B. Campbell, Ed.
Internet-Draft Ping Identity Corp.
Intended status: Standards Track C. Mortimore
Expires: August 7, 2011 Salesforce.com
February 3, 2011
SAML 2.0 Bearer Assertion Grant Type Profile for OAuth 2.0
draft-ietf-oauth-saml2-bearer-02
Abstract
This specification defines the use of a SAML 2.0 Bearer Assertion as
means for requesting an OAuth 2.0 access token.
Status of this Memo
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This Internet-Draft will expire on August 7, 2011.
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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
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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 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 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",
"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.
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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
server.
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
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:
assertion
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
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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.
scope
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
token.
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
"urn:oasis:names:tc:SAML:2.0:nameid-format:entity".
o The Assertion MUST contain a <Subject> element. The subject MAY
identify the resource owner for whom the access token is being
requested.
o The <Subject> element MUST contain at least one
<SubjectConfirmation> element that allows the authorization server
to confirm it as a 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.
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* 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.
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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":"invalid_grant",
"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.
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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 2: Example SAML 2.0 Assertion
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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
grant_type=http%3A%2F%2Foauth.net%2Fgrant_type%2Fassertion%2F
saml%2F2.0%2Fbearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ
[...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
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Appendix B. Document History
[[ to be removed by RFC editor before publication as an RFC ]]
draft-ietf-oauth-saml2-bearer-02
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
http://www.ietf.org/mail-archive/web/oauth/current/msg05253.html
o Editorial changes based on feedback
draft-ietf-oauth-saml2-bearer-01
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
progress
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.
draft-ietf-oauth-saml2-bearer-00
o Added Parameter Registration Request for "assertion" to IANA
Considerations.
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
Framework).
draft-campbell-oauth-saml-01
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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
SubjectConfirmationData.
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
http://oauth.net/grant_type/assertion/saml/2.0/bearer
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).
draft-campbell-oauth-saml-00
o Initial I-D
5. References
5.1. Normative References
[I-D.ietf.oauth-v2]
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.
[OASIS.saml-core-2.0-os]
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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
[OASIS.saml-deleg-cs]
Cantor, S., Ed., "SAML V2.0 Condition for Delegation
Restriction", Nov 2009.
[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]
Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
Authors' Addresses
Brian Campbell (editor)
Ping Identity Corp.
Email: brian.d.campbell@gmail.com
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Chuck Mortimore
Salesforce.com
Email: cmortimore@salesforce.com
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