]> Microsoft

arndts@microsoft.com

Microsoft

pieter.kasselman@microsoft.com

kelley.burgin@gmail.com

NSA-CCSS

mjjenki@cyber.nsa.gov

sec oauth This specification defines a mechanism to preserve identity and call chain information across trust domains that use the OAuth 2.0 Framework.

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. Preserving this information is referred to as identity chaining. This document defines a mechanism for preserving identity chaining information across trust domains using a combination of OAuth 2.0 Token Exchange and Assertion Framework 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 Chaining Across Trust Domains This specification describes a combination of OAuth 2.0 Token Exchange and Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants to achieve identity chaining across trust domains. A client in trust domain A that needs to access a resource server in trust domain B requests an 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. A client in trust domain A that needs to access a resource server in trust domain B requests an 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.

Use Case This section describes two use cases addressed in this specification.

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.

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.

Overview The Identity Chaining flow outlined below describes how a combination of OAuth 2.0 Token Exchange and Assertion Framework 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 Chaining Flow | | | | - - - - - - - - - >| | | | | | | | | (D) present | | | | authorization grant | | | | [RFC 7521] | | | | ------------------->| | | | | | | | (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 an 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 an 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 Authorization Grant.
• (E) Authorization server of Domain B validates the 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 contact the resource server and leverage the WWW-Authentication response (see section 3 of ), maintain a static mapping or use other means to identify the authorization server.

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 an 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 .

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. See Authorization grant type.

scope

OPTIONAL. Additional scopes to indicate scopes included in returned 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.

Authorization grant type The authorization grant format and content is part of a contract between the authorization servers. To achieve a maintainable and flexible systems clients SHOULD NOT request a specific requested_token_type during the token exchange and SHOULD NOT require a certain format or parse the authorization grant (e.g., if the token is encoded as a JWT). The issued_token_type parameter in the response indicates the type and SHOULD be passed into the assertion request. This allows flexibility for authorization servers to change format and content. Authorization servers MAY use an existing grant type such us urn:ietf:params:oauth:grant-type:jwt-bearer to indicate a JWT or urn:ietf:params:oauth:grant-type:saml2-bearer to indicate SAML. Other grant types MAY be used to indicate other formats.

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 authorization grant MUST identify the requested authorization server. This corresponds with RFC 7523 Section 3, Point 3 and is there to reduce missuse 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 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 belows shows the message invoked by the client in trust domain A to perform token exchange with the authorization server in domain A (https://a.org/auth) to receive an authorization grant for the authorization server in trust domain B (https://b.org/auth).

Authorization Grant The client presents the authorization grant it received from the authorization server in its own domain and presents it to the authorization server in the domain of the resources server it wants to access as defined in the "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" .

Request If the authorization grant is in the form of a JWT bearer token, the client SHOULD use the "JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants" as defined in . Otherwise, the client SHOULD request an access token using the "Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants" as defined in (Section 4.1). For the purpose of this specification the following descriptions apply:

grant_type

REQUIRED. In context of this specification clients SHOULD use the type identifier returned by the token exchange (issued_token_type response). See authorization grant type for more details.

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 All of (Section 5.2 in specific) applies, along with the following processing rules:

• The request MUST be denied if the presented authorization grant does not include an "aud" claim identifying the authorization server that processes the request.
• 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).

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://b.org/auth) to receive an access token for a protected resource in trust domain B.

Claims transcription Authorization servers MAY transcribe claims when either producing 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 identifer is reflected in returned 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 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 priveleged than the scopes of presented subject_token.
• Including authorization grant claims: The authorization server performing the assertion flow MAY leverage claims from the presented 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 describes how to use bearer tokens in HTTP requests to access OAuth 2.0 protected resources. Any party in possession of a bearer token (a "bearer") can use it to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer tokens need to be protected from disclosure in storage and in transport. [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 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. 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. 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 yes.com Microsoft Ping Identity n.d. yes.com Yubico Independent Researcher Authlete n.d.

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 appropiate 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 | | | | | | | (B) | | | <- - - - - - - - - - - - - - - - -| | | | | | (C) exchange token | | | | [RFC 8693] | | | |<--------------------| | | | | | | | (D) | | | | - - - - - - - - - ->| | | | | | | | | (E) present | | | | authorization | | | | grant [RFC 7521] | | | | ------------------->| | | | | | | | (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. To receive information about the authorization server which protecs the resource in domain B it calls the resource unauthenticated. (B) The protected resource returns the WWW-Authenticate header to indicate its authorization server. (C) Now, after the resource server has identified the authorization server for Domain B, the resource server requests an authorization grant for the authorization server in Domain B from its own authorization server (Domain A). This happens via the token exchange protocol. (D) If successful, the authorization server returns an authorization grant to the resource server. (E) The resource server presents the authorization grant to the authorization server of Domain B. (F) The authorization server of Domain B uses claims from the authorization grant to identify the user and its access. If access is granted an access token is returned. (G) 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 7521] | | | | --------------------->| | | | | | | | (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 an 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 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 ,

Contributors SGNL

atuls@sgnl.ai

Capital One

george.fletcher@capitalone.com

EY

rifaat.shekh-yusef@ca.ey.com

hannes.tschofenig@gmx.net