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OAuth 2.0 Protected Resource Metadata
draft-ietf-oauth-resource-metadata-05

Document Type Active Internet-Draft (oauth WG)
Authors Michael B. Jones , Phil Hunt , Aaron Parecki
Last updated 2024-06-10 (Latest revision 2024-05-03)
Replaces draft-jones-oauth-resource-metadata
RFC stream Internet Engineering Task Force (IETF)
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draft-ietf-oauth-resource-metadata-05
OAuth Working Group                                           M.B. Jones
Internet-Draft                                    Self-Issued Consulting
Intended status: Standards Track                                 P. Hunt
Expires: 4 November 2024                      Independent Identity, Inc.
                                                              A. Parecki
                                                                    Okta
                                                              3 May 2024

                 OAuth 2.0 Protected Resource Metadata
                 draft-ietf-oauth-resource-metadata-05

Abstract

   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.

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 https://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 4 November 2024.

Copyright Notice

   Copyright (c) 2024 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Protected Resource Metadata . . . . . . . . . . . . . . . . .   4
     2.1.  Signed Protected Resource Metadata  . . . . . . . . . . .   6
   3.  Obtaining Protected Resource Metadata . . . . . . . . . . . .   6
     3.1.  Protected Resource Metadata Request . . . . . . . . . . .   7
     3.2.  Protected Resource Metadata Response  . . . . . . . . . .   8
     3.3.  Protected Resource Metadata Validation  . . . . . . . . .   9
   4.  Authorization Server Metadata . . . . . . . . . . . . . . . .   9
   5.  Use of WWW-Authenticate for Protected Resource Metadata . . .  10
     5.1.  WWW-Authenticate Response . . . . . . . . . . . . . . . .  12
     5.2.  Changes to Resource Metadata  . . . . . . . . . . . . . .  13
     5.3.  Client Identifier and Client Authentication . . . . . . .  13
     5.4.  Compatibility with other authentication methods . . . . .  13
   6.  String Operations . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     7.1.  TLS Requirements  . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Scopes  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Impersonation Attacks . . . . . . . . . . . . . . . . . .  15
     7.4.  Audience-Restricted Access Tokens . . . . . . . . . . . .  15
     7.5.  Publishing Metadata in a Standard Format  . . . . . . . .  16
     7.6.  Authorization Servers . . . . . . . . . . . . . . . . . .  16
     7.7.  Server-Side Request Forgery (SSRF)  . . . . . . . . . . .  17
     7.8.  Phishing  . . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  OAuth Protected Resource Metadata Registry  . . . . . . .  18
       8.1.1.  Registration Template . . . . . . . . . . . . . . . .  18
       8.1.2.  Initial Registry Contents . . . . . . . . . . . . . .  19
     8.2.  OAuth Authorization Server Metadata Registry  . . . . . .  20
       8.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  20
     8.3.  Well-Known URI Registry . . . . . . . . . . . . . . . . .  20
       8.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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1.  Introduction

   This specification defines a metadata format enabling OAuth 2.0
   clients and authorization servers to obtain information needed to
   interact with an OAuth 2.0 protected resource.  This specification is
   intentionally as parallel as possible to "OAuth 2.0 Dynamic Client
   Registration Protocol" [RFC7591], which enables a client to provide
   metadata about itself to an OAuth 2.0 authorization server and to
   OAuth 2.0 Authorization Server Metadata [RFC8414], which enables a
   client to obtain metadata about an OAuth 2.0 authorization server.

   The metadata for a protected resource is retrieved from a well-known
   location as a JSON [RFC7159] document, which declares information
   about its capabilities and optionally, its relationships to other
   services.  This process is described in Section 3.

   This metadata can either be communicated in a self-asserted fashion
   or as a set of signed metadata values represented as claims in a JSON
   Web Token (JWT) [JWT].  In the JWT case, the issuer is vouching for
   the validity of the data about the protected resource.  This is
   analogous to the role that the Software Statement plays in OAuth
   Dynamic Client Registration [RFC7591].

   Each protected resource publishing metadata about itself makes its
   own metadata document available at a well-known location
   deterministically derived from the protected resource's URL, even
   when the resource server implements multiple protected resources.
   This prevents attackers from publishing metadata supposedly
   describing the protected resource, but that is not actually
   authoritative for the protected resource, as described in
   Section 7.3.

   The means by which the client obtains the location of the protected
   resource is out of scope.  In some cases, the location may be
   manually configured into the client.  In other cases, it may be
   dynamically discovered, for instance, through the use of WebFinger
   [RFC7033], in a manner related to the description in Section 2 of
   "OpenID Connect Discovery 1.0" [OpenID.Discovery].

   Section 2 defines metadata values that a protected resource can
   publish, which includes things like which scopes are supported, how a
   client can present an access token, and more.  These values may be
   used by other specifications, such as the jwks_uri used to publish
   public keys the resource server uses to sign resource responses, for
   instance, as described in [FAPI.MessageSigning].

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1.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   All uses of JSON Web Signature (JWS) [JWS] and JSON Web Encryption
   (JWE) [JWE] data structures in this specification utilize the JWS
   Compact Serialization or the JWE Compact Serialization; the JWS JSON
   Serialization and the JWE JSON Serialization are not used.

1.2.  Terminology

   This specification uses the terms "Access Token", "Authorization
   Code", "Authorization Endpoint", "Authorization Grant",
   "Authorization Server", "Client", "Client Authentication", "Client
   Identifier", "Client Secret", "Grant Type", "Protected Resource",
   "Redirection URI", "Refresh Token", "Resource Owner", "Resource
   Server", "Response Type", and "Token Endpoint" defined by OAuth 2.0
   [RFC6749], the terms "Claim Name", "Claim Value", and "JSON Web Token
   (JWT)" defined by JSON Web Token (JWT) [JWT].

   This specification defines the following term:

   Resource Identifier:
      The Protected resource's resource identifier, which is a URL that
      uses the https scheme and has no query or fragment components.
      Protected resource metadata is published at a .well-known location
      [RFC5785] derived from this resource identifier, as described in
      Section 3.

2.  Protected Resource Metadata

   Protected resources can have metadata describing their configuration.
   The following protected resource metadata values are used by this
   specification and are registered in the IANA "OAuth Protected
   Resource Metadata" registry established in Section 8.1:

   resource
      REQUIRED.  The protected resource's resource identifier, which is
      a URL that uses the https scheme and has no query or fragment
      components.  Using these well-known resources is described in
      Section 3.

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   authorization_servers
      OPTIONAL.  JSON array containing a list of OAuth authorization
      server issuer identifiers, as defined in [RFC8414], for
      authorization servers that can be used with this protected
      resource.  Protected resources MAY choose not to advertise some
      supported authorization servers even when this parameter is used.
      In some use cases, the set of authorization servers will not be
      enumerable, in which case this metadata parameter would not be
      used.

   jwks_uri
      OPTIONAL.  URL of the protected resource's JWK Set [JWK] document.
      This contains public keys belonging to the protected resource,
      such as signing key(s) that the resource server uses to sign
      resource responses.  This URL MUST use the https scheme.  When
      both signing and encryption keys are made available, a use (public
      key use) parameter value is REQUIRED for all keys in the
      referenced JWK Set to indicate each key's intended usage.

   scopes_supported
      RECOMMENDED.  JSON array containing a list of the OAuth 2.0
      [RFC6749] scope values that are used in authorization requests to
      request access to this protected resource.  Protected resources
      MAY choose not to advertise some scope values supported even when
      this parameter is used.

   bearer_methods_supported
      OPTIONAL.  JSON array containing a list of the supported methods
      of sending an OAuth 2.0 Bearer Token [RFC6750] to the protected
      resource.  Defined values are ["header", "body", "query"],
      corresponding to Sections 2.1, 2.2, and 2.3 of RFC 6750.

   resource_signing_alg_values_supported
      OPTIONAL.  JSON array containing a list of the JWS [JWS] signing
      algorithms (alg values) [JWA] supported by the protected resource
      for signing resource responses, for instance, as described in
      [FAPI.MessageSigning].  No default algorithms are implied if this
      entry is omitted.  The value none MUST NOT be used.

   resource_documentation
      OPTIONAL.  URL of a page containing human-readable information
      that developers might want or need to know when using the
      protected resource

   resource_policy_uri
      OPTIONAL.  URL that the protected resource provides to read about
      the protected resource's requirements on how the client can use
      the data provided by the protected resource

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   resource_tos_uri
      OPTIONAL.  URL that the protected resource provides to read about
      the protected resource's terms of service

   Additional protected resource metadata parameters MAY also be used.

2.1.  Signed Protected Resource Metadata

   In addition to JSON elements, metadata values MAY also be provided as
   a signed_metadata value, which is a JSON Web Token (JWT) [JWT] that
   asserts metadata values about the protected resource as a bundle.  A
   set of claims that can be used in signed metadata are defined in
   Section 2.  The signed metadata MUST be digitally signed or MACed
   using JSON Web Signature (JWS) [JWS] and MUST contain an iss (issuer)
   claim denoting the party attesting to the claims in the signed
   metadata.  Consumers of the metadata MAY ignore the signed metadata
   if they do not support this feature.  If the consumer of the metadata
   supports signed metadata, metadata values conveyed in the signed
   metadata MUST take precedence over the corresponding values conveyed
   using plain JSON elements.

   Signed metadata is included in the protected resource metadata JSON
   object using this OPTIONAL member:

   signed_metadata
      A JWT containing metadata values about the protected resource as
      claims.  This is a string value consisting of the entire signed
      JWT.  A signed_metadata metadata value SHOULD NOT appear as a
      claim in the JWT.

3.  Obtaining Protected Resource Metadata

   Protected resources supporting metadata MUST make a JSON document
   containing metadata as specified in Section 2 available at a path
   formed by inserting a well-known URI string into the protected
   resource's resource identifier between the host component and the
   path component, if any.  By default, the well-known URI string used
   is /.well-known/oauth-protected-resource.  The syntax and semantics
   of .well-known are defined in [RFC5785].  The well-known URI path
   suffix used MUST be registered in the IANA "Well-Known URIs" registry
   [IANA.well-known].  Examples of this construction can be found in
   Section 3.1.

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   The term "application", as used below (and as used in [RFC8414]),
   encompasses all the components used to accomplish the task for the
   use case.  That can include OAuth clients, authorization servers,
   protected resources, and non-OAuth components, inclusive of the code
   running in each of them.  Applications are built to solve particular
   problems and may utilize many components and services.

   Different applications utilizing OAuth protected resources in
   application-specific ways MAY define and register different well-
   known URI path suffixes for publishing protected resource metadata
   used by those applications.  For instance, if the Example application
   uses an OAuth protected resource in an Example-specific way, and
   there are Example-specific metadata values that it needs to publish,
   then it might register and use the example-protected-resource URI
   path suffix and publish the metadata document at the path formed by
   inserting /.well-known/example-protected-resource between the host
   and path components of the protected resource's resource identifier.
   Alternatively, many such applications will use the default well-known
   URI string /.well-known/oauth-protected-resource, which is the right
   choice for general-purpose OAuth protected resources, and not
   register an application-specific one.

   An OAuth 2.0 application using this specification MUST specify what
   well-known URI suffix it will use for this purpose.  The same
   protected resource MAY choose to publish its metadata at multiple
   well-known locations derived from its resource identifier, for
   example, publishing metadata at both /.well-known/example-protected-
   resource and /.well-known/oauth-protected-resource.

3.1.  Protected Resource Metadata Request

   A protected resource metadata document MUST be queried using an HTTP
   GET request at the previously specified path.

   The consumer of the metadata would make the following request when
   the resource identifier is https://resource.example.com and the well-
   known URI path suffix is oauth-protected-resource to obtain the
   metadata, since the resource identifier contains no path component:

     GET /.well-known/oauth-protected-resource HTTP/1.1
     Host: resource.example.com

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   If the resource identifier value contains a path component, any
   terminating / MUST be removed before inserting /.well-known/ and the
   well-known URI path suffix between the host component and the path
   component.  The consumer of the metadata would make the following
   request when the resource identifier is https://resource.example.com/
   resource1 and the well-known URI path suffix is oauth-protected-
   resource to obtain the metadata, since the resource identifier
   contains a path component:

     GET /.well-known/oauth-protected-resource/resource1 HTTP/1.1
     Host: resource.example.com

   Using path components enables supporting multiple resources per host.
   This is required in some multi-tenant hosting configurations.  This
   use of .well-known is for supporting multiple resources per host;
   unlike its use in [RFC5785], it does not provide general information
   about the host.

3.2.  Protected Resource Metadata Response

   The response is a set of claims about the protected resource's
   configuration.  A successful response MUST use the 200 OK HTTP status
   code and return a JSON object using the application/json content type
   that contains a set of claims as its members that are a subset of the
   metadata values defined in Section 2.  Other claims MAY also be
   returned.

   Claims that return multiple values are represented as JSON arrays.
   Claims with zero elements MUST be omitted from the response.

   An error response uses the applicable HTTP status code value.

   The following is a non-normative example response:

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     HTTP/1.1 200 OK
     Content-Type: application/json

     {
      "resource":
        "https://resource.example.com",
      "authorization_servers":
        ["https://as1.example.com",
         "https://as2.example.net"],
      "bearer_methods_supported":
        ["header", "body"],
      "scopes_supported":
        ["profile", "email", "phone"],
      "resource_documentation":
        "http://resource.example.com/resource_documentation.html"
     }

3.3.  Protected Resource Metadata Validation

   The resource value returned MUST be identical to the protected
   resource's resource identifier value into which the well-known URI
   path suffix was inserted to create the URL used to retrieve the
   metadata.  If these values are not identical, the data contained in
   the response MUST NOT be used.

   If the protected resource metadata was retrieved from a URL returned
   by the protected resource via the WWW-Authenticate resource_metadata
   parameter, then the resource value returned MUST be identical to the
   URL that the client used to make the request to the resource server.
   If these values are not identical, the data contained in the response
   MUST NOT be used.

   These validation actions can thwart impersonation attacks, as
   described in Section 7.3.

4.  Authorization Server Metadata

   To support use cases in which the set of legitimate protected
   resources to use with the authorization server is fixed and
   enumerable, this specification defines the authorization server
   metadata value protected_resources, which enables the authorization
   server to explicitly list the protected resources.  Note that if the
   set of legitimate authorization servers to use with a protected
   resource is also fixed and enumerable, lists in the authorization
   server metadata and protected resource metadata should be cross-
   checked against one another for consistency when these lists are used
   by the application profile.

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   The following authorization server metadata value is defined by this
   specification and is registered in the IANA "OAuth Authorization
   Server Metadata" registry established in OAuth 2.0 Authorization
   Server Metadata [RFC8414].

   protected_resources
      OPTIONAL.  JSON array containing a list of resource identifiers
      for OAuth protected resources for protected resources that can be
      used with this authorization server.  Authorization servers MAY
      choose not to advertise some supported protected resources even
      when this parameter is used.  In some use cases, the set of
      protected resources will not be enumerable, in which case this
      metadata parameter will not be present.

5.  Use of WWW-Authenticate for Protected Resource Metadata

   A protected resource MAY use a WWW-Authenticate response to return a
   URL to its protected resource metadata to the client.  The client can
   then retrieve protected resource metadata as described in Section 3.
   The client might then, for instance, determine what authorization
   server to use for the resource based on protected resource metadata
   retrieved.

   A typical end-to-end flow doing so is as follows.  Note that while
   this example uses the OAuth 2.0 Authorization Code flow, a similar
   sequence could also be implemented with any other OAuth flow.

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        +----------+              +----------+    +---------------+
        |  Client  |              | Resource |    | Authorization |
        |          |              |  Server  |    |    Server     |
        +----+-----+              +----+-----+    +-------+-------+
             |                         |                  |
             |  Resource Request       |                  |
             | ----------------------> |                  |
             |  Without Access Token   |                  |
             |                         |                  |
             |                         |                  |
             |    WWW-Authenticate     |                  |
             | <---------------------- |                  |
             |                         |                  |
             |                         |                  |
             |    Fetch RS Metadata    |                  |
             | ----------------------> |                  |
             |                         |                  |
             |                         |                  |
             |   RS Metadata Response  |                  |
             | <---------------------- |                  |
             |                         |                  |
   +---------+-----------+             |                  |
   |Validate RS Metadata |             |                  |
   |Build AS Metadata URL|             |                  |
   +---------+-----------+             |                  |
             |                         |                  |
             |   Fetch AS Metadata     |                  |
             | ------------------------+----------------> |
             |                         |                  |
             |                         |                  |
             |   AS Metadata Response  |                  |
             | <-----------------------+----------------- |
             |                         |                  |
           +-+-------------------------+------------------+-+
           |       OAuth Authorization Code Flow            |
           |       Client Obtains Access Token              |
           +-+-------------------------+------------------+-+
             |                         |                  |
             |  Resource Request       |                  |
             | ----------------------> |                  |
             |  With Access Token      |                  |
             |                         |                  |
             |                         |                  |
             |   Resource Response     |                  |
             | <---------------------- |                  |
             |                         |                  |

                         Figure 1: Sequence Diagram

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   1.   The client makes a request to a protected resource without
        presenting an access token.

   2.   The resource server responds with a WWW-Authenticate header
        including the URL of the protected resource metadata.

   3.   The client fetches the protected resource metadata from this
        URL.

   4.   The resource server responds with the protected resource
        metadata according to Section 3.2.

   5.   The client validates the protected resource metadata, as
        described in Section 3.3.

   6.   The client builds the authorization server metadata URL from an
        issuer identifier in the resource metadata according to
        [RFC8414] and makes a request to fetch the authorization server
        metadata.

   7.   The authorization server responds with the authorization server
        metadata document according to [RFC8414].

   8.   The client directs the user agent to the authorization server to
        begin the authorization flow.

   9.   The authorization exchange is completed and the authorization
        server returns an access token to the client.

   10.  The client repeats the resource request from step 1, presenting
        the newly obtained access token.

   11.  The resource server returns the requested protected resource.

5.1.  WWW-Authenticate Response

   This specification introduces a new parameter in the WWW-Authenticate
   response to indicate the protected resource metadata URL:

   resource_metadata:
      The URL of the protected resource metadata.

   The response below is an example of a WWW-Authenticate header that
   includes the resource identifier.

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   HTTP/1.1 400 Bad Request
   WWW-Authenticate: Bearer error="invalid_request",
     error_description="No access token was provided in this request",
     resource_metadata=
     "https://resource.example.com/.well-known/oauth-protected-resource"

   The HTTP status code and error string in the response are defined by
   [RFC6750].

   The resource_metadata parameter MAY be combined with other parameters
   defined in other extensions, such as the max_age parameter defined by
   [RFC9470].

5.2.  Changes to Resource Metadata

   At any point, for any reason determined by the protected resource,
   the protected resource MAY respond with a new WWW-Authenticate
   challenge that includes a value for the protected resource metadata
   URL to indicate that its metadata MAY have changed.  If the client
   receives such a WWW-Authenticate response, it SHOULD retrieve the
   updated protected resource metadata and use the new metadata values
   obtained.  Among other things, this enables a resource server to
   change which authorization servers it uses without any other
   coordination with clients.

5.3.  Client Identifier and Client Authentication

   The way in which the client identifier is established at the
   authorization server is out of scope of this specification.

   This specification is intended to be deployed in scenarios where the
   client has no prior knowledge about the resource server, and the
   resource server might or might not have prior knowledge about the
   client.

   There are some existing methods by which an unrecognized client can
   make use of an authorization server, such as using Dynamic Client
   Registration [RFC7591] to register the client prior to initiating the
   authorization flow.  Future extensions might define alternatives,
   such as using URLs to identify clients.

5.4.  Compatibility with other authentication methods

   Resource servers MAY return other WWW-Authenticate headers indicating
   various authentication schemes.  This allows the resource server to
   support clients that may or may not implement this specification, and
   allows clients to choose their preferred authentication scheme.

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6.  String Operations

   Processing some OAuth 2.0 messages requires comparing values in the
   messages to known values.  For example, the member names in the
   metadata response might be compared to specific member names such as
   resource.  Comparing Unicode [UNICODE] strings, however, has
   significant security implications.

   Therefore, comparisons between JSON strings and other Unicode strings
   MUST be performed as specified below:

   1.  Remove any JSON applied escaping to produce an array of Unicode
       code points.

   2.  Unicode Normalization [USA15] MUST NOT be applied at any point to
       either the JSON string or to the string it is to be compared
       against.

   3.  Comparisons between the two strings MUST be performed as a
       Unicode code point to code point equality comparison.

   Note that this is the same equality comparison procedure described in
   Section 8.3 of [RFC8259].

7.  Security Considerations

7.1.  TLS Requirements

   Implementations MUST support TLS.  Which version(s) ought to be
   implemented will vary over time, and depend on the widespread
   deployment and known security vulnerabilities at the time of
   implementation.  Implementations SHOULD follow the guidance in BCP
   195 [RFC8996] [RFC9325], which provides recommendations and
   requirements for improving the security of deployed services that use
   TLS.

   To protect against information disclosure and tampering,
   confidentiality protection MUST be applied using TLS with a
   ciphersuite that provides confidentiality and integrity protection.

7.2.  Scopes

   The scopes_supported parameter is the list of scopes the resource
   server is willing to disclose that it supports.  It is not meant to
   indicate that an OAuth client should request all scopes in the list.
   The client SHOULD still follow OAuth best practices and request
   tokens with as limited scope as possible for the given operation, as
   described in Section 2.3 of OAuth 2.0 Security Best Current Practice

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   [I-D.ietf-oauth-security-topics].

7.3.  Impersonation Attacks

   TLS certificate checking MUST be performed by the client, as
   described in Section 7.1, when making a protected resource metadata
   request.  Checking that the server certificate is valid for the
   resource identifier URL prevents man-in-middle and DNS-based attacks.
   These attacks could cause a client to be tricked into using an
   attacker's resource server, which would enable impersonation of the
   legitimate protected resource.  If an attacker can accomplish this,
   they can access the resources that the affected client has access to
   using the protected resource that they are impersonating.

   An attacker may also attempt to impersonate a protected resource by
   publishing a metadata document that contains a resource claim using
   the resource identifier URL of the protected resource being
   impersonated, but containing information of the attacker's choosing.
   This would enable it to impersonate that protected resource, if
   accepted by the client.  To prevent this, the client MUST ensure that
   the resource identifier URL it is using as the prefix for the
   metadata request exactly matches the value of the resource metadata
   value in the protected resource metadata document received by the
   client, as described in Section 3.3.

7.4.  Audience-Restricted Access Tokens

   If a client expects to interact with multiple resource servers, the
   client SHOULD request audience-restricted access tokens using
   [RFC8707], and the authorization server SHOULD support audience-
   restricted access tokens.

   Without audience-restricted access tokens, a malicious resource
   server (RS1) may be able to use the WWW-Authenticate header to get a
   client to request an access token with a scope used by a legitimate
   resource server (RS2), and after the client sends a request to RS1,
   then RS1 could re-use the access token at RS2.

   While this attack is not explicitly enabled by this specification,
   and is possible in a plain OAuth 2.0 deployment, it is made somewhat
   more likely by the use of dynamically-configured clients.  As such,
   the use of audience-restricted access tokens and Resource Indicators
   [RFC8707] is RECOMMENDED when using the features in this
   specification.

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7.5.  Publishing Metadata in a Standard Format

   Publishing information about the protected resource in a standard
   format makes it easier for both legitimate clients and attackers to
   use the protected resource.  Whether a protected resource publishes
   its metadata in an ad-hoc manner or in the standard format defined by
   this specification, the same defenses against attacks that might be
   mounted that use this information should be applied.

7.6.  Authorization Servers

   To support use cases in which the set of legitimate authorization
   servers to use with the protected resource is fixed and enumerable,
   this specification defines the authorization_servers metadata value,
   which enables explicitly listing them.  Note that if the set of
   legitimate protected resources to use with an authorization server is
   also fixed and enumerable, lists in the protected resource metadata
   and authorization server metadata should be cross-checked against one
   another for consistency when these lists are used by the application
   profile.

   Secure determination of appropriate authorization servers to use with
   a protected resource for all use cases is out of scope of this
   specification.  This specification assumes that the client has a
   means of determining appropriate authorization servers to use with a
   protected resource and that the client is using the correct metadata
   for each protected resource.  Implementers need to be aware that if
   an inappropriate authorization server is used by the client, that an
   attacker may be able to act as a man-in-the-middle proxy to a valid
   authorization server without it being detected by the authorization
   server or the client.

   The ways to determine the appropriate authorization servers to use
   with a protected resource are in general, application-dependent.  For
   instance, some protected resources are used with a fixed
   authorization server or set of authorization servers, the locations
   of which may be well known, or which could be published as metadata
   values by the protected resource.  In other cases, the set of
   authorization servers that can be used with a protected resource can
   by dynamically changed by administrative actions or by changes to the
   set of authorization servers adhering to a trust framework.  Many
   other means of determining appropriate associations between protected
   resources and authorization servers are also possible.

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7.7.  Server-Side Request Forgery (SSRF)

   The OAuth client is expected to fetch the authorization server
   metadata based on the value of the issuer in the resource server
   metadata.  Since this specification enables clients to interoperate
   with RSs and ASs it has no prior knowledge of, this opens a risk for
   SSRF attacks by malicious users or malicious resource servers.
   Clients SHOULD take appropriate precautions against SSRF attacks,
   such as blocking requests to internal IP address ranges.  Further
   recommendations can be found in the OWASP SSRF Prevention Cheat Sheet
   [OWASP.SSRF].

7.8.  Phishing

   This specification may be deployed in a scenario where the desired
   HTTP resource is identified by a user-selected URL.  If this resource
   is malicious or compromised, it could mislead the user into revealing
   their account credentials or authorizing unwanted access to OAuth-
   controlled capabilities.  This risk is reduced, but not eliminated,
   by following best practices for OAuth user interfaces, such as
   providing clear notice to the user, displaying the authorization
   server's domain name, supporting origin-bound phishing-resistant
   authenticators, supporting the use of password managers, and applying
   heuristic checks such as domain reputation.

8.  IANA Considerations

   The following registration procedure is used for the registry
   established by this specification.

   Values are registered on a Specification Required [RFC5226] basis
   after a two-week review period on the oauth-ext-review@ietf.org
   mailing list, on the advice of one or more Designated Experts.
   However, to allow for the allocation of values prior to publication,
   the Designated Experts may approve registration once they are
   satisfied that such a specification will be published.

   Registration requests sent to the mailing list for review should use
   an appropriate subject (e.g., "Request to register OAuth Protected
   Resource Metadata: example").

   Within the review period, the Designated Experts will either approve
   or deny the registration request, communicating this decision to the
   review list and IANA.  Denials should include an explanation and, if
   applicable, suggestions as to how to make the request successful.
   Registration requests that are undetermined for a period longer than
   21 days can be brought to the IESG's attention (using the
   iesg@ietf.org mailing list) for resolution.

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   Criteria that should be applied by the Designated Experts includes
   determining whether the proposed registration duplicates existing
   functionality, determining whether it is likely to be of general
   applicability or whether it is useful only for a single application,
   and whether the registration makes sense.

   IANA must only accept registry updates from the Designated Experts
   and should direct all requests for registration to the review mailing
   list.

   It is suggested that multiple Designated Experts be appointed who are
   able to represent the perspectives of different applications using
   this specification, in order to enable broadly-informed review of
   registration decisions.  In cases where a registration decision could
   be perceived as creating a conflict of interest for a particular
   Expert, that Expert should defer to the judgment of the other
   Experts.

8.1.  OAuth Protected Resource Metadata Registry

   This specification establishes the IANA "OAuth Protected Resource
   Metadata" registry for OAuth 2.0 protected resource metadata names.
   The registry records the protected resource metadata member and a
   reference to the specification that defines it.

8.1.1.  Registration Template

   Metadata Name:
      The name requested (e.g., "resource").  This name is case-
      sensitive.  Names may not match other registered names in a case-
      insensitive manner unless the Designated Experts state that there
      is a compelling reason to allow an exception.

   Metadata Description:
      Brief description of the metadata (e.g., "Resource identifier
      URL").

   Change Controller:
      For Standards Track RFCs, list the "IETF".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.

   Specification Document(s):
      Reference to the document or documents that specify the parameter,
      preferably including URIs that can be used to retrieve copies of
      the documents.  An indication of the relevant sections may also be
      included but is not required.

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8.1.2.  Initial Registry Contents

   *  Metadata Name: resource
   *  Metadata Description: Protected resource's resource identifier URL
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: authorization_servers
   *  Metadata Description: JSON array containing a list of OAuth
      authorization server issuer identifiers
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: jwks_uri
   *  Metadata Description: URL of the protected resource's JWK Set
      document
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: scopes_supported
   *  Metadata Description: JSON array containing a list of the OAuth
      2.0 scope values that are used in authorization requests to
      request access this protected resource
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: bearer_methods_supported
   *  Metadata Description: JSON array containing a list of the OAuth
      2.0 Bearer Token presentation methods that this protected resource
      supports
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: resource_signing_alg_values_supported
   *  Metadata Description: JSON array containing a list of the JWS
      signing algorithms (alg values) supported by the protected
      resource for signed content
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: resource_documentation
   *  Metadata Description: URL of a page containing human-readable
      information that developers might want or need to know when using
      the protected resource
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: resource_policy_uri

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   *  Metadata Description: URL that the protected resource provides to
      read about the protected resource's requirements on how the client
      can use the data provided by the protected resource
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: resource_tos_uri
   *  Metadata Description: URL that the protected resource provides to
      read about the protected resource's terms of service
   *  Change Controller: IETF
   *  Specification Document(s): Section 2 of [[ this specification ]]

   *  Metadata Name: signed_metadata
   *  Metadata Description: Signed JWT containing metadata values about
      the protected resource as claims
   *  Change Controller: IETF
   *  Specification Document(s): Section 2.1 of [[ this specification ]]

8.2.  OAuth Authorization Server Metadata Registry

   The following authorization server metadata value is registered in
   the IANA "OAuth Authorization Server Metadata" registry established
   in OAuth 2.0 Authorization Server Metadata [RFC8414].

8.2.1.  Registry Contents

   *  Metadata Name: protected_resources
   *  Metadata Description: JSON array containing a list of resource
      identifiers for OAuth protected resources
   *  Change Controller: IETF
   *  Specification Document(s): Section 4 of [[ this specification ]]

8.3.  Well-Known URI Registry

   This specification registers the well-known URI defined in Section 3
   in the IANA "Well-Known URIs" registry [IANA.well-known] established
   by [RFC5785].

8.3.1.  Registry Contents

   *  URI suffix: oauth-protected-resource
   *  Change controller: IETF
   *  Specification document: Section 3 of [[ this specification ]]
   *  Related information: (none)

9.  References

9.1.  Normative References

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   [JWA]      Jones, M.B., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://tools.ietf.org/html/rfc7518>.

   [JWE]      Jones, M.B. and J. Hildebrand, "JSON Web Encryption
              (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://tools.ietf.org/html/rfc7516>.

   [JWK]      Jones, M.B., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://tools.ietf.org/html/rfc7517>.

   [JWS]      Jones, M.B., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://tools.ietf.org/html/rfc7515>.

   [JWT]      Jones, M.B., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://tools.ietf.org/html/rfc7519>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <https://www.rfc-editor.org/info/rfc5785>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
              2013, <https://www.rfc-editor.org/info/rfc7033>.

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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/info/rfc7591>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC8996]  Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

   [USA15]    Davis, M. and K. Whistler, "Unicode Normalization Forms",
              Unicode Standard Annex 15, 1 June 2015,
              <http://www.unicode.org/reports/tr15/>.

9.2.  Informative References

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   [FAPI.MessageSigning]
              Tonge, D. and D. Fett, "FAPI 2.0 Message Signing", 24
              March 2023,
              <https://openid.net/specs/fapi-2_0-message-signing.html>.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-23, 5 June 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-23>.

   [IANA.well-known]
              IANA, "Well-Known URIs",
              <http://www.iana.org/assignments/well-known-uris>.

   [OpenID.Discovery]
              Sakimura, N., Bradley, J., Jones, M.B., and E. Jay,
              "OpenID Connect Discovery 1.0", 8 November 2014,
              <http://openid.net/specs/openid-connect-discovery-
              1_0.html>.

   [OWASP.SSRF]
              OWASP, "OWASP SSRF Prevention Cheat Sheet",
              <https://cheatsheetseries.owasp.org/cheatsheets/
              Server_Side_Request_Forgery_Prevention_Cheat_Sheet.html>.

   [RFC9470]  Bertocci, V. and B. Campbell, "OAuth 2.0 Step Up
              Authentication Challenge Protocol", RFC 9470,
              DOI 10.17487/RFC9470, September 2023,
              <https://www.rfc-editor.org/info/rfc9470>.

Appendix A.  Acknowledgements

   The authors of this specification would like to thank the attendees
   of the IETF 115 OAuth and HTTP API Working Group meetings and the
   attendees of subsequent OAuth Working Group meetings for their input
   on this specification.  We would would also like to thank Brian
   Campbell, Vladimir Dzhuvinov, George Fletcher, Pieter Kasselman, Tony
   Nadalin, Filip Skokan, and Atul Tulshibagwale for their contributions
   to the specification.

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   -05

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   *  Added SVG diagram

   -04

   *  Applied working group last call suggestions by Atul Tulshibagwale.

   *  Better described the purpose of
      resource_signing_alg_values_supported and removed
      resource_encryption_alg_values_supported and
      resource_encryption_enc_values_supported, per WGLC comments by
      Vladimir Dzhuvinov and Brian Campbell.

   *  Applied suggestions by Pieter Kasselman.

   -03

   *  Applied correction by Filip Skokan.

   -02

   *  Switched from concatenating .well-known to the end of the resource
      identifier to inserting it between the host and path components of
      it.

   *  Have WWW-Authenticate return resource_metadata rather than
      resource.

   -01

   *  Renamed scopes_provided to scopes_supported.

   *  Added security consideration for scopes_supported.

   *  Use BCP 195 for TLS recommendations.

   *  Clarified that resource metadata can be used by clients and
      authorization servers.

   *  Updated references.

   *  Added security consideration recommending audience-restricted
      access tokens.

   *  Mention FAPI Message Signing as a use case for publishing signing
      keys.

   -00

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   *  Initial working group version based on draft-jones-oauth-resource-
      metadata-04.

Authors' Addresses

   Michael B. Jones
   Self-Issued Consulting
   Email: michael_b_jones@hotmail.com
   URI:   https://self-issued.info/

   Phil Hunt
   Independent Identity, Inc.
   Email: phil.hunt@yahoo.com

   Aaron Parecki
   Okta
   Email: aaron@parecki.com
   URI:   https://aaronparecki.com/

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