Internet Engineering Task Force (IETF) B. Campbell Request for Comments: 7522 Ping Identity Category: Standards Track C. Mortimore ISSN: 2070-1721 Salesforce M. Jones Microsoft May 2015
Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants
Abstract
This specification defines the use of a Security Assertion Markup Language (SAML) 2.0 Bearer Assertion as a means for requesting an OAuth 2.0 access token as well as for client authentication.
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/rfc7522.
Copyright Notice
Copyright (c) 2015 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.
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 (SSO), 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. (Some examples include [OASIS.WSS-SAMLTokenProfile] and [OASIS.WS-Fed].) An Assertion is generally issued by an Identity Provider and consumed by a Service Provider that 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
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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.
"Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7521] 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 OAuth Assertion Framework [RFC7521] 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 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 it 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.
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.
All terms are as defined in the following specifications: "The OAuth 2.0 Authorization Framework" [RFC6749], the OAuth Assertion Framework [RFC7521], and "Assertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0" [OASIS.saml-core-2.0-os].
2. HTTP Parameter Bindings for Transporting Assertions
The OAuth Assertion Framework [RFC7521] 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, the client uses an access token request as defined in Section 4 of the OAuth Assertion Framework [RFC7521] with the following specific parameter values and encodings.
The value of the "grant_type" parameter is "urn:ietf:params:oauth:grant-type:saml2-bearer".
The value of the "assertion" parameter contains a single SAML 2.0 Assertion. It MUST NOT contain more than one 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 MUST NOT be line wrapped and pad characters ("=") MUST NOT be included.
The "scope" parameter may be used, as defined in the OAuth Assertion Framework [RFC7521], to indicate the requested scope.
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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 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, the client uses the following parameter values and encodings.
The value of the "client_assertion_type" parameter is "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 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=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4& client_assertion_type=urn%3Aietf%3Aparams%3Aoauth %3Aclient-assertion-type%3Asaml2-bearer& client_assertion=PHNhbW...[omitted for brevity]...ZT
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. In the absence of an application profile specifying otherwise, compliant applications MUST compare Issuer values using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986].
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 (SAML) V2.0" [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. The authorization server MUST reject any Assertion that does not contain its own identity as the intended audience. In the absence of an application profile specifying otherwise, compliant applications MUST compare the Audience values using the Simple String Comparison method defined in Section 6.2.1 of RFC 3986 [RFC3986]. As noted in Section 5, the precise strings to be used as the Audience for a given authorization server must be configured out of band by the authorization server and the issuer of the Assertion.
3. The Assertion MUST contain a <Subject> element identifying the principal that is the subject of the Assertion. Additional information identifying the subject/principal MAY be included in an <AttributeStatement>.
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, it 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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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 has a Method attribute with a value of "urn:oasis:names:tc:SAML:2.0:cm:bearer". If the Assertion does not have a suitable NotOnOrAfter attribute on the <Conditions> element, the <SubjectConfirmation> element MUST contain a <SubjectConfirmationData> element. 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 reject the entire Assertion if the NotOnOrAfter instant on the <Conditions> element has passed (subject to allowable clock skew between systems). The authorization server MUST reject the <SubjectConfirmation> (but MAY still use the rest of the Assertion) if the NotOnOrAfter instant on the <SubjectConfirmationData> has passed (subject to allowable clock skew). Note that 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 directly authenticated the subject, the Assertion SHOULD contain a single <AuthnStatement> representing that authentication event. If the Assertion was issued with the intention that the client act autonomously on behalf of the subject, an <AuthnStatement> SHOULD NOT be included and the client presenting the Assertion 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].
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8. Other statements, in particular <AttributeStatement> elements, MAY be included in the Assertion.
9. The Assertion MUST be digitally signed or have a Message Authentication Code (MAC) applied by the issuer. The authorization server MUST reject Assertions with an invalid signature or MAC.
10. Encrypted elements MAY appear in place of their plaintext counterparts as defined in [OASIS.saml-core-2.0-os].
11. The authorization server MUST reject an Assertion that is not valid in all other respects per [OASIS.saml-core-2.0-os], such as (but not limited to) all content within the Conditions element including the NotOnOrAfter and NotBefore attributes, unknown condition types, etc.
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 constructs 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
If the client Assertion is not valid (including if its subject confirmation requirements cannot be met), the authorization server constructs 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.
The following examples illustrate what a conforming Assertion and an 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".
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Below is an example SAML 2.0 Assertion (whitespace formatting is for display purposes only):
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-
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] specifies one common method of exchanging SAML-related information about system entities.
The RSA-SHA256 algorithm, from [RFC6931], is a mandatory-to-implement XML signature algorithm for this profile.
The security considerations described within the following specifications are all applicable to this document: "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" [RFC7521], "The OAuth 2.0 Authorization Framework" [RFC6749], and "Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0" [OASIS.saml-sec-consider-2.0-os].
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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.
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 Transport Layer Security (TLS). In cases where it is desirable to prevent disclosure of certain information to 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 by 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 OAuth Assertion Framework [RFC7521].
8. IANA Considerations
8.1. Sub-Namespace Registration of urn:ietf:params:oauth:grant-type:saml2-bearer
This section registers the value "grant-type:saml2-bearer" in the IANA "OAuth URI" registry established by "An IETF URN Sub-Namespace for OAuth" [RFC6755].
o URN: urn:ietf:params:oauth:grant-type:saml2-bearer
o Common Name: SAML 2.0 Bearer Assertion Grant Type Profile for OAuth 2.0
8.2. Sub-Namespace Registration of urn:ietf:params:oauth:client-assertion-type:saml2-bearer
This section registers the value "client-assertion-type:saml2-bearer" in the IANA "OAuth URI" registry established by "An IETF URN Sub- Namespace for OAuth" [RFC6755].
o URN: urn:ietf:params:oauth:client-assertion-type:saml2-bearer
o Common Name: SAML 2.0 Bearer Assertion Profile for OAuth 2.0 Client Authentication
[OASIS.saml-core-2.0-os] Cantor, S., Kemp, J., Philpott, R., and E. Maler, "Assertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0", OASIS Standard saml-core-2.0-os, March 2005, <http://docs.oasis-open.org/ security/saml/v2.0/saml-core-2.0-os.pdf>.
[OASIS.saml-deleg-cs] Cantor, S., Ed., "SAML V2.0 Condition for Delegation Restriction Version 1", Committee Specification 01, November 2009, <http://docs.oasis-open.org/ security/saml/Post2.0/sstc-saml-delegation-cs-01.html>.
[OASIS.saml-sec-consider-2.0-os] Hirsch, F., Philpott, R., and E. Maler, "Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0", OASIS Standard saml-sec-consider-2.0-os, March 2005, <http://docs.oasis-open.org/security/saml/v2.0/ saml-sec-consider-2.0-os.pdf>.
[OASIS.WS-Fed] Goodner, M. and A. Nadalin, "Web Services Federation Language (WS-Federation) Version 1.2", OASIS Standard, May 2009, <http://docs.oasis-open.org/wsfed/ federation/v1.2/os/ws-federation-1.2-spec-os.html>.
[OASIS.WSS-SAMLTokenProfile] Monzillo, R., Kaler, C., Nadalin, T., Hallam-Baker, P., and C. Milono, "Web Services Security SAML Token Profile Version 1.1.1", OASIS Standard, May 2012, <http://docs.oasis-open.org/wss-m/wss/v1.1.1/ wss-SAMLTokenProfile-v1.1.1.html>.
[OASIS.saml-metadata-2.0-os] Cantor, S., Moreh, J., Philpott, R., and E. Maler, "Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0", OASIS Standard saml-metadata-2.0-os, March 2005, <http://docs.oasis-open.org/security/saml/v2.0/ saml-metadata-2.0-os.pdf>.
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[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, <http://docs.oasis-open.org/security/saml/v2.0/ saml-profiles-2.0-os.pdf>.
[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, <http://www.w3.org/TR/1999/REC-html401-19991224>.
Acknowledgements
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.