



Web Authorization Protocol                                    A. Parecki
Internet-Draft                                                      Okta
Intended status: Informational                                   D. Fett
Expires: 5 January 2026                                        J. Heenan
                                                                Authlete
                                                             4 July 2025


                       OAuth 2.0 Client ID Prefix
                draft-parecki-oauth-client-id-prefix-00

Abstract

   This specification defines the concept of a Client Identifier Prefix
   to enable Authorization Servers and Clients to use more than one
   mechanism to obtain and validate Client metadata.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://drafts.aaronpk.com/oauth-client-id-prefix/draft-parecki-
   oauth-client-id-prefix.html.  Status information for this document
   may be found at https://datatracker.ietf.org/doc/draft-parecki-oauth-
   client-id-prefix/.

   Discussion of this document takes place on the Web Authorization
   Protocol Working Group mailing list (mailto:oauth@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/oauth/.
   Subscribe at https://www.ietf.org/mailman/listinfo/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/aaronpk/oauth-client-id-prefix.

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/.







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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Client Identifier Prefix  . . . . . . . . . . . . . . . . . .   3
     3.1.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Fallback for Unrecognized Client ID Prefixes  . . . . . .   4
       3.2.1.  Example . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Defined Client Identifier Prefixes  . . . . . . . . . . .   5
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Authorization Server Metadata . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  Client Identifier Mixups  . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  OAuth Authorization Server Metadata Registry  . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8










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1.  Introduction

   A Client Identifier is used by an OAuth 2.0 Client to identify itself
   to an Authorization Server.  The Client Identifier is used in the
   Authorization Request and various other places throughout OAuth
   flows.  In ecosystems where more than one method of obtaining and
   validating Client metadata is used, it is necessary to indicate
   unambiguously which method is used.  This specification defines a
   structure for Client Identifiers that includes a prefix indicating
   the Client Identifier Prefix.

2.  Conventions and Definitions

   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.

3.  Client Identifier Prefix

   This specification defines the concept of a Client Identifier Prefix
   that indicates how an Authorization Server is supposed to interpret
   the Client Identifier and associated data in the process of Client
   identification, authentication, and authorization.  The Client
   Identifier Prefix enables deployments of this specification to use
   different mechanisms to obtain and validate metadata of the Client
   beyond the scope of [RFC6749].

   The Client Identifier Prefix is a string that MAY be communicated by
   the Client in a prefix within the client_id parameter in the
   Authorization Request.  A fallback to pre-registered Clients as in
   [RFC6749] or a default Client Identifier Prefix is in place as a
   default mechanism in case no Client Identifier Prefix was provided.
   A certain Client Identifier Prefix may require the Client to sign the
   Authorization Request as means of authentication and/or pass
   additional parameters and require the Authorization Server to process
   them.

3.1.  Syntax

   In the client_id Authorization Request parameter and other places
   where the Client Identifier is used, the Client Identifier Prefixes
   are prefixed to the usual Client Identifier, separated by a : (colon)
   character:

   <client_id_prefix>:<orig_client_id>




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   Here, <client_id_prefix> is the Client Identifier Prefix and
   <orig_client_id> is an identifier for the Client within the namespace
   of that prefix.  See Section 3.3 for Client Identifier Prefixes
   defined by this specification.

   Authorization Servers MUST use the presence of a : (colon) character
   and the content preceding it to determine whether a Client Identifier
   Prefix is used.  If a : character is present, and the content
   preceding it is a recognized and supported Client Identifier Prefix
   value, the Authorization Server MUST interpret the Client Identifier
   according to the given Client Identifier Prefix.  The Client
   Identifier Prefix is defined as the string before the (first) :
   character.  If the Authorization Server does not support the Client
   Identifier Prefix, the Authorization Server MUST refuse the request.

   For example, an Authorization Request might contain
   client_id=client_attestation:example-client to indicate that the
   client_attestation Client Identifier Prefix is to be used and that
   within this prefix, the Client can be identified by the string
   example-client.

   Note that the Client may need to determine which Client Identifier
   Prefixes the Authorization Server supports prior to sending the
   Authorization Request in order to ensure the client's preferred
   prefix is supported.

3.2.  Fallback for Unrecognized Client ID Prefixes

   If a : character is not present in the Client Identifier, the
   Authorization Server MUST treat the Client Identifier as referencing
   a pre-registered client.  This is equivalent to the [RFC6749] default
   behavior, i.e., the Client Identifier needs to be known to the
   Authorization Server in advance of the Authorization Request.  The
   Client metadata is pre-registered using [RFC7591] or through out-of-
   band mechanisms.

   For example, if an Authorization Request contains client_id=example-
   client, the Authorization Server would interpret the Client
   Identifier as referring to a pre-registered client.

   If a : character is present in the Client Identifier but the value
   preceding it is not a recognized and supported Client Identifier
   Prefix value, the Authorization Server MAY treat the Client
   Identifier as having a default Client Identifier Prefix.







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   For example, an Authorization Request containing a client_id value of
   https://client.example.com/metadata.json could be interpreted by the
   Authorization Server as referring to a Client ID Metadata Document
   [I-D.draft-parecki-oauth-client-id-metadata-document], with the
   default Client Identifier Prefix being client-id-metadata-document.

   From this definition, it follows that pre-registered clients MUST NOT
   contain a : character preceded immediately by a supported Client
   Identifier Prefix value in the first part of their Client Identifier.

3.2.1.  Example

   Deployments that use https URLs as client IDs and that have only one
   way to resolve client metadata from the URL, MAY use only the full
   https URL as the client ID.  If there is only one way to resolve
   client metadata then there is no ambiguity in which metadata
   retrieval method to use, and are not susceptible to client identifier
   mixup attacks as described in Section 6.1.

   For example, an authorization server using only the Client ID
   Metadata Document
   [I-D.draft-parecki-oauth-client-id-metadata-document] method to
   retrieve client metadata MAY accept client IDs such as:

   https://client.example.com/metadata.json

   This results in this non-normative example of an authorization
   request:

   GET /authorize?
     response_type=code
     &client_id=https%3A%2F%2Fclient.example.org%2Fmetadata.json
     &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcallback
     &code_challenge=GdE4nqBrwRxQfN2Y8fq3rrYk_kkpwg6tQ74J94-2nHw
     &code_challenge_method=S256
     &scope=write

3.3.  Defined Client Identifier Prefixes

   This specification defines the following Client Identifier Prefixes,
   followed by the examples where applicable:

   *  redirect_uri: This value indicates that the Client Identifier
      (without the prefix redirect_uri:) is the Client's Redirect URI
      (or Response URI when Response Mode direct_post is used).  The
      Authorization Request MUST NOT be signed.  The Client MAY omit the
      redirect_uri Authorization Request parameter.  Example Client
      Identifier: redirect_uri:https%3A%2F%2Fclient.example.org%2Fcb.



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   *  client_id_metadata_document: This value indicates that the Client
      Identifer (without the prefix client_id_metadata_document:) is the
      client's Client ID Metadata Document
      [I-D.draft-parecki-oauth-client-id-metadata-document].

   *  https: This Client Identifier Prefix MUST NOT be registered.

4.  Example

   The following is a non-normative example of an authorization request
   with the client_id_metadata_document Client ID Prefix:

GET /authorize?
  response_type=code
  &client_id=client_id_metadata_document:https%3A%2F%2Fclient.example.org%2Fmetadata.json
  &redirect_uri=https%3A%2F%2Fclient.example.org%2Fredirect
  &code_challenge=GdE4nqBrwRxQfN2Y8fq3rrYk_kkpwg6tQ74J94-2nHw
  &code_challenge_method=S256
  &scope=write

5.  Authorization Server Metadata

   Authorization servers that publish Authorization Server Metadata
   ([RFC8414]) MUST include the following properties to indicate support
   for client ID prefixes as described in this specification.

   client_id_prefixes_supported:  REQUIRED.  A JSON array of strings
      indicating the registered client ID prefixes supported by this
      authorization server.

6.  Security Considerations

6.1.  Client Identifier Mixups

   Confusing Clients using a Client Identifier Prefix with those using
   none can lead to various mixup attacks.  Therefore, Authorization
   Servers MUST always use the full Client Identifier, including the
   prefix if provided, within the context of the Authorization Server or
   its responses to identify the client.  This refers in particular to
   places where the Client Identifier is used in [RFC6749] as well as in
   any artifacts such as the aud claim of JWT access tokens [RFC9068].

7.  IANA Considerations








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7.1.  OAuth Authorization Server Metadata Registry

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

   *  Metadata Name: client_id_prefixes_supported

   *  Metadata Description: A JSON array of strings indicating the
      client ID prefixes supported by the authorization server.

   *  Change Controller: IETF

   *  Specification Document: Section 5 of [[ this specification ]]

8.  References

8.1.  Normative References

   [DID-Core] "DID Core", 19 July 2022,
              <https://www.w3.org/TR/did-core/>.

   [OpenID.Federation]
              Hedberg, R., Jones, M.B., Solberg, A.Å., Bradley, J.,
              Marco, G. D., and V. Dzhuvinov, "OpenID Federation 1.0",
              17 May 2024,
              <https://openid.net/specs/openid-federation-1_0.html>.

   [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/rfc/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

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

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/rfc/rfc7515>.




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   [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/rfc/rfc8174>.

   [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/rfc/rfc8414>.

8.2.  Informative References

   [I-D.draft-parecki-oauth-client-id-metadata-document]
              Parecki, A. and E. Smith, "OAuth Client ID Metadata
              Document", Work in Progress, Internet-Draft, draft-
              parecki-oauth-client-id-metadata-document-02, 9 January
              2025, <https://datatracker.ietf.org/doc/html/draft-
              parecki-oauth-client-id-metadata-document-02>.

   [OpenID]   Sakimura, N., Bradley, J., Jones, M., Medeiros, B. de.,
              and C. Mortimore, "OpenID Connect Core 1.0", 15 December
              2023,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [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/rfc/rfc7591>.

   [RFC9068]  Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
              Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
              2021, <https://www.rfc-editor.org/rfc/rfc9068>.

Acknowledgments

   The authors would like to thank the following people for their
   contributions and reviews of this specification:

   Brian Campbell, Emelia Smith.

Authors' Addresses

   Aaron Parecki
   Okta
   Email: aaron@parecki.com


   Daniel Fett
   Authlete



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   Email: mail@danielfett.de


   Joseph Heenan
   Authlete
   Email: joseph@heenan.me.uk













































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