OAuth 2.0 Assertion ProfileSalesforce.comcmortimore@salesforce.comMicrosoftmbj@microsoft.comPing Identity Corp.brian.d.campbell@gmail.comMicrosoftyarong@microsoft.com
Internet
OAuthSAMLAssertionThis specification provides a general framework for the use of
assertions as client credentials and/or authorization grants with OAuth
2.0. It includes a generic mechanism for transporting assertions during
interactions with a token endpoint, as wells as rules for the content
and processing of those assertions. The intent is to provide an enhanced
security profile by using derived values such as signatures or HMACs, as
well as facilitate the use of OAuth 2.0 in client-server integration
scenarios where the end-user may not be present.This specification only defines abstract message flow and assertion
content. Actual use requires implementation of a companion protocol
binding specification. Additional profile documents provide standard
representations in formats such as SAML and JWT.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 .Throughout this document, values are quoted to indicate that they are
to be taken literally. When using these values in protocol messages, the
quotes MUST NOT be used as part of the value.The OAuth 2.0 Authorization Protocol
provides a method for making authenticated HTTP requests to a
resource using an access token. Access tokens are issued to clients by
an authorization server with the (sometimes implicit) approval of the
resource owner. These access tokens are typically obtained by exchanging
an authorization grant representing authorization by the resource owner
or privileged administrator. 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.In scenarios where security is at a premium one wants to avoid
sending secrets across the Internet, even on encrypted connections.
Instead one wants to send values derived from the secret that prove to
the receiver that the sender is in possession of the secret without
actually sending the secret. Typically the way derived values are
created is by generating an assertion that is then either HMAC’ed
or digitally signed using an agreed upon secret. By validating the HMAC
or digital signature on the assertion, the receiver can prove to
themselves that the entity that generated the assertion was in
possession of the secret without actually communicating the secret
directly.This specification provides a general framework for the use of
assertions as client credentials and/or authorization grants with OAuth
2.0. It includes a generic mechanism for transporting assertions during
interactions with a token endpoint, as wells as rules for the content
and processing of those assertions. The intent is to provide an enhanced
security profile by using derived values such as signatures or HMACs, as
well as facilitate the use of OAuth 2.0 in client-server integration
scenarios where the end-user may not be present.This specification only defines abstract message flow and assertion
content. Actual use requires implementation of a companion protocol
binding specification. Additional profile documents provide standard
representations in formats such as SAML and JWT.This specification provides a model for using assertions for
authentication of an OAuth client during interactions with an
Authorization Server, as well as the use of assertions as authorization
grants. It is important to note that the use of assertions for client
authentication is orthogonal and separable from using assertions as an
authorization grant and can be used either in combination or in
isolation. In addition, in scenarios when assertion based authentication
and authorization are used in combination, the assertion format and
processing may be redundant; under such circumstances, the protocol may
be optimized to present a single assertion.This section defines generic HTTP parameters for transporting
assertions during interactions with a token endpoint.In scenarios where one wants to avoid sending secrets, one wants to
send values derived from the secret that prove to the receiver that
the sender is in possession of the secret without actually sending the
secret.For example, a client can establish a secret using an out-of-band
mechanism with a resource server. As part of this out-of-band
communication the client and resource server agree that the client
will authenticate itself using an assertion with agreed upon
parameters that will be signed by the provisioned secret. Later on,
the client might send an access token request to the token endpoint
for the resource server that includes an authorization code, as well
as a client_assertion that is signed with the previously agreed key
and parameters. The client_assertion proves to the token endpoint the
identity of the client and the authorization code provides the link to
the end-user authorization.The following section defines the use of assertions as client
credentials as an extension of Section 2.3 of OAuth 2.0. When using
assertions as client credentials, the client includes the assertion
and related information using the following HTTP request parameters:OPTIONAL. The client identifier as
described in Section 2.2 of OAuth
2.0. When present, the client_id MUST identify the client to the authorization server.REQUIRED. The format of the
assertion as defined by the authorization server. The value MUST
be an absolute URI.REQUIRED. The assertion being used
to authenticate the client. Specific serialization of the
assertion is defined by profile documents. The serialization MUST
be encoded for transport within HTTP forms. It is RECOMMENDED that
base64url be used.The following non-normative example demonstrates a client
authenticating using an assertion during an Authorization Code Access
Token Request as defined in Section 4.1.3 of
OAuth 2.0
(with line breaks for display purposes only):Token endpoints can differentiate between assertion based
credentials and other client credential types by looking for the
presence of the client_assertion and
client_assertion_type parameters,
which will only be present when using assertions for client
authentication.If an assertion is invalid for any reason or if more than one client authentication mechanism is used, the Authorization
Server MUST construct an error response as defined in OAuth 2.0. The value of the error
parameter MUST be the invalid_client error code. The
authorization server MAY include additional information regarding the
reasons the client assertion was considered invalid using the error_description
or error_uri parameters.For example:An assertion can be used to request an access token when a client
wishes to utilize an existing trust relationship. This may be done
through the semantics of (and a digital signature/HMAC calculated
over) the assertion, and expressed to the authorization server
through an extension authorization grant type.
The processes by which authorization is previously granted, and by
which the client obtains the assertion prior to exchanging it with the
authorization server, are out of scope.The following defines the use of assertions as authorization grants
as an extension of OAuth 2.0,
Section 4.5. When using assertions as authorization grants, the client
includes the assertion and related information using the following HTTP request
parameters:REQUIRED. The format of the assertion as
defined by the authorization server. The value MUST be an absolute
URI.REQUIRED. The assertion being used as an
authorization grant. Specific serialization of the assertion is
defined by profile documents. The serialization MUST be encoded
for transport within HTTP forms. It is RECOMMENDED that base64url
be used.OPTIONAL. The requested scope as
described in Section 3.3 of OAuth
2.0. When
exchanging assertions for access tokens, the authorization for the
token has been previously granted through some other mechanism. As
such, the requested scope SHOULD be equal or lesser than the scope
originally granted to the authorized accessor. If the scope
parameter and/or value is omitted, the scope SHOULD be treated as
equal to the scope originally granted to the authorized accessor.
The Authorization Server SHOULD limit the scope of the issued
access token to be equal or lesser than the scope originally
granted to the authorized accessor.The following non-normative example demonstrates an assertion being
used as an authorization grant
(with line breaks for display purposes only):An assertion used in this context is generally a short lived representation
of the authorization grant and authorization servers SHOULD NOT issue tokens
that exceed that lifetime by a significant period. In practice, that will
usually mean that refresh tokens are not issued in response to assertion
grant requests and access tokens will be issued with a reasonably limited lifetime.
Clients can refresh an expired access token by requesting a new one using the same
assertion, if it is still valid, or with a new assertion.
If an assertion is not valid or has expired, the Authorization Server
MUST construct an error response as defined in OAuth 2.0. 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:This section provides a general content and processing model for the
use of assertions in OAuth
2.0.The following are entities and metadata involved in the issuance,
exchange and processing of assertions in OAuth 2.0. These are general
terms, abstract from any particular assertion format. Mappings of
these terms into specific representations are provided by profiles of
this specification.The unique identifier for the entity that
issued the assertion. Generally this is the entity that holds the
keying material used to generate the assertion. In some use cases
Issuers may be either OAuth Clients (when assertions are
self-asserted) or a Security Token Service (STS) (an entity capable of
issuing, renewing, transforming and validating of security
tokens).A unique identifier for the subject of the
assertion. When using assertions for client authentication, the
Principal SHOULD be the client_id of the OAuth client. When using
assertions as an authorization grant, the Principal MUST identify
an authorized accessor for whom the access token is being
requested (typically the resource owner, or an authorized
delegate).A URI that identifies the party intended to
process the assertion. The audience SHOULD be the URL of
the Token Endpoint as defined in Section 3.2 of OAuth 2.0.The time at which the assertion was
issued. While the serialization may differ by assertion format,
this is always expressed in UTC with no time zone component.The time at which the assertion expires.
While the serialization may differ by assertion format, this is
always expressed in UTC with no time zone component.A nonce or unique identifier for the
assertion. The Assertion ID may be used by implementations
requiring message de-duplication for one-time use assertions. Any
entity that assigns an identifier MUST ensure that there is
negligible probability that that entity or any other entity will
accidentally assign the same identifier to a different data
object.The following are general format and processing rules for the use
of assertions in OAuth:The assertion MUST contain an Issuer. The Issuer MUST identify
the entity that issued the assertion as recognized by the
Authorization Server. If an assertion is self-asserted, the Issuer
SHOULD be the client_id.The assertion SHOULD contain a Principal. The Principal MUST
identify an authorized accessor for whom the access token is being
requested (typically the resource owner, or an authorized
delegate). When the client is acting on behalf of itself, the
Principal SHOULD be the client_id.The assertion MUST contain an Audience that identifies the
Authorization Server as the intended audience. The Authorization
Server MUST verify that it is an intended audience for the
assertion. The Audience SHOULD be the URL of the Authorization
Server's Token Endpoint.The assertion MUST contain an Expires At entity that limits the
time window during which the assertion can be used. The
authorization server MUST verify that the expiration time has not
passed, subject to allowable clock skew between systems. The
authorization server SHOULD reject assertions with an Expires At
attribute value that is unreasonably far in the future.The assertion MAY contain an Issued At entity containing the
UTC time at which the assertion was issued.The assertion MAY contain an Assertion ID. An Authorization
Server MAY dictate that Assertion ID is mandatory.The Authorization Server MUST validate the assertion's signature
in order to verify the Issuer of the assertion. The algorithm used to validate the
assertion, and the mechanism for designating the secret used to
generate the assertion, are beyond the scope of this specification.The following clarifies the format and processing rules defined in
and
for a number of common use cases:When a client authenticates to a token service using an assertion,
it SHOULD do so according to . The following format and
processing rules apply.The client_assertion_type HTTP parameter MUST identify the
assertion format being used for authentication.The client_assertion HTTP parameter MUST contain the serialized
assertion in a format indicated by the client_assertion_type
parameter.The Principal SHOULD be the client_id.The Issuer of the assertion MUST identify the entity that issued
the assertion as recognized by the Authorization Server. If the
assertion is self-asserted, the Issuer SHOULD be the client_id.The Audience of the assertion MUST identify the Authorization
Server and SHOULD be the URL of the Token Endpoint.The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion.The following non-normative example demonstrates the use of a
client authentication using an assertion during an Authorization Code
Access Token Request as defined in Section 4.1.3 of
OAuth 2.0
(with line breaks for display purposes only):When a client is accessing resources on behalf of itself, it SHOULD
do so in a manner analogous to the Client Credentials flow defined in
Section 4.4 of OAuth 2.0. This
is a special case that combines both the authentication and
authorization grant usage patterns. In this case, the interactions
with the authorization server SHOULD be treated as using an assertion
for Client Authentication according to , with the addition
of a grant_type parameter. The following format and processing rules
apply.The grant_type HTTP request parameter MUST be
client_credentials.The client_assertion_type HTTP parameter MUST identify the
assertion format.The client_assertion HTTP parameter MUST contain the serialized
assertion as in a format indicated by the client_assertion_type
parameter.The Issuer of the assertion MUST identify the entity that
issued the assertion as recognized by the Authorization Server. If
the assertion is self-asserted, the Issuer SHOULD be the
client_id. If the assertion was issued by a Security Token
Service (STS), the Issuer SHOULD identify the STS as recognized by the
Authorization Server.The Principal SHOULD be the client_id.The Audience of the assertion MUST identify the Authorization
Server and SHOULD be the URL of the Token Endpoint.The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion.The following non-normative example demonstrates the use of a
sample assertion being used for a Client Credentials Access Token
Request as defined in Section 4.4.2 of
OAuth 2.0
(with line breaks for display purposes only):When a client is accessing resources on behalf of a user, it SHOULD
be treated as using an assertion as an Authorization Grant according
to . The following format and processing rules apply.The grant_type HTTP request parameter MUST indicate the
assertion format.The assertion HTTP parameter MUST contain the serialized
assertion as in a format indicated by the grant_type
parameter.The Issuer of the assertion MUST identify the entity that
issued the assertion as recognized by the Authorization Server. If
the assertion is self-asserted, the Issuer SHOULD be the
client_id. If the assertion was issued by a
Security Token Service (STS), the Issuer SHOULD
identify the STS as recognized by the Authorization Server.The Principal MUST identify an authorized accessor for whom the
access token is being requested (typically the resource owner, or
an authorized delegate).The Audience of the assertion MUST identify the Authorization
Server and MAY be the URL of the Token Endpoint.The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion.The following non-normative example demonstrates the use of a
client authenticating using an assertion during an Authorization Code
Access Token Request as defined in Section 4.1.3 of OAuth 2.0
(with line breaks for display purposes only):When a client is accessing resources on behalf of an anonymous
user, the following format and processing rules apply.The grant_type HTTP request parameter MUST indicate the
assertion format.The assertion HTTP parameter MUST contain the serialized
assertion as in a format indicated by the grant_type
parameter.The Issuer of the assertion MUST identify the entity that
issued the assertion as recognized by the Authorization Server. If
the assertion is self-asserted, the Issuer SHOULD be the
client_id. If the assertion was issued by a Security Token
Service (STS), the Issuer SHOULD identify the STS as recognized by the
Authorization Server.The Principal SHOULD indicate to the Authorization Server that
the client is acting on-behalf of an anonymous user as defined by
the Authorization Server. It is implied that authorization is based
upon additional criteria, such as additional attributes or claims
provided in the assertion. For example, a client may present an
assertion from a trusted issuer asserting that the bearer is over
18 via an included claim. In this case, no additional information
about the user's identity is included yet all the data needed to
issue an access token is present.The Audience of the assertion MUST identify the Authorization
Server and MAY be the URL of the Token Endpoint.The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion.Authorization Providers concerned with preventing replay attacks may choose to implement using replay detection using a combination of the Assertion ID and Issued At/Expires At attributes. Previously processed assertions MAY be de-duped and rejected based on the Assertion ID. The addition of the validity window relieves the authorization server from maintaining an infinite state table of processed assertion IDs.Authorization Servers SHOULD consider the amount of information exposed in error responses, and the risk associated with exposing details of specific processing errors. In addition, Authorization Servers SHOULD prevent timing attacks related to cryptographic processing of the assertion.There are no additional security considerations beyond those described within
OAuth 2.0, Section 11.
The following is the parameter registration request, as
defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization
Protocol, for the
assertion parameter:
Parameter name: assertionParameter usage location: client authentication, token request
Change controller: IETFSpecification document(s): [[this document]]
The following is the parameter registration request, as
defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization
Protocol, for the
client_assertion parameter:
Parameter name: client_assertionParameter usage location: client authentication, token request
Change controller: IETFSpecification document(s): [[this document]]
The following is the parameter registration request, as
defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization
Protocol, for the
client_assertion_type parameter:
Parameter name: client_assertion_typeParameter usage location: client authentication, token request
Change controller: IETFSpecification document(s): [[this document]]The authors wish to thank the following people that have influenced
or contributed this specification: Paul Madsen, Eric Sachs, Jian Cai,
Tony Nadalin, the authors of OAuth WRAP, and those in the OAuth working group.
[[ to be removed by RFC editor before publication as an RFC ]]
draft-ietf-oauth-assertions-03
updated reference to draft-ietf-oauth-v2 from -25 to -26
draft-ietf-oauth-assertions-02
Added text about limited lifetime ATs and RTs per http://www.ietf.org/mail-archive/web/oauth/current/msg08298.html.Changed the line breaks in some examples to avoid awkward rendering to text format. Also removed encoded '=' padding from a few examples because both known derivative specs, SAML and JWT, omit the padding char in serialization/encoding.Remove section 7 on error responses and move that (somewhat modified) content into subsections of section 4 broken up by authn/authz per http://www.ietf.org/mail-archive/web/oauth/current/msg08735.html.Rework the text about "MUST validate ... in order to establish a mapping between ..." per http://www.ietf.org/mail-archive/web/oauth/current/msg08872.html and http://www.ietf.org/mail-archive/web/oauth/current/msg08749.html.Change "The Principal MUST identify an authorized accessor. If the
assertion is self-issued, the Principal SHOULD be the client_id" in 6.1 per http://www.ietf.org/mail-archive/web/oauth/current/msg08873.html.Update reference in 4.1 to point to 2.3 (rather than 3.2) of oauth-v2 (rather than self) http://www.ietf.org/mail-archive/web/oauth/current/msg08874.html.Move the "Section 3 of" out of the xref to hopefully fix the link in 4.1 and remove the client_id bullet from 4.2 per http://www.ietf.org/mail-archive/web/oauth/current/msg08875.html.Add ref to Section 3.3 of oauth-v2 for scope definition and remove some then redundant text per http://www.ietf.org/mail-archive/web/oauth/current/msg08890.html.Change "The following format and processing rules SHOULD be applied" to "The following format and processing rules apply" in sections 6.x to remove conflicting normative qualification of other normative statements per http://www.ietf.org/mail-archive/web/oauth/current/msg08892.html.Add text the client_id must id the client to 4.1 and remove similar text from other places per http://www.ietf.org/mail-archive/web/oauth/current/msg08893.html.Remove the MUST from the text prior to the HTTP parameter definitions per http://www.ietf.org/mail-archive/web/oauth/current/msg08920.html.Updated examples to use grant_type and client_assertion_type values from the OAuth SAML Assertion Profiles spec.