Internet Engineering Task Force (IETF) B. Campbell
Request for Comments: 7521 Ping Identity
Category: Standards Track C. Mortimore
ISSN: 2070-1721 Salesforce
M. Jones
Y. Goland
Microsoft
May 2015
Assertion Framework for OAuth 2.0 Client Authentication and
Authorization Grants
Abstract
This specification provides a framework for the use of assertions
with OAuth 2.0 in the form of a new client authentication mechanism
and a new authorization grant type. Mechanisms are specified for
transporting assertions during interactions with a token endpoint;
general processing rules are also specified.
The intent of this specification is to provide a common framework for
OAuth 2.0 to interwork with other identity systems using assertions
and to provide alternative client authentication mechanisms.
Note that this specification only defines abstract message flows and
processing rules. In order to be implementable, companion
specifications are necessary to provide the corresponding concrete
instantiations.
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/rfc7521.
Campbell, et al. Standards Track [Page 1]RFC 7521 OAuth Assertion Framework May 2015
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.
Table of Contents
1. Introduction ....................................................3
2. Notational Conventions ..........................................4
3. Framework .......................................................4
4. Transporting Assertions .........................................7
4.1. Using Assertions as Authorization Grants ...................7
4.1.1. Error Responses .....................................8
4.2. Using Assertions for Client Authentication .................9
4.2.1. Error Responses ....................................10
5. Assertion Content and Processing ...............................10
5.1. Assertion Metamodel .......................................10
5.2. General Assertion Format and Processing Rules .............12
6. Common Scenarios ...............................................12
6.1. Client Authentication .....................................13
6.2. Client Acting on Behalf of Itself .........................13
6.3. Client Acting on Behalf of a User .........................13
6.3.1. Client Acting on Behalf of an Anonymous User .......14
7. Interoperability Considerations ................................14
8. Security Considerations ........................................15
8.1. Forged Assertion ..........................................15
8.2. Stolen Assertion ..........................................15
8.3. Unauthorized Disclosure of Personal Information ...........16
8.4. Privacy Considerations ....................................17
9. IANA Considerations ............................................17
9.1. "assertion" Parameter Registration ........................17
9.2. "client_assertion" Parameter Registration .................18
9.3. "client_assertion_type" Parameter Registration ............18
10. References ....................................................18
10.1. Normative References .....................................18
10.2. Informative References ...................................18
Acknowledgements ..................................................20
Authors' Addresses ................................................20
Campbell, et al. Standards Track [Page 2]RFC 7521 OAuth Assertion Framework May 2015
1. Introduction
An assertion is a package of information that facilitates the sharing
of identity and security information across security domains.
Section 3 provides a more detailed description of the concept of an
assertion for the purpose of this specification.
OAuth 2.0 [RFC6749] is an authorization framework that enables a
third-party application to obtain limited access to a protected HTTP
resource. In OAuth, those third-party applications are called
clients; they access protected resources by presenting an access
token to the HTTP resource. 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, which represents the authorization
granted by the resource owner (or by a privileged administrator).
Several authorization grant types are defined to support a wide range
of client types and user experiences. OAuth also provides an
extensibility mechanism for defining additional grant types, which
can serve as a bridge between OAuth and other protocol frameworks.
This specification provides a general framework for the use of
assertions as authorization grants with OAuth 2.0. It also provides
a framework for assertions to be used for client authentication. It
provides generic mechanisms for transporting assertions during
interactions with an authorization server's token endpoint as well as
general rules for the content and processing of those assertions.
The intent is to provide an alternative client authentication
mechanism (one that doesn't send client secrets) and to 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 flows and processing
rules. In order to be implementable, companion specifications are
necessary to provide the corresponding concrete instantiations. For
instance, "Security Assertion Markup Language (SAML) 2.0 Profile for
OAuth 2.0 Client Authentication and Authorization Grants" [RFC7522]
defines a concrete instantiation for Security Assertion Markup
Language (SAML) 2.0 Assertions and "JSON Web Token (JWT) Profile for
OAuth 2.0 Client Authentication and Authorization Grants" [RFC7523]
defines a concrete instantiation for JWTs.
Note: The use of assertions for client authentication is orthogonal
to and separable from using assertions 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
Campbell, et al. Standards Track [Page 3]
RFC 7521 OAuth Assertion Framework May 2015
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.
2. 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 [RFC2119].
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.
3. Framework
An assertion is a package of information that allows identity and
security information to be shared across security domains. An
assertion typically contains information about a subject or
principal, information about the party that issued the assertion and
when was it issued, and the conditions under which the assertion is
to be considered valid, such as when and where it can be used.
The entity that creates and signs or integrity-protects the assertion
is typically known as the "Issuer", and the entity that consumes the
assertion and relies on its information is typically known as the
"Relying Party". In the context of this document, the authorization
server acts as a relying party.
Assertions used in the protocol exchanges defined by this
specification MUST always be integrity protected using a digital
signature or Message Authentication Code (MAC) applied by the issuer,
which authenticates the issuer and ensures integrity of the assertion
content. In many cases, the assertion is issued by a third party,
and it must be protected against tampering by the client that
presents it. An assertion MAY additionally be encrypted, preventing
unauthorized parties (such as the client) from inspecting the
content.
Although this document does not define the processes by which the
client obtains the assertion (prior to sending it to the
authorization server), there are two common patterns described below.
Campbell, et al. Standards Track [Page 4]
RFC 7521 OAuth Assertion Framework May 2015
In the first pattern, depicted in Figure 1, the client obtains an
assertion from a third-party entity capable of issuing, renewing,
transforming, and validating security tokens. Typically, such an
entity is known as a "security token service" (STS) or just "token
service", and a trust relationship (usually manifested in the
exchange of some kind of key material) exists between the token
service and the relying party. The token service is the assertion
issuer; its role is to fulfill requests from clients, which present
various credentials, and mint assertions as requested, fill them with
appropriate information, and integrity-protect them with a signature
or message authentication code. WS-Trust [OASIS.WS-Trust] is one
available standard for requesting security tokens (assertions).
Relying
Party Client Token Service
| | |
| | 1) Request Assertion |
| |------------------------>|
| | |
| | 2) Assertion |
| |<------------------------|
| 3) Assertion | |
|<-------------------------| |
| | |
| 4) OK or Failure | |
|------------------------->| |
| | |
| | |
Figure 1: Assertion Created by Third Party
In the second pattern, depicted in Figure 2, the client creates
assertions locally. To apply the signatures or message
authentication codes to assertions, it has to obtain key material:
either symmetric keys or asymmetric key pairs. The mechanisms for
obtaining this key material are beyond the scope of this
specification.
Although assertions are usually used to convey identity and security
information, self-issued assertions can also serve a different
purpose. They can be used to demonstrate knowledge of some secret,
such as a client secret, without actually communicating the secret
directly in the transaction. In that case, additional information
included in the assertion by the client itself will be of limited
value to the relying party, and for this reason, only a bare minimum
of information is typically included in such an assertion, such as
information about issuing and usage conditions.
Campbell, et al. Standards Track [Page 5]RFC 7521 OAuth Assertion Framework May 2015
Relying
Party Client
| |
| | 1) Create
| | Assertion
| |--------------+
| | |
| | 2) Assertion |
| |<-------------+
| 3) Assertion |
|<-------------------------|
| |
| 4) OK or Failure |
|------------------------->|
| |
| |
Figure 2: Self-Issued Assertion
Deployments need to determine the appropriate variant to use based on
the required level of security, the trust relationship between the
entities, and other factors.
From the perspective of what must be done by the entity presenting
the assertion, there are two general types of assertions:
1. Bearer Assertions: Any entity in possession of a bearer assertion
(the bearer) can use it to get access to the associated resources
(without demonstrating possession of a cryptographic key). To
prevent misuse, bearer assertions need to be protected from
disclosure in storage and in transport. Secure communication
channels are required between all entities to avoid leaking the
assertion to unauthorized parties.
2. Holder-of-Key Assertions: To access the associated resources, the
entity presenting the assertion must demonstrate possession of
additional cryptographic material. The token service thereby
binds a key identifier to the assertion, and the client has to
demonstrate to the relying party that it knows the key
corresponding to that identifier when presenting the assertion.
The protocol parameters and processing rules defined in this document
are intended to support a client presenting a bearer assertion to an
authorization server. They are not directly suitable for use with
holder-of-key assertions. While they could be used as a baseline for
a holder-of-key assertion system, there would be a need for
Campbell, et al. Standards Track [Page 6]RFC 7521 OAuth Assertion Framework May 2015
additional mechanisms (to support proof-of-possession of the secret
key), and possibly changes to the security model (e.g., to relax the
requirement for an Audience).
4. Transporting Assertions
This section defines HTTP parameters for transporting assertions
during interactions with a token endpoint of an OAuth authorization
server. Because requests to the token endpoint result in the
transmission of clear-text credentials (in both the HTTP request and
response), all requests to the token endpoint MUST use Transport
Layer Security (TLS), as mandated in Section 3.2 of OAuth 2.0
[RFC6749].
4.1. Using Assertions as Authorization Grants
This section defines the use of assertions as authorization grants,
based on the definition provided in Section 4.5 of OAuth 2.0
[RFC6749]. When using assertions as authorization grants, the client
includes the assertion and related information using the following
HTTP request parameters:
grant_type
REQUIRED. The format of the assertion as defined by the
authorization server. The value will be an absolute URI.
assertion
REQUIRED. The assertion being used as an authorization grant.
Specific serialization of the assertion is defined by profile
documents.
scope
OPTIONAL. The requested scope as described in Section 3.3 of
OAuth 2.0 [RFC6749]. When exchanging assertions for access
tokens, the authorization for the token has been previously
granted through some out-of-band mechanism. As such, the
requested scope MUST be equal to or less than the scope originally
granted to the authorized accessor. The authorization server MUST
limit the scope of the issued access token to be equal to or less
than the scope originally granted to the authorized accessor.
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.
Campbell, et al. Standards Track [Page 7]RFC 7521 OAuth Assertion Framework May 2015
The following example demonstrates an assertion being used as an
authorization grant (with extra line breaks for display purposes
only):
POST /token HTTP/1.1
Host: server.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
An assertion used in this context is generally a short-lived
representation of the authorization grant, and authorization servers
SHOULD NOT issue access tokens with a lifetime that exceeds the
validity period of the assertion 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 short 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.
An IETF URN for use as the "grant_type" value can be requested using
the template in [RFC6755]. A URN of the form
urn:ietf:params:oauth:grant-type:* is suggested.
4.1.1. Error Responses
If an assertion is not valid or has expired, 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
{
"error":"invalid_grant",
"error_description":"Audience validation failed"
}
Campbell, et al. Standards Track [Page 8]RFC 7521 OAuth Assertion Framework May 2015
4.2. Using Assertions for Client Authentication
The following section defines the use of assertions as client
credentials as an extension of Section 2.3 of OAuth 2.0 [RFC6749].
When using assertions as client credentials, the client includes the
assertion and related information using the following HTTP request
parameters:
client_assertion_type
REQUIRED. The format of the assertion as defined by the
authorization server. The value will be an absolute URI.
client_assertion
REQUIRED. The assertion being used to authenticate the client.
Specific serialization of the assertion is defined by profile
documents.
client_id
OPTIONAL. The client identifier as described in Section 2.2 of
OAuth 2.0 [RFC6749]. The "client_id" is unnecessary for client
assertion authentication because the client is identified by the
subject of the assertion. If present, the value of the
"client_id" parameter MUST identify the same client as is
identified by the client assertion.
The following example demonstrates a client authenticating using an
assertion during an access token request, as defined in Section 4.1.3
of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes
only):
POST /token HTTP/1.1
Host: server.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
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.
An IETF URN for use as the "client_assertion_type" value may be
requested using the template in [RFC6755]. A URN of the form
urn:ietf:params:oauth:client-assertion-type:* is suggested.
Campbell, et al. Standards Track [Page 9]RFC 7521 OAuth Assertion Framework May 2015
4.2.1. Error Responses
If an assertion is invalid for any reason or if more than one client
authentication mechanism is used, 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 client 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_client"
"error_description":"assertion has expired"
}
5. Assertion Content and Processing
This section provides a general content and processing model for the
use of assertions in OAuth 2.0 [RFC6749].
5.1. Assertion Metamodel
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.
Issuer
A unique identifier for the entity that issued the assertion.
Generally, this is the entity that holds the key material used to
sign or integrity-protect the assertion. Examples of issuers are
OAuth clients (when assertions are self-issued) and third-party
security token services. If the assertion is self-issued, the
Issuer value is the client identifier. If the assertion was
issued by a security token service (STS), the Issuer should
identify the STS in a manner recognized by the authorization
server. 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].
Campbell, et al. Standards Track [Page 10]RFC 7521 OAuth Assertion Framework May 2015
Subject
A unique identifier for the principal that is the subject of the
assertion.
* When using assertions for client authentication, the Subject
identifies the client to the authorization server using the
value of the "client_id" of the OAuth client.
* When using assertions as an authorization grant, the Subject
identifies an authorized accessor for which the access token is
being requested (typically, the resource owner or an authorized
delegate).
Audience
A value that identifies the party or parties intended to process
the assertion. The URL of the token endpoint, as defined in
Section 3.2 of OAuth 2.0 [RFC6749], can be used to indicate that
the authorization server is a valid intended audience of the
assertion. 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].
Issued At
The time at which the assertion was issued. While the
serialization may differ by assertion format, it is REQUIRED that
the time be expressed in UTC with no time zone component.
Expires At
The time at which the assertion expires. While the serialization
may differ by assertion format, it is REQUIRED that the time be
expressed in UTC with no time zone component.
Assertion ID
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 for
that entity or any other entity to accidentally assign the same
identifier to a different data object.
Campbell, et al. Standards Track [Page 11]RFC 7521 OAuth Assertion Framework May 2015
5.2. General Assertion Format and Processing Rules
The following are general format and processing rules for the use of
assertions in OAuth:
o The assertion MUST contain an Issuer. The Issuer identifies the
entity that issued the assertion as recognized by the
authorization server. If an assertion is self-issued, the Issuer
MUST be the value of the client's "client_id".
o The assertion MUST contain a Subject. 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. When the client is acting
on behalf of itself, the Subject MUST be the value of the client's
"client_id".
o The assertion MUST contain an Audience that identifies the
authorization server as the intended audience. The authorization
server MUST reject any assertion that does not contain its own
identity as the intended audience.
o The assertion MUST contain an Expires At entity that limits the
time window during which the assertion can be used. The
authorization server MUST reject assertions that have expired
(subject to allowable clock skew between systems). Note that the
authorization server may reject assertions with an Expires At
attribute value that is unreasonably far in the future.
o The assertion MAY contain an Issued At entity containing the UTC
time at which the assertion was issued.
o The authorization server MUST reject assertions with an invalid
signature or MAC. The algorithm used to validate the signature or
message authentication code and the mechanism for designating the
secret used to generate the signature or message authentication
code over the assertion are beyond the scope of this
specification.
6. Common Scenarios
The following provides additional guidance, beyond the format and
processing rules defined in Sections 4 and 5, on assertion use for a
number of common use cases.
Campbell, et al. Standards Track [Page 12]RFC 7521 OAuth Assertion Framework May 2015
6.1. Client Authentication
A client uses an assertion to authenticate to the authorization
server's token endpoint by using the "client_assertion_type" and
"client_assertion" parameters as defined in Section 4.2. The Subject
of the assertion identifies the client. If the assertion is self-
issued by the client, the Issuer of the assertion also identifies the
client.
The example in Section 4.2 shows a client authenticating using an
assertion during an access token request.
6.2. Client Acting on Behalf of Itself
When a client is accessing resources on behalf of itself, it does so
in a manner analogous to the Client Credentials Grant defined in
Section 4.4 of OAuth 2.0 [RFC6749]. 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 Section 4.2, while using the "grant_type"
parameter with the value "client_credentials" to indicate that the
client is requesting an access token using only its client
credentials.
The following example demonstrates an assertion being used for a
client credentials access token request, as defined in Section 4.4.2
of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes
only):
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials&
client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
%3Aclient-assertion-type%3Asaml2-bearer&
client_assertion=PHNhbW...[omitted for brevity]...ZT
6.3. Client Acting on Behalf of a User
When a client is accessing resources on behalf of a user, it does so
by using the "grant_type" and "assertion" parameters as defined in
Section 4.1. The Subject identifies an authorized accessor for which
the access token is being requested (typically, the resource owner or
an authorized delegate).
Campbell, et al. Standards Track [Page 13]RFC 7521 OAuth Assertion Framework May 2015
The example in Section 4.1 shows a client making an access token
request using an assertion as an authorization grant.
6.3.1. Client Acting on Behalf of an Anonymous User
When a client is accessing resources on behalf of an anonymous user,
a mutually agreed-upon Subject identifier indicating anonymity is
used. The Subject value might be an opaque persistent or transient
pseudonymous identifier for the user or be an agreed-upon static
value indicating an anonymous user (e.g., "anonymous"). The
authorization may be based upon additional criteria, such as
additional attributes or claims provided in the assertion. For
example, a client might 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.
More information about anonymity, pseudonymity, and privacy
considerations in general can be found in [RFC6973].
7. Interoperability Considerations
This specification defines a framework for using assertions with
OAuth 2.0. However, as an abstract framework in which the data
formats used for representing many values are not defined, on its
own, this specification is not sufficient to produce interoperable
implementations.
Two other specifications that profile this framework for specific
assertions have been developed: [RFC7522] uses SAML 2.0 Assertions
and [RFC7523] uses JSON Web Tokens (JWTs). These two instantiations
of this framework specify additional details about the assertion
encoding and processing rules for using those kinds of assertions
with OAuth 2.0.
However, even when profiled for specific assertion types, agreements
between system entities regarding identifiers, keys, and endpoints
are required in order to achieve interoperable deployments. Specific
items that require agreement are as follows: values for the Issuer
and Audience identifiers, supported assertion and client
authentication types, the location of the token endpoint, the key
used to apply and verify the digital signature or MAC 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 the scope of this specification. Deployments for particular trust
frameworks, circles of trust, or other uses cases will need to agree
Campbell, et al. Standards Track [Page 14]
RFC 7521 OAuth Assertion Framework May 2015
among the participants on the kinds of values to be used for some
abstract fields defined by this specification. In some cases,
additional profiles may be created that constrain or prescribe these
values or specify how they are to be exchanged. The "OAuth 2.0
Dynamic Client Registration Core Protocol" [OAUTH-DYN-REG] is one
such profile that enables OAuth Clients to register metadata about
themselves at an authorization server.
8. Security Considerations
This section discusses security considerations that apply when using
assertions with OAuth 2.0 as described in this document. As
discussed in Section 3, there are two different ways to obtain
assertions: either as self-issued or obtained from a third-party
token service. While the actual interactions for obtaining an
assertion are outside the scope of this document, the details are
important from a security perspective. Section 3 discusses the high-
level architectural aspects. Many of the security considerations
discussed in this section are applicable to both the OAuth exchange
as well as the client obtaining the assertion.
The remainder of this section focuses on the exchanges that concern
presenting an assertion for client authentication and for the
authorization grant.
8.1. Forged Assertion
Threat:
An adversary could forge or alter an assertion in order to obtain
an access token (in the case of the authorization grant) or to
impersonate a client (in the case of the client authentication
mechanism).
Countermeasures:
To avoid this kind of attack, the entities must assure that proper
mechanisms for protecting the integrity of the assertion are
employed. This includes the issuer digitally signing the
assertion or computing a MAC over the assertion.
8.2. Stolen Assertion
Threat:
An adversary may be able obtain an assertion (e.g., by
eavesdropping) and then reuse it (replay it) at a later point in
time.
Campbell, et al. Standards Track [Page 15]RFC 7521 OAuth Assertion Framework May 2015
Countermeasures:
The primary mitigation for this threat is the use of secure
communication channels with server authentication for all network
exchanges.
An assertion may also contain several elements to prevent replay
attacks. There is, however, a clear trade-off between reusing an
assertion for multiple exchanges and obtaining and creating new,
fresh assertions.
Authorization servers and resource servers may use a combination
of the Assertion ID and Issued At/Expires At attributes for replay
protection. Previously processed assertions may be 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.
8.3. Unauthorized Disclosure of Personal Information
Threat:
The ability for other entities to obtain information about an
individual, such as authentication information, role in an
organization, or other authorization-relevant information, raises
privacy concerns.
Countermeasures:
To address this threat, two cases need to be differentiated:
First, a third party that did not participate in any of the
exchange is prevented from eavesdropping on the content of the
assertion by employing confidentiality protection of the exchange
using TLS. This ensures that an eavesdropper on the wire is
unable to obtain information. However, this does not prevent
legitimate protocol entities from obtaining information that they
are not allowed to possess from assertions. Some assertion
formats allow for the assertion to be encrypted, preventing
unauthorized parties from inspecting the content.
Second, an authorization server may obtain an assertion that was
created by a third-party token service and that token service may
have placed attributes into the assertion. To mitigate potential
privacy problems, prior consent for the release of such attribute
information from the resource owner should be obtained. OAuth
itself does not directly provide such capabilities, but this
consent approval may be obtained using other identity management
protocols or user consent interactions; it may also be obtained in
an out-of-band fashion.
Campbell, et al. Standards Track [Page 16]RFC 7521 OAuth Assertion Framework May 2015
For the cases where a third-party token service creates assertions
to be used for client authentication, privacy concerns are
typically lower, since many of these clients are Web servers
rather than individual devices operated by humans. If the
assertions are used for client authentication of devices or
software that can be closely linked to end users, then privacy
protection safeguards need to be taken into consideration.
Further guidance on privacy friendly protocol design can be found
in [RFC6973].
8.4. Privacy Considerations
An 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 to
the client, the assertion (or portions of it) should be encrypted to
the authorization server.
Deployments should determine the minimum amount of information
necessary to complete the exchange and include only such information
in the assertion. In some cases, the Subject identifier can be a
value representing an anonymous or pseudonymous user, as described in
Section 6.3.1.
9. IANA Considerations
This section registers three values, as listed in the subsections
below, in the IANA "OAuth Parameters" registry established by RFC
6749 [RFC6749].
9.1. "assertion" Parameter Registration
o Name: assertion
o Parameter Usage Location: token request
o Change Controller: IESG
o Specification Document(s): RFC 7521
Campbell, et al. Standards Track [Page 17]RFC 7521 OAuth Assertion Framework May 2015
9.2. "client_assertion" Parameter Registration
o Name: client_assertion
o Parameter Usage Location: token request
o Change Controller: IESG
o Specification Document(s): RFC 7521
9.3. "client_assertion_type" Parameter Registration
o Name: client_assertion_type
o Parameter Usage Location: token request
o Change Controller: IESG
o Specification Document(s): RFC 7521
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
10.2. Informative References
[OASIS.WS-Trust]
Nadalin, A., Ed., Goodner, M., Ed., Gudgin, M., Ed.,
Barbir, A., Ed., and H. Granqvist, Ed., "WS-Trust",
February 2009, <http://docs.oasis-open.org/ws-sx/
ws-trust/v1.4/ws-trust.html>.
Campbell, et al. Standards Track [Page 18]RFC 7521 OAuth Assertion Framework May 2015
[OAUTH-DYN-REG]
Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
Work in Progress, draft-ietf-oauth-dyn-reg-29, May 2015.
[RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
<http://www.rfc-editor.org/info/rfc6755>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7522] Campbell, B., Mortimore, C., and M. Jones, "Security
Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0
Client Authentication and Authorization Grants", RFC 7522,
DOI 10.17487/RFC7522, May 2015,
<http://www.rfc-editor.org/info/rfc7522>.
[RFC7523] Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
2015, <http://www.rfc-editor.org/info/rfc7523>.
Campbell, et al. Standards Track [Page 19]RFC 7521 OAuth Assertion Framework May 2015
Acknowledgements
The authors wish to thank the following people who have influenced or
contributed to this specification: Paul Madsen, Eric Sachs, Jian Cai,
Tony Nadalin, Hannes Tschofenig, the authors of the OAuth WRAP
specification, and the members of the OAuth working group.
Authors' Addresses
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/
Yaron Y. Goland
Microsoft
EMail: yarong@microsoft.com
Campbell, et al. Standards Track [Page 20]
