OAuth 2.0 Pushed Authorization Requests
draft-ietf-oauth-par-03

Web Authorization Protocol                                T. Lodderstedt
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: 1 February 2021                                   Ping Identity
                                                             N. Sakimura
                                                          NAT.Consulting
                                                                D. Tonge
                                           Moneyhub Financial Technology
                                                               F. Skokan
                                                                   Auth0
                                                            31 July 2020

                OAuth 2.0 Pushed Authorization Requests
                        draft-ietf-oauth-par-03

Abstract

   This document defines the pushed authorization request endpoint,
   which allows clients to push the payload of an OAuth 2.0
   authorization request to the authorization server via a direct
   request and provides them with a request URI that is used as
   reference to the data in a subsequent authorization request.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 1 February 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   5
   2.  Pushed Authorization Request Endpoint . . . . . . . . . . . .   5
     2.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Successful Response . . . . . . . . . . . . . . . . . . .   8
     2.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .   9
   3.  "request" Parameter . . . . . . . . . . . . . . . . . . . . .  10
   4.  Authorization Request . . . . . . . . . . . . . . . . . . . .  12
   5.  Authorization Server Metadata . . . . . . . . . . . . . . . .  13
   6.  Client Metadata . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     7.1.  Request URI Guessing  . . . . . . . . . . . . . . . . . .  14
     7.2.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Request Object Replay . . . . . . . . . . . . . . . . . .  14
     7.4.  Client Policy Change  . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  OAuth Authorization Server Metadata . . . . . . . . . . .  15
     9.2.  OAuth Dynamic Client Registration Metadata  . . . . . . .  15
     9.3.  OAuth URI Registration  . . . . . . . . . . . . . . . . .  15
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  15
   11. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   In OAuth [RFC6749] authorization request parameters are typically
   sent as URI query parameters via redirection in the user-agent.  This
   is simple but also yields challenges:

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   *  There is no cryptographic integrity and authenticity protection.
      An attacker could, for example, modify the ACR value requested by
      the client or swap the context of a payment transaction
      authorization by changing scope values.  Although clients should
      detect such changes by inspecting the token response data,
      preventing such modifications early in the process would be a
      better solution.

   *  There is no mechanism to ensure confidentiality of the request
      parameters.  This obviously is an issue if personal identifiable
      information is sent in the authorization request, which might be
      the case in identity and open banking scenarios.

   *  Authorization request URLs can become quite large, especially in
      scenarios requiring fine-grained authorization data, which might
      cause errors in request processing.

   JWT Secured Authorization Request (JAR) [I-D.ietf-oauth-jwsreq]
   provides solutions for the security challenges by allowing OAuth
   clients to wrap authorization request parameters in a signed, and
   optionally encrypted, JSON Web Token (JWT), the so-called "Request
   Object".  In order to cope with the size restrictions, JAR introduces
   the "request_uri" parameter that allows clients to send a reference
   to a request object instead of the request object itself.

   This document complements JAR by providing an interoperable way to
   push the payload of a request object directly to the authorization
   server in exchange for a "request_uri".

   It also allows for clients to push the form encoded authorization
   request parameters to the authorization server in order to exchange
   them for a request URI that the client can use in a subsequent
   authorization request.

   For example, a client typically initiates an authorization request by
   directing the user-agent to make an HTTP request like the following:

     GET /authorize?response_type=code
      &client_id=s6BhdRkqt3&state=af0ifjsldkj
      &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb HTTP/1.1
     Host: as.example.com

   Such a request could instead be pushed directly to the authorization
   server by the client as follows:

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     POST /as/par HTTP/1.1
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded
     Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3

     response_type=code
     &client_id=s6BhdRkqt3&state=af0ifjsldkj
     &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb

   The authorization server responds with a request URI:

     HTTP/1.1 201 Created
     Cache-Control: no-cache, no-store
     Content-Type: application/json

     {

       "request_uri": "urn:example:bwc4JK-ESC0w8acc191e-Y1LTC2",
       "expires_in": 90
     }

   The client uses the request URI value to create the subsequent
   authorization request and directing the user-agent to make an HTTP
   request like the following:

     GET /authorize?client_id=s6BhdRkqt3&
     request_uri=urn%3Aexample%3Abwc4JK-ESC0w8acc191e-Y1LTC2 HTTP/1.1
     Host: as.example.com

   The pushed authorization request endpoint fosters OAuth security by
   providing all clients a simple means for a confidential and integrity
   protected authorization request, but it also allows clients requiring
   an even higher security level, especially cryptographically confirmed
   non-repudiation, to explicitly adopt JWT-based request objects.

   As a further benefit, the pushed authorization request allows the
   authorization server to authenticate the clients before any user
   interaction happens, i.e., the authorization server may refuse
   unauthorized requests much earlier in the process and has much higher
   confidence in the client's identity in the authorization process than
   before.  This generally improves security since it prevents attempts
   to spoof confidential clients early in the process.

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   This is directly utilized by this draft to allow confidential clients
   to set the redirect URI for every authorization request, which gives
   them more flexibility in building redirect URI.  And if the client
   IDs and credentials are managed by some external authority (e.g. a
   certification authority), explicit client registration with the
   particular authorization server could practically be skipped.

   Note: HTTP POST requests to the authorization endpoint as described
   in Section 3.1 of [RFC6749] and Section 3.1.2.1 of [OIDC] could also
   be used to cope with the request size limitations described above.
   Although this is a viable option for traditional web applications,
   it's difficult to use with mobile apps.  Those apps typically invoke
   a custom tab with an URL that is translated into a GET request.
   Using POST would require the app to first open a web page under its
   control in the custom tab that in turn would initiate the form POST
   towards the authorization server.  PAR is simpler to use and has
   additional security benefits as described above.

1.1.  Conventions and Terminology

   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.

   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 [RFC6749].

2.  Pushed Authorization Request Endpoint

   The pushed authorization request endpoint is an HTTP API at the
   authorization server that accepts POST requests with parameters in
   the HTTP request entity-body using the "application/x-www-form-
   urlencoded" format with a character encoding of UTF-8 as described in
   Appendix B of [RFC6749].  The pushed authorization request endpoint
   URL MUST use the "https" scheme.

   Authorization servers supporting pushed authorization requests SHOULD
   include the URL of their pushed authorization request endpoint in
   their authorization server metadata document [RFC8414] using the
   "pushed_authorization_request_endpoint" parameter as defined in
   Section 5.

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   The endpoint accepts the parameters defined in [RFC6749] for the
   authorization endpoint as well as all applicable extensions defined
   for the authorization endpoint.  Some examples of such extensions
   include PKCE [RFC7636], Resource Indicators [RFC8707], and OpenID
   Connect [OIDC].  The endpoint also supports sending all authorization
   request parameters as request object according to
   [I-D.ietf-oauth-jwsreq].

   The rules for client authentication as defined in [RFC6749] for token
   endpoint requests, including the applicable authentication methods,
   apply for the pushed authorization request endpoint as well.  If
   applicable, the "token_endpoint_auth_method" client metadata
   parameter indicates the registered authentication method for the
   client to use when making direct requests to the authorization
   server, including requests to the pushed authorization request
   endpoint.

   Note that there's some potential ambiguity around the appropriate
   audience value to use when JWT client assertion based authentication
   is employed.  To address that ambiguity the issuer identifier URL of
   the authorization server according to [RFC8414] SHOULD be used as the
   value of the audience.  In order to facilitate interoperability the
   authorization server MUST accept its issuer identifier, token
   endpoint URL, or pushed authorization request endpoint URL as values
   that identify it as an intended audience.

2.1.  Request

   A client can send all the parameters that usually comprise an
   authorization request to the pushed authorization request endpoint.
   A basic parameter set will typically include:

   *  "client_id"

   *  "response_type"

   *  "redirect_uri"

   *  "scope"

   *  "state"

   *  "code_challenge"

   *  "code_challenge_method"

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   Depending on client type and authentication method, the request might
   also include other parameters for client authentication such as the
   "client_secret" parameter, the "client_assertion" parameter and the
   "client_assertion_type" parameter.  The "request_uri" authorization
   request parameter MUST NOT be provided in this case (see Section 3).

   The client adds the parameters in "x-www-form-urlencoded" format with
   a character encoding of UTF-8 as described in Appendix B of [RFC6749]
   to the body of an HTTP POST request.  If applicable, the client also
   adds client credentials to the request header or the request body
   using the same rules as for token endpoint requests.

   This is illustrated by the following example:

     POST /as/par HTTP/1.1
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded
     Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3

     response_type=code&
     state=af0ifjsldkj&
     client_id=s6BhdRkqt3&
     redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb&
     code_challenge=K2-ltc83acc4h0c9w6ESC_rEMTJ3bww-uCHaoeK1t8U&
     code_challenge_method=S256&
     scope=ais

   The authorization server MUST process the request as follows:

   1.  Authenticate the client in the same way as at the token endpoint.

   2.  Reject the request if the "request_uri" authorization request
       parameter is provided.

   3.  Validate the pushed request as it would an authorization request
       sent to the authorization endpoint.  For example, the
       authorization server checks whether the redirect URI matches one
       of the redirect URIs configured for the client and also checks
       whether the client is authorized for the scope for which it is
       requesting access.  This validation allows the authorization
       server to refuse unauthorized or fraudulent requests early.  The
       authorization server MAY omit validation steps that it is unable
       to perform when processing the pushed request, however such
       checks MUST then be performed at the authorization endpoint.

   The authorization server MAY allow confidential clients to establish
   per-authorization request redirect URIs with every pushed
   authorization request.  This is possible since, in contrast to

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   [RFC6749], this specification gives the authorization server the
   ability to authenticate and authorize clients before the actual
   authorization request is performed.

   This feature gives clients more flexibility in building redirect URIs
   and, if the client IDs and credentials are managed by some authority
   (CA or other type), the explicit client registration with the
   particular authorization server (manually or via dynamic client
   registration [RFC7591]) could practically be skipped.  This makes
   this mechanism especially useful for clients interacting with a
   federation of authorization servers (or OpenID Connect Providers),
   for example in Open Banking, where the certificate is provided as
   part of a federated PKI.

2.2.  Successful Response

   If the verification is successful, the server MUST generate a request
   URI and return a JSON response with the following members at the top
   level with "201 Created" HTTP response code.

   *  "request_uri" : The request URI corresponding to the authorization
      request posted.  This URI is used as reference to the respective
      request data in the subsequent authorization request only.  The
      way the authorization process obtains the authorization request
      data is at the discretion of the authorization server and out of
      scope of this specification.  There is no need to make the
      authorization request data available to other parties via this
      URI.

   *  "expires_in" : A JSON number that represents the lifetime of the
      request URI in seconds.  The request URI lifetime is at the
      discretion of the authorization server and will typically be
      relatively short.

   The format of the "request_uri" value is at the discretion of the
   authorization server but it MUST contain some part generated using a
   cryptographically strong pseudorandom algorithm such that it is
   computationally infeasible to predict or guess a valid value.  The
   authorization server MAY construct the "request_uri" value using the
   form "urn:ietf:params:oauth:request_uri:<reference-value>" with
   "<reference-value>" as the random part of the URI that references the
   respective authorization request data.  The string representation of
   a UUID as a URN per [RFC4122] is also an option for authorization
   servers to construct "request_uri" values.

   The "request_uri" MUST be bound to the client that posted the
   authorization request.

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   Since parts of the request content, e.g. the "code_challenge"
   parameter value, is unique to a certain authorization request, a
   "request_uri" SHOULD be limited to one-time use.

   The following is an example of such a response:

    HTTP/1.1 201 Created
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
      "request_uri":
        "urn:ietf:params:oauth:request_uri:bwc4JK-ESC0w8acc191e-Y1LTC2",
      "expires_in": 60
    }

2.3.  Error Response

   For an error the authorization server sets an appropriate HTTP status
   code and MAY include additional error parameters in the entity-body
   of the HTTP response using the format specified for the token
   endpoint in Section 5.2 of [RFC6749].

   If the authorization server sets an error code, it SHOULD be one of
   the defined codes for the token endpoint in Section 5.2 or for the
   authorization endpoint in Sections 4.1.2.1 and 4.2.2.1 of [RFC6749],
   or by an OAuth extension if one is involved in the initial processing
   of authorization request that was pushed.  Since initial processing
   of the pushed authorization request doesn't involve resource owner
   interaction, error codes related to user interaction, such as
   "consent_required" defined by [OIDC], are not returned.

   If the client is required to use signed request objects, either by
   authorization server or client policy (see [I-D.ietf-oauth-jwsreq],
   section 10.5), the authorization server MUST only accept requests
   complying with the definition given in Section 3 and MUST refuse any
   other request with HTTP status code 400 and error code
   "invalid_request".

   In addition to the error codes above, the pushed authorization
   request endpoint can also make use of the following HTTP status
   codes:

   *  405: If the request did not use POST, the authorization server
      responds with an HTTP 405 (Method Not Allowed) status code.

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   *  413: If the request size was beyond the upper bound that the
      authorization server allows, the authorization server responds
      with an HTTP 413 (Payload Too Large) status code.

   *  429: If the request from the client for a time period goes beyond
      the number the authorization server allows, the authorization
      server responds with an HTTP 429 (Too Many Requests) status code.

   The following is an example of an error response from the pushed
   authorization request endpoint:

     HTTP/1.1 400 Bad Request
     Content-Type: application/json
     Cache-Control: no-cache, no-store

     {
       "error": "invalid_request",
       "error_description":
         "The redirect_uri is not valid for the given client"
     }

3.  "request" Parameter

   Clients MAY use the "request" parameter as defined in JAR
   [I-D.ietf-oauth-jwsreq] to push a request object JWT to the
   authorization server.  The rules for processing, signing, and
   encryption of the request object as defined in JAR
   [I-D.ietf-oauth-jwsreq] apply.  When the "application/x-www-form-
   urlencoded" HTTP entity-body "request" parameter is used, the request
   object MUST contain all the authorization request parameters as
   claims of the JWT.  Additional request parameters as required by the
   given client authentication method are to be included as
   'application/x-www-form-urlencoded' parameters in the HTTP request
   entity-body (e.g.  Mutual TLS client authentication
   [I-D.ietf-oauth-mtls] uses the "client_id" HTTP request parameter
   while JWT assertion based client authentication [RFC7523] uses
   "client_assertion" and "client_assertion_type").

   The following is an example of a pushed authorization request using a
   signed request object.  The client is authenticated by its client
   secret using the HTTP Basic Authentication scheme specified in
   Section 2.3.1 of [RFC6749]:

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     POST /as/par HTTP/1.1
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded
     Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3

     request=eyJraWQiOiJrMmJkYyIsImFsZyI6IlJTMjU2In0.eyJpc3MiOiJzNkJoZ
     FJrcXQzIiwiYXVkIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJyZXNwb2
     5zZV90eXBlIjoiY29kZSIsImNsaWVudF9pZCI6InM2QmhkUmtxdDMiLCJyZWRpcmV
     jdF91cmkiOiJodHRwczovL2NsaWVudC5leGFtcGxlLm9yZy9jYiIsInNjb3BlIjoi
     YWlzIiwic3RhdGUiOiJhZjBpZmpzbGRraiIsImNvZGVfY2hhbGxlbmdlIjoiSzItb
     HRjODNhY2M0aDBjOXc2RVNDX3JFTVRKM2J3dy11Q0hhb2VLMXQ4VSIsImNvZGVfY2
     hhbGxlbmdlX21ldGhvZCI6IlMyNTYifQ.O49ffUxRPdNkN3TRYDvbEYVr1CeAL64u
     W4FenV3n9WlaFIRHeFblzv-wlEtMm8-tusGxeE9z3ek6FxkhvvLEqEpjthXnyXqqy
     Jfq3k9GSf5ay74ml_0D6lHE1hy-kVWg7SgoPQ-GB1xQ9NRhF3EKS7UZIrUHbFUCF0
     MsRLbmtIvaLYbQH_Ef3UkDLOGiU7exhVFTPeyQUTM9FF-u3K-zX-FO05_brYxNGLh
     VkO1G8MjqQnn2HpAzlBd5179WTzTYhKmhTiwzH-qlBBI_9GLJmE3KOipko9TfSpa2
     6H4JOlMyfZFl0PCJwkByS0xZFJ2sTo3Gkk488RQohhgt1I0onw

   The authorization server needs to take the following steps beyond the
   processing rules defined in Section 2.1:

   1.  If applicable, decrypt the request object as specified in JAR
       [I-D.ietf-oauth-jwsreq], section 6.1.

   2.  Validates the request object signature as specified in JAR
       [I-D.ietf-oauth-jwsreq], section 6.2.

   3.  If the client is a confidential client, the authorization server
       MUST check whether the authenticated "client_id" matches the
       "client_id" claim in the request object.  If they do not match,
       the authorization server MUST refuse to process the request.  It
       is at the authorization server's discretion to require the "iss"
       claim to match the "client_id" as well.

   The following RSA key pair, represented in JWK [RFC7517] format, can
   be used to validate or recreate the request object signature in the
   above example (line wraps within values for display purposes only):

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    {
      "kty": "RSA",
      "kid":"k2bdc",
      "n": "y9Lqv4fCp6Ei-u2-ZCKq83YvbFEk6JMs_pSj76eMkddWRuWX2aBKGHAtKlE
            5P7_vn__PCKZWePt3vGkB6ePgzAFu08NmKemwE5bQI0e6kIChtt_6KzT5Oa
            aXDFI6qCLJmk51Cc4VYFaxgqevMncYrzaW_50mZ1yGSFIQzLYP8bijAHGVj
            dEFgZaZEN9lsn_GdWLaJpHrB3ROlS50E45wxrlg9xMncVb8qDPuXZarvghL
            L0HzOuYRadBJVoWZowDNTpKpk2RklZ7QaBO7XDv3uR7s_sf2g-bAjSYxYUG
            sqkNA9b3xVW53am_UZZ3tZbFTIh557JICWKHlWj5uzeJXaw",
      "e": "AQAB",
      "d": "LNwG_pCKrwowALpCpRdcOKlSVqylSurZhE6CpkRiE9cpDgGKIkO9CxPlXOL
            zjqxXuQc8MdMqRQZTnAwgd7HH0B6gncrruV3NewI-XQV0ckldTjqNfOTz1V
            Rs-jE-57KAXI3YBIhu-_0YpIDzdk_wBuAk661Svn0GsPQe7m9DoxdzenQu9
            O_soewUhlPzRrTH0EeIqYI715rwI3TYaSzoWBmEPD2fICyj18FF0MPy_SQz
            k3noVUUIzfzLnnJiWy_p63QBCMqjRoSHHdMnI4z9iVpIwJWQ3jO5n_2lC2-
            cSgwjmKsFzDBbQNJc7qMG1N6EssJUwgGJxz1eAUFf0w4YAQ",
      "qi": "J-mG0swR4FTy3atrcQ7dd0hhYn1E9QndN-
            -sDG4EQO0RnFj6wIefCvwIc4
            7hCtVeFnCTPYJNc_JyV-mU-9vlzS5GSNuyR5qdpsMZXUMpEvQcwKt23ffPZ
            YGaqfKyEesmf_Wi8fFcE68H9REQjnniKrXm7w2-IuG_IrVJA9Ox-uU",
      "q": "4hlMYAGa0dvogdK1jnxQ7J_Lqpqi99e-AeoFvoYpMPhthChTzwFZO9lQmUo
            BpMqVQTws_s7vWGmt7ZAB3ywkurf0pV7BD0fweJiUzrWk4KJjxtmP_auuxr
            jvm3s2FUGn6f0wRY9Z8Hj9A7C72DnYCjuZiJQMYCWDsZ8-d-L1a-s",
      "p": "5sd9Er3I2FFT9R-gy84_oakEyCmgw036B_nfYEEOCwpSvi2z7UcIVK3bSEL
            5WCW6BNgB3HDWhq8aYPirwQnqm0K9mX1E-4xM10WWZ-rP3XjYpQeS0Snru5
            LFVWsAzi-FX7BOqBibSAXLdEGXcXa44l08iec_bPD3xduq5V_1YoE",
      "dq": "Nz2PF3XM6bEc4XsluKZO70ErdYdKgdtIJReUR7Rno_tOZpejwlPGBYVW19
            zpAeYtCT82jxroB2XqhLxGeMxEPQpsz2qTKLSe4BgHY2ml2uxSDGdjcsrbb
            NoKUKaN1CuyZszhWl1n0AT_bENl4bJgQj_Fh0UEsQj5YBBUJt5gr_k",
      "dp": "Zc877jirkkLOtyTs2vxyNe9KnMNAmOidlUc2tE_-0gAL4Lpo1hSwKCtKwe
            ZJ-gkqt1hT-dwNx_0Xtg_-NXsadMRMwJnzBMYwYAfjApUkfqABc0yUCJJl3
            KozRCugf1WXkU9GZAH2_x8PUopdNUEa70ISowPRh04HANKX4fkjWAE"
     }

4.  Authorization Request

   The client uses the "request_uri" value returned by the authorization
   server to build an authorization request as defined in
   [I-D.ietf-oauth-jwsreq].  This is shown in the following example
   where the client directs the user-agent to make the following HTTP
   request:

     GET /authorize?client_id=s6BhdRkqt3&request_uri=urn%3Aietf%3Aparams
     %3Aoauth%3Arequest_uri%3Abwc4JK-ESC0w8acc191e-Y1LTC2 HTTP/1.1
     Host: as.example.com

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   The authorization server MUST validate authorization requests arising
   from a pushed request as it would any other authorization request.
   The authorization server MAY omit validation steps that it performed
   when the request was pushed, provided that it can validate that the
   request was a pushed request, and that the request or the
   authorization server's policy has not been modified in a way that
   would affect the outcome of the omitted steps.

   Authorization server policy MAY dictate, either globally or on a per-
   client basis, that pushed authorization requests are the only means
   for a client to pass authorization request data.  In this case, the
   authorization server will refuse, using the "invalid_request" error
   code, to process any request to the authorization endpoint that does
   not have a "request_uri" parameter with a value obtained from the
   pushed authorization request endpoint.

   Note: authorization server and clients MAY use metadata as defined in
   Section 5 and Section 6 to signal the desired behavior.

5.  Authorization Server Metadata

   The following authorization server metadata [RFC8414] parameters are
   introduced to signal the server's capability and policy with respect
   to pushed authorization requests.

   "pushed_authorization_request_endpoint"  The URL of the pushed
      authorization request endpoint at which the client can post an
      authorization request and get a request URI in exchange.

   "require_pushed_authorization_requests"  Boolean parameter indicating
      whether the authorization server accepts authorization request
      data only via the pushed authorization request method.  If
      omitted, the default value is "false".

6.  Client Metadata

   The Dynamic Client Registration Protocol [RFC7591] defines an API for
   dynamically registering OAuth 2.0 client metadata with authorization
   servers.  The metadata defined by [RFC7591], and registered
   extensions to it, also imply a general data model for clients that is
   useful for authorization server implementations even when the Dynamic
   Client Registration Protocol isn't in play.  Such implementations
   will typically have some sort of user interface available for
   managing client configuration.  The following client metadata
   parameter is introduced by this document to indicate whether pushed
   authorization requests are reqired for the given client.

   "require_pushed_authorization_requests"  Boolean parameter indicating

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      whether the only means of initiating an authorization request the
      client is allowed to use is a pushed authorization request.

7.  Security Considerations

7.1.  Request URI Guessing

   An attacker could attempt to guess and replay a valid request URI
   value and try to impersonate the respective client.  The
   authorization server MUST consider the considerations given in JAR
   [I-D.ietf-oauth-jwsreq], section 10.2, clause (d) on request URI
   entropy.

7.2.  Open Redirection

   An attacker could try register a redirect URI pointing to a site
   under his control in order to obtain authorization codes or lauch
   other attacks towards the user.  The authorization server MUST only
   accept new redirect URIs in the PAR request from confidential clients
   after successful authentication and authorization.

7.3.  Request Object Replay

   An attacker could replay a request URI captured from a legitimate
   authorization request.  In order to cope with such attacks, the
   authorization server SHOULD make the request URIs one-time use.

7.4.  Client Policy Change

   The client policy might change between the lodging of the request
   object and the authorization request using a particular request
   object.  It is therefore recommended that the authorization server
   check the request parameter against the client policy when processing
   the authorization request.

8.  Acknowledgements

   This specification is based on the work towards Pushed Request Object
   (https://bitbucket.org/openid/fapi/src/master/
   Financial_API_Pushed_Request_Object.md) conducted at the Financial-
   grade API working group at the OpenID Foundation.  We would like to
   thank the members of the WG for their valuable contributions.

   We would like to thank Vladimir Dzhuvinov, Aaron Parecki, Justin
   Richer, Sascha Preibisch, Daniel Fett, Michael B.  Jones, Annabelle
   Backman, Joseph Heenan, Sean Glencross, Maggie Hung, Neil Madden, and
   Takahiko Kawasaki for their valuable feedback on this draft.

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9.  IANA Considerations

9.1.  OAuth Authorization Server Metadata

   This specification requests registration of the following values in
   the IANA "OAuth Authorization Server Metadata" registry of
   [IANA.OAuth.Parameters] established by [RFC8414].

   Metadata Name:  "pushed_authorization_request_endpoint"
   Metadata Description:  URL of the authorization server's pushed
      authorization request endpoint
   Change Controller:  IESG
   Specification Document(s):  Section 5 of [[ this document ]]
 
   Metadata Name:  "require_pushed_authorization_requests"
   Metadata Description:  Indicates whether the authorization server
      accepts authorization request only via the pushed authorization
      request method.
   Change Controller:  IESG
   Specification Document(s):  Section 5 of [[ this document ]]

9.2.  OAuth Dynamic Client Registration Metadata

   This specification requests registration of the following value in
   the IANA "OAuth Dynamic Client Registration Metadata" registry of
   [IANA.OAuth.Parameters] established by [RFC7591].

   Metadata Name:  "require_pushed_authorization_requests"
   Metadata Description:  Indicates whether the client is required to
      use the pushed authorization request method to initiate
      authorization requests.
   Change Controller:  IESG
   Specification Document(s):  Section 6 of [[ this document ]]

9.3.  OAuth URI Registration

   This specification requests registration of the following value in
   the "OAuth URI" registry of [IANA.OAuth.Parameters] established by
   [RFC6755].

   URN:  "urn:ietf:params:oauth:request_uri:"
   Common Name:  A URN Sub-Namespace for OAuth Request URIs.
   Change Controller:  IESG
   Specification Document(s):  Section 2.2 of [[ this document ]]

10.  Normative References

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   [I-D.ietf-oauth-jwsreq]
              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)", Work
              in Progress, Internet-Draft, draft-ietf-oauth-jwsreq-26,
              27 July 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-26>.

   [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>.

   [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>.

   [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>.

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", 8 November 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [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>.

11.  Informative References

   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/info/rfc7523>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [I-D.ietf-oauth-mtls]
              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", Work in Progress,
              Internet-Draft, draft-ietf-oauth-mtls-17, 23 August 2019,
              <https://tools.ietf.org/html/draft-ietf-oauth-mtls-17>.

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   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

   [IANA.OAuth.Parameters]
              IANA, "OAuth Parameters",
              <http://www.iana.org/assignments/oauth-parameters>.

   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
              <https://www.rfc-editor.org/info/rfc6755>.

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,
              <https://www.rfc-editor.org/info/rfc7636>.

   [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>.

   [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>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -03

   *  Editorial updates

   *  Mention that https is required for the PAR endpoint

   *  Add some discussion of browser form posting an authz request vs.
      the benefits of PAR for any application

   *  Added text about motivations behind PAR - integrity,
      confidentiality and early client auth

   *  Better explain one-time use recommendation of the request_uri

   *  Drop the section on special error responses for request objects

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   *  Clarify authorization request examples to say that the client
      directs the user-agent to make the HTTP GET request (vs. making
      the request itself)

   -02

   *  Update Resource Indicators reference to the somewhat recently
      published RFC 8707

   *  Added metadata in support of pushed authorization requests only
      feature

   *  Update to comply with draft-ietf-oauth-jwsreq-21, which requires
      "client_id" in the authorization request in addition to the
      "request_uri"

   *  Clarified timing of request validation

   *  Add some guidance/options on the request URI structure

   *  Add the key used in the request object example so that a reader
      could validate or recreate the request object signature

   *  Update to draft-ietf-oauth-jwsreq-25 and added note regarding
      "require_signed_request_object"

   -01

   *  Use the newish RFC v3 XML and HTML format
   *  Added IANA registration request for
      "pushed_authorization_request_endpoint"
   *  Changed abbrev to "OAuth PAR"

   -00 (WG draft)

   *  Reference RFC6749 sec 2.3.1 for client secret basic rather than
      RFC7617
   *  further clarify that a request object JWT contains all the
      authorization request parameters while client authentication
      params, if applicable, are outside that JWT as regular form
      encoded params in HTTP body

   -01

   *  List "client_id" as one of the basic parameters
   *  Explicitly forbid "request_uri" in the processing rules
   *  Clarification regarding client authentication and that public
      clients are allowed

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   *  Added option to let clients register per-authorization request
      redirect URIs
   *  General clean up and wording improvements

   -00

   *  first draft

Authors' Addresses

   Torsten Lodderstedt
   yes.com

   Email: torsten@lodderstedt.net

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

   Nat Sakimura
   NAT.Consulting

   Email: nat@sakimura.org

   Dave Tonge
   Moneyhub Financial Technology

   Email: dave@tonge.org

   Filip Skokan
   Auth0

   Email: panva.ip@gmail.com

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