OAuth Working Group B. Campbell Internet-Draft Ping Intended status: Standards Track C. Mortimore Expires: January 3, 2013 Salesforce M. Jones Y. Goland Microsoft July 2, 2012 Assertion Framework for OAuth 2.0 draft-ietf-oauth-assertions-04 Abstract This specification provides a framework for the use of assertions with OAuth 2.0 in the form of new client authentication mechanism and a new authorization grant type. Mechanisms are specifies for transporting assertions during interactions with a token endpoint, as well as general processing rules. 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 and that, in order to be implementable, companion specifications are necessary to provide the corresponding instantiation. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 3, 2013. Copyright Notice Campbell, et al. Expires January 3, 2013 [Page 1]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 Copyright (c) 2012 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . 8 4.2.1. Error Responses . . . . . . . . . . . . . . . . . . . 9 5. Assertion Content and Processing . . . . . . . . . . . . . . . 10 5.1. Assertion Metamodel . . . . . . . . . . . . . . . . . . . 10 5.2. General Assertion Format and Processing Rules . . . . . . 11 6. Specific Assertion Format and Processing Rules . . . . . . . . 12 6.1. Client Authentication . . . . . . . . . . . . . . . . . . 12 6.2. Client Acting on Behalf of Itself . . . . . . . . . . . . 13 6.3. Client Acting on Behalf of a User . . . . . . . . . . . . 14 6.4. Client Acting on Behalf of an Anonymous User . . . . . . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 7.1. Forged Assertion . . . . . . . . . . . . . . . . . . . . . 16 7.2. Stolen Assertion . . . . . . . . . . . . . . . . . . . . . 16 7.3. Unauthorized Disclosure of Personal Information . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 8.1. assertion Parameter Registration . . . . . . . . . . . . . 18 8.2. client_assertion Parameter Registration . . . . . . . . . 18 8.3. client_assertion_type Parameter Registration . . . . . . . 18 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 9.1. Normative References . . . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . . 19 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19 Appendix B. Document History . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Campbell, et al. Expires January 3, 2013 [Page 2]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 1. Introduction OAuth 2.0 [I-D.ietf-oauth-v2] is an authorization framework that enables a third-party application to obtain limited access to a protected HTTP resource. In OAuth those third-party application are called clients and 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 trust 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 over HTTP), 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 flows and processing rules and that, in order to be implementable, companion specifications are necessary to provide the corresponding instantiation. Note: 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. 2. Terminology 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. Campbell, et al. Expires January 3, 2013 [Page 3]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 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 who issued the assertion and when was it issued, as well as conditions under which the assertion is to be considered valid like when and where it can be used. The entity that creates and signs the assertion is typically known as the 'Issuer' and the entity that consumes the assertion and relies on its information is 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 protected against tampering using a digital signature or a keyed message digest applied by the issuer. An assertion MAY additionally be encrypted to prevent unauthorized parties 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 that are worth mentioning. 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' or just 'Token Service' and a trust relationship (usually manifested in the exchange of some kind of keying material) exists between the token service and the relying party. The token service is the assertion issuer and its role is to fulfill requests from clients, who present various credentials, and mint assertions as requested, fill them with verified information, and sign them. WS-Trust [OASIS.WT-Trust] is one available standard for requesting security tokens (assertions). Campbell, et al. Expires January 3, 2013 [Page 4]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 Relying Party Client Token Service | | | | | 1) Request Assertion | | |------------------------>| | | | | | 2) Assertion | | |<------------------------| | 3) Assertion | | |<-------------------------| | | | | | 4) OK or Failure | | |------------------------->| | | | | | | | Figure 1: Third Party created Assertion In Figure 2 the second pattern is depicted where the client creates assertions locally. In order to sign the assertions it has to obtain keying material, either a symmetric key or an asymmetric key pair, out-of-band or though some other process. Although assertions are usually used to convey identity and security information, self-created 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 is included in such an assertion, such as issuing and conditions information. Campbell, et al. Expires January 3, 2013 [Page 5]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 Relying Party Client | | | | 1) Create | | Assertion | |--------------+ | | | | | 2) Assertion | | |<-------------+ | 3) Assertion | |<-------------------------| | | | 4) OK or Failure | |------------------------->| | | | | Figure 2: Self-Created Assertion Deployments need to determine the best possible variant based on the required level of security, 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 (e.g. 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. A secure communication channel is required between all entities to avoid leaking the assertion to unauthorized parties. 2. Holder-of-key assertions. In order to access to the associated resources, the entity presenting the assertion must somehow 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. This mechanism provides additional security properties. The protocol parameters and processing rules defined in this document are intended to support a client presenting a bearer assertion to an authorization server. The use of holder-of-key assertions are not precluded by this document but additional protocol details would need to be specified. Campbell, et al. Expires January 3, 2013 [Page 6]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 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 require the use of TLS as mandated in Section 3.2 of OAuth 2.0 [I-D.ietf-oauth-v2]. 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 [I-D.ietf-oauth-v2]. 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 MUST be an absolute URI. assertion 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. scope OPTIONAL. The requested scope as described in Section 3.3 of OAuth 2.0 [I-D.ietf-oauth-v2]. 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 or lesser than the scope originally granted to the authorized accessor. If the scope parameter and/or value is omitted, the scope MUST be treated as equal to the scope originally granted to the authorized accessor. The Authorization Server MUST 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): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded client_id=s6BhdRkqt3& grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer& assertion=PHNhbWxwOl...[omitted for brevity]...ZT4 Campbell, et al. Expires January 3, 2013 [Page 7]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 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 IEFT URN for use as the "grant_type" value can be requested using the template in An IETF URN Sub-Namespace for OAuth [I-D.ietf-oauth-urn-sub-ns]. A URN of the from 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 MUST construct an error response as defined in OAuth 2.0 [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" } 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 [I-D.ietf-oauth-v2]. When using assertions as client credentials, the client includes the assertion and related information using the following HTTP request parameters: client_id OPTIONAL. The client identifier as described in Section 2.2 of OAuth 2.0 [I-D.ietf-oauth-v2]. When present, the "client_id" MUST identify the client to the authorization server. Campbell, et al. Expires January 3, 2013 [Page 8]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 client_assertion_type REQUIRED. The format of the assertion as defined by the authorization server. The value MUST 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. 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 Token Request as defined in Section 4.1.3 of OAuth 2.0 [I-D.ietf-oauth-v2] (with 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=i1WsRn1uB1& client_id=s6BhdRkqt3& 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 IEFT URN for use as the "client_assertion_type" value may be requested using the template in An IETF URN Sub-Namespace for OAuth [I-D.ietf-oauth-urn-sub-ns]. A URN of the from urn:ietf:params:oauth:client-assertion-type:* is suggested. 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 MUST construct an error response as defined in OAuth 2.0 [I-D.ietf-oauth-v2]. 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: Campbell, et al. Expires January 3, 2013 [Page 9]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 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 [I-D.ietf-oauth-v2]. 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 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. The issuer may be either an OAuth client (when assertions are self-created) or a third party token service. Principal 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). Audience 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 [I-D.ietf-oauth-v2]. Issued At 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. Campbell, et al. Expires January 3, 2013 [Page 10]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 Expires At 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. 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 that that entity or any other entity will accidentally assign the same identifier to a different data object. 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 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". o 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". o 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. 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 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. o The assertion MAY contain an Issued At entity containing the UTC time at which the assertion was issued. o The assertion MAY contain an Assertion ID. An Authorization Server MAY dictate that Assertion ID is mandatory. o The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion. The algorithm Campbell, et al. Expires January 3, 2013 [Page 11]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 used to validate the assertion, and the mechanism for designating the secret used to generate the assertion, are beyond the scope of this specification. 6. Specific Assertion Format and Processing Rules The following clarifies the format and processing rules defined in Section 4 and Section 5 for a number of common use cases: 6.1. Client Authentication When a client uses an assertion for authentication it SHOULD do so according to Section 4.2. The following format and processing rules apply. o The "client_assertion_type" HTTP parameter MUST identify the assertion format being used for authentication. o The "client_assertion" HTTP parameter MUST contain the serialized assertion in a format indicated by the "client_assertion_type" parameter. o The Principal SHOULD be the "client_id". o The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-created, the Issuer SHOULD be the "client_id". o The Audience of the assertion MUST identify the Authorization Server and SHOULD be the URL of the Token Endpoint. o The Authorization Server MUST verify the assertion's signature or keyed message digest in order to determine the validity of the issuer and the content of the assertion. The following non-normative example demonstrates the use of a client authentication using an assertion during an Access Token Request as defined in Section 4.1.3 of OAuth 2.0 [I-D.ietf-oauth-v2] (with line breaks for display purposes only): Campbell, et al. Expires January 3, 2013 [Page 12]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=authorization_code& code=i1WsRn1uB1& client_id=s6BhdRkqt3& client_assertion_type=urn%3Aietf%3Aparams%3Aoauth %3Aclient-assertion-type%3Asaml2-bearer& client_assertion=PHNhb...[omitted for brevity]...ZT4 6.2. Client Acting on Behalf of Itself 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 [I-D.ietf-oauth-v2]. 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, with the addition of a grant_type parameter. The following format and processing rules apply. o The grant_type HTTP request parameter MUST be "client_credentials". o The "client_assertion_type" HTTP parameter MUST identify the assertion format. o The "client_assertion" HTTP parameter MUST contain the serialized assertion as in a format indicated by the "client_assertion_type" parameter. o The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-created, 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. o The Principal SHOULD be the "client_id". o The Audience of the assertion MUST identify the Authorization Server and SHOULD be the URL of the Token Endpoint. o The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion. Campbell, et al. Expires January 3, 2013 [Page 13]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 The following non-normative example demonstrates the use of a an assertion being used for a Client Credentials Access Token Request as defined in Section 4.4.2 of OAuth 2.0 [I-D.ietf-oauth-v2] (with line breaks for display purposes only): POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded client_id=s6BhdRkqt3& 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 SHOULD be treated as using an assertion as an Authorization Grant according to Section 4.1. The following format and processing rules apply. o The grant_type HTTP request parameter MUST indicate the assertion format. o The assertion HTTP parameter MUST contain the serialized assertion as in a format indicated by the grant_type parameter. o The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-issued, 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. o The Principal MUST identify an authorized accessor for whom the access token is being requested (typically the resource owner, or an authorized delegate). o The Audience of the assertion MUST identify the Authorization Server and MAY be the URL of the Token Endpoint. o The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion. The following non-normative example demonstrates a client presenting as assertion during an Authorization Grant during an Access Token Request as defined in Section 4.1.3 of OAuth 2.0 [I-D.ietf-oauth-v2] (with line breaks for display purposes only): Campbell, et al. Expires January 3, 2013 [Page 14]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded client_id=s6BhdRkqt3& grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer& assertion=PHNhbWxwOl...[omitted for brevity]...ZT 6.4. Client Acting on Behalf of an Anonymous User When a client is accessing resources on behalf of an anonymous user, the following format and processing rules apply. o The grant_type HTTP request parameter MUST indicate the assertion format. o The assertion HTTP parameter MUST contain the serialized assertion as in a format indicated by the grant_type parameter. o 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. o 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. o The Audience of the assertion MUST identify the Authorization Server and MAY be the URL of the Token Endpoint. o The Authorization Server MUST validate the assertion's signature in order to verify the Issuer of the assertion. 7. Security Considerations This section discusses security considerations that apply when using assertions with OAuth 2.0 as proposed in this document. As described in Section 3 there are two different ways to obtain assertions, Campbell, et al. Expires January 3, 2013 [Page 15]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 either as self-created 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 and 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 concerns presenting an assertion for client authentication and for the authorization grant. 7.1. Forged Assertion Threat: An adversary could forge or alter an assertion in order to obtain an access token (in case of the authorization grant) or to impersonate a client (in 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 digitally signing the assertion by the issuer or computing a keyed message digest over the assertion. 7.2. Stolen Assertion Threat: An adversary may be able obtain an assertion (e.g., by eavesdropping) and to then re-use it (replay it) at a later point in time. Countermeasures: The primary mitigation for this threat is the use of a secure communication channel with server authentication for all network exchanges. An assertion may also contain several elements to prevent replay attacks. There is, however, a clear tradeoff between re-using an assertion for multiple exchanges and obtaining and creating new fresh assertions. Authorization Servers and Resource Servers may use a combination Campbell, et al. Expires January 3, 2013 [Page 16]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 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. 7.3. Unauthorized Disclosure of Personal Information Threat: The ability for other entities to obtain additional information about an individual, such as authentication information, role in an organization or other authorization relevant information raises privacy concerns. Countermeasures: To address the threats 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 HTTP signaling 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 from an assertion they may not have been allowed to obtain. Some assertion formats allow for the assertion to be encrypted to prevent 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 from the resource owner has to be obtained and OAuth itself provides such capabilities but this consent approval may be obtained using other identity management protocols or in an out-of-band fashion. For the cases where a third party token service creates assertions for usage with 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 [I-D.iab-privacy-considerations]. Campbell, et al. Expires January 3, 2013 [Page 17]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 8. IANA Considerations 8.1. assertion Parameter Registration 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): [[this document]] 8.2. client_assertion Parameter Registration 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 "client_assertion" parameter: o Parameter name: "client_assertion" o Parameter usage location: token request o Change controller: IETF o Specification document(s): [[this document]] 8.3. client_assertion_type Parameter Registration 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 "client_assertion_type" parameter: o Parameter name: "client_assertion_type" o Parameter usage location: token request o Change controller: IETF o Specification document(s): [[this document]] 9. References Campbell, et al. Expires January 3, 2013 [Page 18]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 9.1. Normative References [I-D.ietf-oauth-v2] Hammer-Lahav, E., Recordon, D., and D. Hardt, "The OAuth 2.0 Authorization Framework", draft-ietf-oauth-v2-28 (work in progress), June 2012. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 9.2. Informative References [I-D.iab-privacy-considerations] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., and J. Morris, "Privacy Considerations for Internet Protocols", draft-iab-privacy-considerations-02 (work in progress), March 2012. [I-D.ietf-oauth-urn-sub-ns] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace for OAuth", draft-ietf-oauth-urn-sub-ns-05 (work in progress), June 2012. [OASIS.WT-Trust] Nadalin, A., Ed., Goodner, M., Ed., Gudgin, M., Ed., Barbir, A., Ed., and H. Granqvist, Ed., "WS-Trust", Feb 2009. Appendix A. Acknowledgements The authors wish to thank the following people that have influenced or contributed this specification: Paul Madsen, Eric Sachs, Jian Cai, Tony Nadalin, Hannes Tschofenig the authors of OAuth WRAP, and those in the OAuth working group. Appendix B. Document History [[ to be removed by RFC editor before publication as an RFC ]] draft-ietf-oauth-assertions-04 o Updated document to incorporate the review comments from the shepherd - thread and alternative draft at http://www.ietf.org/mail-archive/web/oauth/current/msg09437.html Campbell, et al. Expires January 3, 2013 [Page 19]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 o Added reference to draft-ietf-oauth-urn-sub-ns and include suggestions on urn:ietf:params:oauth:[grant-type|client-assertion-type]:* URNs draft-ietf-oauth-assertions-03 o updated reference to draft-ietf-oauth-v2 from -25 to -26 draft-ietf-oauth-assertions-02 o Added text about limited lifetime ATs and RTs per http://www.ietf.org/mail-archive/web/oauth/current/msg08298.html. o 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. o 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. o 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. o 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. o 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. o 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. o 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. o Change "The following format and processing rules SHOULD be applied" to "The following format and processing rules apply" in Campbell, et al. Expires January 3, 2013 [Page 20]
Internet-Draft Assertion Framework for OAuth 2.0 July 2012 sections 6.x to remove conflicting normative qualification of other normative statements per http://www.ietf.org/mail-archive/web/oauth/current/msg08892.html. o 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. o Remove the MUST from the text prior to the HTTP parameter definitions per http://www.ietf.org/mail-archive/web/oauth/current/msg08920.html. o Updated examples to use grant_type and client_assertion_type values from the OAuth SAML Assertion Profiles spec. Authors' Addresses Brian Campbell Ping Identity Corp. Email: brian.d.campbell@gmail.com Chuck Mortimore Salesforce.com Email: cmortimore@salesforce.com Michael B. Jones Microsoft Email: mbj@microsoft.com Yaron Y. Goland Microsoft Email: yarong@microsoft.com Campbell, et al. Expires January 3, 2013 [Page 21]