]> Microsoft

arndts@microsoft.com

Microsoft

pieter.kasselman@microsoft.com

MITRE

kburgin@mitre.org

NSA-CCSS

mjjenki@cyber.nsa.gov

Ping Identity

bcampbell@pingidentity.com

sec oauth This specification defines a mechanism to preserve identity information and federate authorization across trust domains that use the OAuth 2.0 Framework. Discussion Venues Discussion of this document takes place on the Web Authorization Protocol Working Group mailing list (oauth@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Applications often require access to resources that are distributed across multiple trust domains where each trust domain has its own OAuth 2.0 authorization server. As a result, developers are often faced with the situation that a protected resource is located in a different trust domain and thus protected by a different authorization server. A request may transverse multiple resource servers in multiple trust domains before completing. All protected resources involved in such a request need to know on whose behalf the request was originally initiated (i.e. the user), what authorization was granted and optionally which other resource servers were called prior to making an authorization decision. This information needs to be preserved, even when a request crosses one or more trust domains. This document referrers to this as "chaining" and defines a mechanism for preserving identity and authorization information across domains using a combination of OAuth 2.0 Token Exchange and JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants .

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Identity and Authorization Chaining Across Domains This specification describes a combination of OAuth 2.0 Token Exchange and JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants to achieve identity and authorization chaining across domains. A client in trust domain A that needs to access a resource server in trust domain B requests a JWT authorization grant from the authorization server for trust domain A via a token exchange. The client in trust domain A presents the received grant as an assertion to the authorization server in domain B in order to obtain an access token for the protected resource in domain B. The client in domain A may be a resource server, or it may be the authorization server itself.

Overview The identity and authorization chaining flow outlined below describes how a combination of OAuth 2.0 Token Exchange and JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants are used to address the use cases identified. The appendix include two additional examples that describe how this flow is used. In one example, the resource server acts as the client and in the other, the authorization server acts as the client.

Identity and Authorization Chaining Flow | | | | - - - - - - - - - >| | | | | | | | | (D) present | | | | authorization grant | | | | [RFC 7523] | | | | ------------------->| | | | | | | | (E) | | | | <- - - - - - - - - -| | | | | | | | (F) access | | | --------------------------------->| | | | | | | | | ]]>

The flow illustrated in Figure 1 shows the steps the client in trust Domain A needs to perform to access a protected resource in trust domain B. In this flow, the client has a way to discover the authorization server in Domain B and a trust relationship exists between Domain A and Domain B (e.g., through federation). It includes the following:

• (A) The client of Domain A needs to discover the authorization server of Domain B. See Authorization Server Discovery.
• (B) The client exchanges its token at the authorization server of its own domain (Domain A) for a JWT authorization grant that can be used at the authorization server in Domain B. See Token Exchange.
• (C) The authorization server of Domain A processes the request and returns a JWT authorization grant that the client can use with the authorization server of Domain B. This requires a trust relationship between Domain A and Domain B (e.g., through federation).
• (D) The client presents the authorization grant to the authorization server of Domain B. See Access Token Request.
• (E) Authorization server of Domain B validates the JWT authorization grant and returns an access token.
• (F) The client now possesses an access token to access the protected resource in Domain B.

Authorization Server Discovery This specification does not define authorization server discovery. A client MAY maintain a static mapping or use other means to identify the authorization server. The authorization_servers property in MAY be used.

Token Exchange The client performs token exchange as defined in with the authorization server for its own domain (e.g., Domain A) in order to obtain a JWT authorization grant that can be used with the authorization server of a different domain (e.g., Domain B) as specified in section 1.3 of .

Token Exchange Request The parameters described in section 2.1 of apply here with the following restrictions:

requested_token_type

OPTIONAL according to . In the context of this specification this parameter SHOULD NOT be used.

scope

OPTIONAL. Additional scopes to indicate scopes included in the returned JWT authorization grant. See Claims transcription.

resource

REQUIRED if audience is not set. URI of authorization server of targeting domain (domain B).

audience

REQUIRED if resource is not set. Well known/logical name of authorization server of targeting domain (domain B).

Processing rules

• If the request itself is not valid or if the given resource or audience are unknown, or are unacceptable based on policy, the authorization server MUST deny the request.
• The authorization server MAY add, remove or change claims. See Claims transcription.

Token Exchange Response All of section 2.2 of applies. In addition, the following applies to implementations that conform to this specification.

• The "aud" claim in the returned JWT authorization grant MUST identify the requested authorization server. This corresponds with RFC 7523 Section 3, Point 3 and is there to reduce misuse and to prevent clients from presenting access tokens as an authorization grant to an authorization server in a different domain.
• The "aud" claim included in the returned JWT authorization grant MAY identify multiple authorization servers, provided that trust relationships exist with them (e.g. through federation). It is RECOMMENDED that the "aud" claim is restricted to a single authorization server to prevent an authorization server in one domain from presenting the client's authorization grant to an authorization server in a different trust domain. For example, this will prevent the authorization server in Domain B from presenting the authorization grant it received from the client in Domain A to the authorization server for Domain C.

Example The example below shows the message invoked by the client in trust domain A to perform token exchange with the authorization server in domain A (https://as.a.org/auth) to receive a JWT authorization grant for the authorization server in trust domain B (https://as.b.org/auth).

Token exchange request

Token exchange response

JWT Authorization Grant The client presents the JWT authorization grant it received from the authorization server in its own domain as an authorization grant to the authorization server in the domain of the resource server it wants to access as defined in .

Access Token Request The authorization grant is a JWT bearer token, which the client uses to request an access token as described in the JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants . For the purpose of this specification the following descriptions apply:

grant_type

REQUIRED. As defined in Section 2.1 of the value urn:ietf:params:oauth:grant-type:jwt-bearer indicates the request is a JWT bearer assertion authorization grant.

assertion

REQUIRED. Authorization grant returned by the token exchange (access_token response).

scope

OPTIONAL.

The client MAY indicate the audience it is trying to access through the scope parameter or the resource parameter defined in .

Processing rules The authorization server MUST validate the JWT authorization grant as specified in Sections 3 and 3.1 of . The following processing rules also apply:

• The "aud" claim MUST identify the Authorization Server as a valid intended audience of the assertion using either the token endpoint as described Section 3 or the issuer identifier as defined in Section 2 of .
• The authorization server SHOULD deny the request if it is not able to identify the subject.
• Due to policy the request MAY be denied (for instance if the federation from domain A is not allowed).

Access Token Response The authorization server responds with an access token as described in section 5.1 of .

Example The example belows shows how the client in trust domain A presents an authorization grant to the authorization server in trust domain B (https://as.b.org/auth) to receive an access token for a protected resource in trust domain B.

Assertion request

Assertion response

Claims transcription Authorization servers MAY transcribe claims when either producing JWT authorization grants in the token exchange flow or access tokens in the assertion flow.

• Transcribing the subject identifier: Subject identifier can differ between the parties involved. For instance: A user is known at domain A by "johndoe@a.org" but in domain B by "doe.john@b.org". The mapping from one identifier to the other MAY either happen in the token exchange step and the updated identifier is reflected in returned JWT authorization grant or in the assertion step where the updated identifier would be reflected in the access token. To support this both authorization servers MAY add, change or remove claims as described above.
• Selective disclosure: Authorization servers MAY remove or hide certain claims due to privacy requirements or reduced trust towards the targeting trust domain. To hide and enclose claims MAY be used.
• Controlling scope: Clients MAY use the scope parameter to control transcribed claims (e.g. downscoping). Authorization Servers SHOULD verify that requested scopes are not higher privileged than the scopes of presented subject_token.
• Including JWT authorization grant claims: The authorization server performing the assertion flow MAY leverage claims from the presented JWT authorization grant and include them in the returned access token. The populated claims SHOULD be namespaced or validated to prevent the injection of invalid claims.

The representation of transcribed claims and their format is not defined in this specification.

IANA Considerations To be added.

Security Considerations

Client Authentication Authorization Servers SHOULD follow the OAuth 2.0 Security Best Current Practice for client authentication.

References Normative References The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] This specification defines a protocol for an HTTP- and JSON-based Security Token Service (STS) by defining how to request and obtain security tokens from OAuth 2.0 authorization servers, including security tokens employing impersonation and delegation. This specification defines the use of a JSON Web Token (JWT) Bearer Token as a means for requesting an OAuth 2.0 access token as well as for client authentication. This document specifies an extension to the OAuth 2.0 Authorization Framework defining request parameters that enable a client to explicitly signal to an authorization server about the identity of the protected resource(s) to which it is requesting access. This specification defines a metadata format that an OAuth 2.0 client can use to obtain the information needed to interact with an OAuth 2.0 authorization server, including its endpoint locations and authorization server capabilities. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Authlete Microsoft Ping Identity This specification defines a mechanism for selective disclosure of individual elements of a JSON object used as the payload of a JSON Web Signature (JWS) structure. It encompasses various applications, including but not limited to the selective disclosure of JSON Web Token (JWT) claims. SPRIND Yubico Independent Researcher Authlete This document describes best current security practice for OAuth 2.0. It updates and extends the threat model and security advice given in [RFC6749], [RFC6750], and [RFC6819] to incorporate practical experiences gathered since OAuth 2.0 was published and covers new threats relevant due to the broader application of OAuth 2.0. It further deprecates some modes of operation that are deemed less secure or even insecure. Self-Issued Consulting Independent Identity, Inc. Okta This specification defines a metadata format that an OAuth 2.0 client or authorization server can use to obtain the information needed to interact with an OAuth 2.0 protected resource.

Use cases This sections outlines some use cases where the identity and authorization chaining described in this document can be applied. This section is not complete and other use cases not mentioned here are also valid.

Preserve User Context across Multi-cloud, Multi-Hybrid environments A user attempts to access a service that is implemented as a number of on-premise and cloud-based microservices. Both the on-premise and cloud-based services are segmented by multiple trust boundaries that span one or more on-premise or cloud service environments. Every microservice can apply an authorization policy that takes the context of the original user, as well as intermediary microservices into account, irrespective of where the microservices are running and even when a microservice in one trust domain calls another service in another trust domain.

Continuous Integration Accessing External Resources A continuous integration system needs to access external resources, for example to upload an artifact or to run tests. These resources are protected by different authorization servers. The identity information of the build, for example metadata such as commit hashes or repository, should be preserved and carried across the domain boundary. This not just prevents maintaining credentials it also allows fine grained access control at the resource.

API Security Use Case A home devices company provides a "Camera API" to enable access to home cameras. Partner companies use this Camera API to integrate the camera feeds into their security dashboards. Using OAuth between the partner and the Camera API, a partner can request the feed from a home camera to be displayed in their dashboard. The user has an account with the camera provider. The user may be logged in to view the partner provided dashboard, or they may authorize emergency access to the camera. The home devices company must be able to independently verify that the request originated and was authorized by a user who is authorized to view the feed of the requested home camera.

Examples This section contains two examples, demonstrating how this specification may be used in different environments with specific requirements. The first example shows the resource server acting as the client and the second example shows the authorization server acting as the client.

Resource server acting as client Resources servers may act as clients if the following is true:

• Authorization Server B is reachable by the resource server by network and is able to perform the appropriate client authentication (if required).
• The resource server has the ability to determine the authorization server of the protected resource outside its trust domain.

The flow would look like this:

Resource server acting as client | | | <- - - - - - - - - - - - - - - - -| | | | | | (C) exchange token | | | | [RFC 8693] | | | |<--------------------| | | | | | | | (D) | | | | - - - - - - - - - ->| | | | | | | | | (E) present | | | | authorization | | | | grant [RFC 7523] | | | | ------------------->| | | | | | | | (F) | | | | <- - - - - - - - - -| | | | | | | | (G) access | | | --------------------------------->| | | | | | | | | ]]>

The flow contains the following steps: (A) The resource server of domain A needs to access protected resource in Domain B. It requires an access token to do so which it does not possess. In this example is used to receive information about the authorization server which protects the resource in domain B. This step MAY be skipped if discovery is not needed and other means of discovery MAY be used. The protected resource returns its metadata along with the authorization server information. (B) Now, after the resource server has identified the authorization server for Domain B, the resource server requests a JWT authorization grant for the authorization server in Domain B from its own authorization server (Domain A). This happens via the token exchange protocol. (C) If successful, the authorization server returns a JWT authorization grant to the resource server. (D) The resource server presents the JWT authorization grant to the authorization server of Domain B. (E) The authorization server of Domain B uses claims from the JWT authorization grant to identify the user and its access. If access is granted an access token is returned. (F) The resource server uses the access token to access the protected resource at Domain B.

Authorization server acting as client Authorization servers may act as clients too. This can be necessary because of following reasons:

• Resource servers may not have knowledge of authorization servers.
• Resource servers may not have network access to other authorization servers.
• A strict access control on resources outside the trust domain is required and enforced by authorization servers.
• Authorization servers require client authentication. Managing clients for resource servers outside of the trust domain is not intended.

The flow when authorization servers act as client would look like this:

Authorization server acting as client | | | | | | | | |----+ | | | | | (B) determine | | | |<---+ authorization | | | | server B | | | | | | | | | | | |----+ | | | | | (C) issue | | | |<---+ authorization | | | | grant ("internal | | | | token exchange") | | | | | | | | | | | | (D) present | | | | authorization grant | | | | [RFC 7523] | | | | --------------------->| | | | | | | | (E) | | | | <- - - - - - - - - - -| | | | | | | (F) | | | | <- - - - - - - - - - | | | | | | | | | (G) access | | | ---------------------------------------------------------->| | | | | | | | | ]]>

The flow contains the following steps: (A) The resource server of Domain A requests a token for the protected resource in Domain B from the authorization server in Domain A. This specification does not cover this step. A profile of Token Exchange may be used. (B) The authorization server (of Domain A) determines the authorization server (of Domain B). This could have been passed by the client, is statically maintained or dynamically resolved. (C) Once the authorization server is determined a JWT authorization grant is issued internally. This reflects to Token exchange of this specification and can be seen as an "internal token exchange". (D) The issued JWT authorization grant is presented to the authorization server of Domain B. This presentation happens between the authorization servers and authorization server A may be required to perform client authentication while doing so. (E) The authorization server of Domain B returns an access token for access to the protected resource in Domain B to the authorization server in Domain A. (F) The authorization server of Domain A returns that access token to the resource server in Domain A. (G) The resource server in Domain A uses the received access token to access the protected resource in Domain B.

Acknowledgements The editors would like to thank Joe Jubinski, Justin Richer, Aaron Parecki and others (please let us know, if you've been mistakenly omitted) for their valuable input, feedback and general support of this work.

Document History [[ To be removed from the final specification ]] -01

• limit the authorization grant format to RFC7523 JWT
• minor example fixes
• editorial fixes
• added Aaron Parecki to acknowledgements
• renamed section headers to be more explicit
• use more specific term "JWT authorization grant"
• changed name to "OAuth Identity and Authorization Chaining Across Domains"
• move use cases to appendix and add continuous integration use case

-00

• initial working group version (previously draft-schwenkschuster-oauth-identity-chaining)

Contributors SGNL

atuls@sgnl.ai

Capital One

george.fletcher@capitalone.com

EY

rifaat.shekh-yusef@ca.ey.com

hannes.tschofenig@gmx.net