The exchange of such information is explicitly out of scope for this specification and typical deployment of it will be done alongside existing SAML Web SSO deployments that have already established a means of exchanging such information. Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0 [OASIS.saml‑metadata‑2.0‑os] (Cantor, S., Moreh, J., Philpott, R., and E. Maler, “Metadata for the Security Assertion Markup Language (SAML) V2.0,” March 2005.) is one common method of exchanging SAML related information about system entities.
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The security considerations described within the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I‑D.ietf‑oauth‑assertions] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.), The OAuth 2.0 Authorization Framework [RFC6749] (Hardt, D., “The OAuth 2.0 Authorization Framework,” October 2012.), and the Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0 [OASIS.saml‑sec‑consider‑2.0‑os] (Hirsch, F., Philpott, R., and E. Maler, “Security and Privacy Considerations for the OASIS Security Markup Language (SAML) V2.0,” March 2005.) specifications are all applicable to this document.
The specification does not mandate replay protection for the SAML assertion usage for either the authorization grant or for client authentication. It is an optional feature, which implementations may employ at their own discretion.
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A SAML Assertion may contain privacy-sensitive information and, to prevent disclosure of such information to unintended parties, should only be transmitted over encrypted channels, such as TLS. In cases where it is desirable to prevent disclosure of certain information the client, the Subject and/or individual attributes of a SAML Assertion should be encrypted to the authorization server.
Deployments should determine the minimum amount of information necessary to complete the exchange and include only that information in an Assertion (typically by limiting what information is included in an <AttributeStatement> or omitting it altogether). In some cases, the Subject can be a value representing an anonymous or pseudonymous user, as described in Section 6.3.1 of the Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants [I‑D.ietf‑oauth‑assertions] (Campbell, B., Mortimore, C., Jones, M., and Y. Goland, “Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants,” July 2014.).
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This is a request to IANA to please register the value grant-type:saml2-bearer in the registry urn:ietf:params:oauth established in An IETF URN Sub-Namespace for OAuth [RFC6755] (Campbell, B. and H. Tschofenig, “An IETF URN Sub-Namespace for OAuth,” October 2012.).
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This is a request to IANA to please register the value client-assertion-type:saml2-bearer in the registry urn:ietf:params:oauth established in An IETF URN Sub-Namespace for OAuth [RFC6755] (Campbell, B. and H. Tschofenig, “An IETF URN Sub-Namespace for OAuth,” October 2012.).
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[OASIS.saml-deleg-cs] | Cantor, S., Ed., “SAML V2.0 Condition for Delegation Restriction,” Nov 2009. |
[OASIS.saml-metadata-2.0-os] | Cantor, S., Moreh, J., Philpott, R., and E. Maler, “Metadata for the Security Assertion Markup Language (SAML) V2.0,” OASIS Standard saml-metadata-2.0-os, March 2005. |
[OASIS.saml-profiles-2.0-os] | 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. |
[OASIS.saml-sec-consider-2.0-os] | 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. |
[W3C.REC-html401-19991224] | Raggett, D., Hors, A., and I. Jacobs, “HTML 4.01 Specification,” World Wide Web Consortium Recommendation REC-html401-19991224, December 1999 (HTML). |
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The following people contributed wording and concepts to this document: Paul Madsen, Patrick Harding, Peter Motykowski, Eran Hammer, Peter Saint-Andre, Ian Barnett, Eric Fazendin, Torsten Lodderstedt, Susan Harper, Scott Tomilson, Scott Cantor, Hannes Tschofenig, David Waite, Phil Hunt, and Mukesh Bhatnagar.
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[[ to be removed by RFC editor before publication as an RFC ]]
draft-ietf-oauth-saml2-bearer-21
draft-ietf-oauth-saml2-bearer-20
draft-ietf-oauth-saml2-bearer-19
draft-ietf-oauth-saml2-bearer-18
draft-ietf-oauth-saml2-bearer-17
draft-ietf-oauth-saml2-bearer-16
draft-ietf-oauth-saml2-bearer-15
draft-ietf-oauth-saml2-bearer-14
draft-ietf-oauth-saml2-bearer-13
draft-ietf-oauth-saml2-bearer-12
draft-ietf-oauth-saml2-bearer-11
draft-ietf-oauth-saml2-bearer-10
draft-ietf-oauth-saml2-bearer-09
draft-ietf-oauth-saml2-bearer-08
draft-ietf-oauth-saml2-bearer-07
draft-ietf-oauth-saml2-bearer-06
draft-ietf-oauth-saml2-bearer-05
draft-ietf-oauth-saml2-bearer-04
draft-ietf-oauth-saml2-bearer-03
draft-ietf-oauth-saml2-bearer-02
draft-ietf-oauth-saml2-bearer-01
draft-ietf-oauth-saml2-bearer-00
draft-campbell-oauth-saml-01
draft-campbell-oauth-saml-00
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Brian Campbell | |
Ping Identity | |
Email: | brian.d.campbell@gmail.com |
Chuck Mortimore | |
Salesforce.com | |
Email: | cmortimore@salesforce.com |
Michael B. Jones | |
Microsoft | |
Email: | mbj@microsoft.com |
URI: | http://self-issued.info/ |