Ping Identity

brian.d.campbell@gmail.com

Yubico

ve7jtb@ve7jtb.com

Arm Limited

hannes.tschofenig@gmx.net

Security OAuth Working Group OAuth Resource Audience 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.

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 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2020 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 () 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

• .  Introduction
  • .  Requirements Notation and Conventions
  • .  Terminology
• .  Resource Parameter
  • .  Authorization Request
  • .  Access Token Request
• .  Security Considerations
• .  Privacy Considerations
• .  IANA Considerations
  • .  OAuth Parameters Registration
  • .  OAuth Extensions Error Registration
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction Several years of deployment and implementation experience with the OAuth 2.0 Authorization Framework has uncovered a need (in some circumstances, such as an authorization server servicing a significant number of diverse resources) for the client to explicitly signal to the authorization server where it intends to use the access token it is requesting. Knowing the protected resource (a.k.a. resource server, application, API, etc.) that will process the access token enables the authorization server to construct the token as necessary for that entity. Properly encrypting the token (or content within the token) to a particular resource, for example, requires knowing which resource will receive and decrypt the token. Furthermore, various resources oftentimes have different requirements with respect to the data contained in (or referenced by) the token, and knowing the resource where the client intends to use the token allows the authorization server to mint the token accordingly. Specific knowledge of the intended recipient(s) of the access token also helps facilitate improved security characteristics of the token itself. Bearer tokens, currently the most commonly utilized type of OAuth access token, allow any party in possession of a token to get access to the associated resources. To prevent misuse, several important security assumptions must hold, one of which is that an access token must only be valid for use at a specific protected resource and for a specific scope of access. , for example, prescribes including the token's intended recipients within the token to prevent token redirect. When the authorization server is informed of the resource that will process the access token, it can restrict the intended audience of that token to the given resource such that the token cannot be used successfully at other resources. OAuth scope, from , is sometimes overloaded to convey the location or identity of the protected resource, however, doing so isn't always feasible or desirable. Scope is typically about what access is being requested rather than where that access will be redeemed (e.g., email, admin:org, user_photos, channels:read, and channels:write are a small sample of scope values in use in the wild that convey only the type of access and not the location or identity). In some circumstances and for some deployments, a means for the client to signal to the authorization server where it intends to use the access token it's requesting is important and useful. A number of implementations and deployments of OAuth 2.0 have already employed proprietary parameters toward that end. Going forward, this specification aspires to provide a standardized and interoperable alternative to the proprietary approaches.

Requirements Notation and Conventions 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.

Terminology This specification uses the terms "access token", "refresh token", "authorization server", "resource server", "authorization endpoint", "authorization request", "authorization response", "token endpoint", "grant type", "access token request", "access token response", and "client" defined by The OAuth 2.0 Authorization Framework .

Resource Parameter In requests to the authorization server, a client MAY indicate the protected resource (a.k.a. resource server, application, API, etc.) to which it is requesting access by including the following parameter in the request.

resource

Indicates the target service or resource to which access is being requested. Its value MUST be an absolute URI, as specified by . The URI MUST NOT include a fragment component. It SHOULD NOT include a query component, but it is recognized that there are cases that make a query component a useful and necessary part of the resource parameter, such as when one or more query parameters are used to scope requests to an application. The resource parameter URI value is an identifier representing the identity of the resource, which MAY be a locator that corresponds to a network-addressable location where the target resource is hosted. Multiple resource parameters MAY be used to indicate that the requested token is intended to be used at multiple resources.

The parameter value identifies a resource to which the client is requesting access. The parameter can carry the location of a protected resource, typically as an https URL or a more abstract identifier. This enables the authorization server to apply policy as appropriate for the resource, such as determining the type and content of tokens to be issued, if and how tokens are encrypted, and applying appropriate audience restrictions. The client SHOULD provide the most specific URI that it can for the complete API or set of resources it intends to access. In practice, a client will know a base URI for the application or resource that it interacts with, which is appropriate to use as the value of the resource parameter. The client SHOULD use the base URI of the API as the resource parameter value unless specific knowledge of the resource dictates otherwise. For example, the value https://api.example.com/ would be used for a resource that is the exclusive application on that host; however, if the resource is one of many applications on that host, something like https://api.example.com/app/ would be used as a more specific value. Another example is when an API has multiple endpoints, e.g., when System for Cross-domain Identity Management (SCIM) has endpoints such as https://apps.example.com/scim/Users, https://apps.example.com/scim/Groups, and https://apps.example.com/scim/Schemas. The client would use https://apps.example.com/scim/ as the resource so that the issued access token is valid for all the endpoints of the SCIM API. The following error code is provided for an authorization server to indicate problems with the requested resource(s) in response to an authorization request or access token request. It can also be used to inform the client that it has requested an invalid combination of resource and scope.

invalid_target

The requested resource is invalid, missing, unknown, or malformed.

The authorization server SHOULD audience-restrict issued access tokens to the resource(s) indicated by the resource parameter. Audience restrictions can be communicated in JSON Web Tokens with the aud claim and the top-level member of the same name provides the audience restriction information in a Token Introspection response. The authorization server may use the exact resource value as the audience or it may map from that value to a more general URI or abstract identifier for the given resource.

Authorization Request When the resource parameter is used in an authorization request to the authorization endpoint, it indicates the identity of the protected resource(s) to which access is being requested. When an access token will be returned directly from the authorization endpoint via the implicit flow (OAuth 2.0), the requested resource is applicable to that access token. In the code flow (OAuth 2.0) where an intermediate representation of the authorization grant (the authorization code) is returned from the authorization endpoint, the requested resource is applicable to the full authorization grant. For an authorization request sent as a JSON Web Token (JWT), such as when using the JWT Secured Authorization Request , a single resource parameter value is represented as a JSON string while multiple values are represented as an array of strings. If the client omits the resource parameter when requesting authorization, the authorization server MAY process the request with no specific resource or by using a predefined default resource value. Alternatively, the authorization server MAY require clients to specify the resource(s) they intend to access and MAY fail requests that omit the parameter with an invalid_target error. The authorization server might use this data to inform the user about the resources the client is going to access on their behalf, to apply policy (e.g., refuse the request due to unknown resources), and to determine the set of resources that can be used in subsequent access token requests. If the authorization server fails to parse the provided value(s) or does not consider the resource(s) acceptable, it should reject the request with an error response using the error code invalid_target as the value of the error parameter and can provide additional information regarding the reasons for the error using the error_description. An example of an authorization request where the client tells the authorization server that it wants an access token for use at https://api.example.com/app/ is shown in below (extra line breaks and indentation are for display purposes only).

Implicit Flow Authorization Request GET /as/authorization.oauth2?response_type=token &client_id=example-client &state=XzZaJlcwYew1u0QBrRv_Gw &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb &resource=https%3A%2F%2Fapi.example.com%2Fapp%2F HTTP/1.1 Host: authorization-server.example.com

Below in is an example of an authorization request using the code response type where the client is requesting access to the resource owner's contacts and calendar data at https://cal.example.com/ and https://contacts.example.com/ (extra line breaks and indentation are for display purposes only).

Code Flow Authorization Request GET /as/authorization.oauth2?response_type=code &client_id=s6BhdRkqt3 &state=tNwzQ87pC6llebpmac_IDeeq-mCR2wLDYljHUZUAWuI &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb &scope=calendar%20contacts &resource=https%3A%2F%2Fcal.example.com%2F &resource=https%3A%2F%2Fcontacts.example.com%2F HTTP/1.1 Host: authorization-server.example.com

Access Token Request When the resource parameter is used on an access token request made to the token endpoint, for all grant types, it indicates the target service or protected resource where the client intends to use the requested access token. The resource value(s) that is acceptable to an authorization server in fulfilling an access token request is at its sole discretion based on local policy or configuration. In the case of a refresh_token or authorization_code grant type request, such policy may limit the acceptable resources to those that were originally granted by the resource owner or a subset thereof. In the authorization_code case where the requested resources are a subset of the set of resources originally granted, the authorization server will issue an access token based on that subset of requested resources, whereas any refresh token that is returned is bound to the full original grant. When requesting a token, the client can indicate the desired target service(s) where it intends to use that token by way of the resource parameter and can indicate the desired scope of the requested token using the scope parameter. The semantics of such a request are that the client is asking for a token with the requested scope that is usable at all the requested target services. Effectively, the requested access rights of the token are the cartesian product of all the scopes at all the target services. To the extent possible, when issuing access tokens, the authorization server should downscope the scope value associated with an access token to the value the respective resource is able to process and needs to know. (Here, "downscope" means give fewer permissions than originally authorized by the resource owner.) This further improves privacy as a list of scope values is an indication that the resource owner uses the multiple various services listed; downscoping a token to only that which is needed for a particular service can limit the extent to which such information is revealed across different services. As specified in , the authorization server must indicate the access token's effective scope to the client in the scope response parameter value when it differs from the scope requested by the client. Following from the code flow authorization request shown in , the below examples show an authorization_code grant type access token request () and response () where the client tells the authorization server that it wants the access token for use at https://cal.example.com/ (extra line breaks and indentation are for display purposes only).

Access Token Request POST /as/token.oauth2 HTTP/1.1 Host: authorization-server.example.com Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ Content-Type: application/x-www-form-urlencoded grant_type=authorization_code &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb &code=10esc29BWC2qZB0acc9v8zAv9ltc2pko105tQauZ &resource=https%3A%2F%2Fcal.example.com%2F

Access Token Response HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-cache, no-store { "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgwMCwic2NvcGUiOiJjYWxlbmRhciIs ImF1ZCI6Imh0dHBzOi8vY2FsLmV4YW1wbGUuY29tLyJ9.nNWJ2dXSxaDRdMUK lzs-cYIj8MDoM6Gy7pf_sKrLGsAFf1C2bDhB60DQfW1DZL5npdko1_Mmk5sUf zkiQNVpYw", "token_type":"Bearer", "expires_in":3600, "refresh_token":"4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2", "scope":"calendar" }

A subsequent access token request, using the refresh token, where the client tells the authorization server that it wants an access token for use at https://contacts.example.com/ is shown in below with the response shown in (extra line breaks and indentation are for display purposes only).

Access Token Request POST /as/token.oauth2 HTTP/1.1 Host: authorization-server.example.com Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ Content-Type: application/x-www-form-urlencoded grant_type=refresh_token &refresh_token=4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2 &resource=https%3A%2F%2Fcontacts.example.com%2F

Access Token Response HTTP/1.1 200 OK Content-Type: application/json Cache-Control: no-cache, no-store { "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgyNiwic2NvcGUiOiJjb250YWN0cyIs ImF1ZCI6Imh0dHBzOi8vY29udGFjdHMuZXhhbXBsZS5jb20vIn0.5f4yhqazc OSlJw4y94KPeWNEFQqj2cfeO8x4hr3YbHtIl3nQXnBMw5wREY5O1YbZED-GfH UowfmtNaA5EikYAw", "token_type":"Bearer", "expires_in":3600, "scope":"contacts" }

Security Considerations An audience-restricted access token that is legitimately presented to a resource cannot then be taken by that resource and presented elsewhere for illegitimate access to other resources. The resource parameter enables a client to indicate the protected resources where the requested access token will be used, which in turn enables the authorization server to apply the appropriate audience restrictions to the token. Some servers may host user content or be multi-tenant. In order to avoid attacks where one tenant uses an access token to illegitimately access resources owned by a different tenant, it is important to use a specific resource URI including any portion of the URI that identifies the tenant, such as a path component. This will allow access tokens to be audience-restricted in a way that identifies the tenant and prevents their use, due to an invalid audience, at resources owned by a different tenant. Although multiple occurrences of the resource parameter may be included in a token request, using only a single resource parameter is encouraged. If a bearer token has multiple intended recipients (audiences), then the token is valid at more than one protected resource and can be used by any one of those resources to access any of the others. Thus, a high degree of trust between the involved parties is needed when using access tokens with multiple audiences. Furthermore, an authorization server may be unwilling or unable to fulfill a token request with multiple resources. Whenever feasible, the resource parameter should correspond to the network-addressable location of the protected resource. This makes it possible for the client to validate that the resource being requested controls the corresponding network location, reducing the risk of malicious endpoints obtaining tokens meant for other resources. If the resource parameter contains an abstract identifier, it is the client's responsibility to validate out of band that any network endpoint to which tokens are sent are the intended audience for that identifier.

Privacy Considerations In typical OAuth deployments the authorization sever is in a position to observe and track a significant amount of user and client behavior. It is largely just inherent to the nature of OAuth, and this document does little to affect that. In some cases, however, such as when access token introspection is not being used, use of the resource parameter defined herein may allow for tracking behavior at a somewhat more granular and specific level than would otherwise be possible in its absence.

IANA Considerations

OAuth Parameters Registration This specification updates the following value in the IANA "OAuth Parameters" registry established by .

Parameter name:

resource

Parameter usage location:

authorization request, token request

Change controller:

IESG

Specification document(s):

RFC 8707

OAuth Extensions Error Registration This specification updates the following error in the IANA "OAuth Extensions Error Registry" established by .

Error name:

invalid_target

Error usage location:

implicit grant error response, token error response

Related protocol extension:

resource parameter

Change controller:

IESG

Specification document(s):

RFC 8707

References Normative References IANA 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. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] 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] 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 The authorization request in OAuth 2.0 described in RFC 6749 utilizes query parameter serialization, which means that Authorization Request parameters are encoded in the URI of the request and sent through user agents such as web browsers. While it is easy to implement, it means that (a) the communication through the user agents are not integrity protected and thus the parameters can be tainted, and (b) the source of the communication is not authenticated. Because of these weaknesses, several attacks to the protocol have now been put forward. This document introduces the ability to send request parameters in a JSON Web Token (JWT) instead, which allows the request to be signed with JSON Web Signature (JWS) and encrypted with JSON Web Encryption (JWE) so that the integrity, source authentication and confidentiality property of the Authorization Request is attained. The request can be sent by value or by reference. Work in Progress 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] JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. The System for Cross-domain Identity Management (SCIM) specification is an HTTP-based protocol that makes managing identities in multi-domain scenarios easier to support via a standardized service. Examples include, but are not limited to, enterprise-to-cloud service providers and inter-cloud scenarios. The specification suite seeks to build upon experience with existing schemas and deployments, placing specific emphasis on simplicity of development and integration, while applying existing authentication, authorization, and privacy models. SCIM's intent is to reduce the cost and complexity of user management operations by providing a common user schema, an extension model, and a service protocol defined by this document. This specification defines a method for a protected resource to query an OAuth 2.0 authorization server to determine the active state of an OAuth 2.0 token and to determine meta-information about this token. OAuth 2.0 deployments can use this method to convey information about the authorization context of the token from the authorization server to the protected resource.

Acknowledgements This specification was developed within the OAuth Working Group under the chairmanship of and with , , and serving as Security Area Directors. Additionally, the following individuals contributed ideas, feedback, and wording that helped shape this specification: , , , , , , , , , , , , , , , , , , , and .

Authors' Addresses Ping Identity

brian.d.campbell@gmail.com

Yubico

ve7jtb@ve7jtb.com

Arm Limited

hannes.tschofenig@gmx.net