OAuth 2.0 Pushed Authorization Requests
draft-ietf-oauth-par-07

Web Authorization Protocol                                T. Lodderstedt
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: 14 October 2021                                   Ping Identity
                                                             N. Sakimura
                                                          NAT.Consulting
                                                                D. Tonge
                                           Moneyhub Financial Technology
                                                               F. Skokan
                                                                   Auth0
                                                           12 April 2021

                OAuth 2.0 Pushed Authorization Requests
                        draft-ietf-oauth-par-07

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 call to the authorization
   endpoint.

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 14 October 2021.

Copyright Notice

   Copyright (c) 2021 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.  Introductory Example  . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions and Terminology . . . . . . . . . . . . . . .   5
   2.  Pushed Authorization Request Endpoint . . . . . . . . . . . .   5
     2.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Successful Response . . . . . . . . . . . . . . . . . . .   8
     2.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Management of Client Redirect URIs  . . . . . . . . . . .  10
   3.  The "request" Request Parameter . . . . . . . . . . . . . . .  11
   4.  Authorization Request . . . . . . . . . . . . . . . . . . . .  13
   5.  Authorization Server Metadata . . . . . . . . . . . . . . . .  14
   6.  Client Metadata . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Request URI Guessing  . . . . . . . . . . . . . . . . . .  15
     7.2.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  15
     7.3.  Request Object Replay . . . . . . . . . . . . . . . . . .  16
     7.4.  Client Policy Change  . . . . . . . . . . . . . . . . . .  16
     7.5.  Request URI Swapping  . . . . . . . . . . . . . . . . . .  16
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  OAuth Authorization Server Metadata  . . . . . . . . . .  17
     10.2.  OAuth Dynamic Client Registration Metadata . . . . . . .  17
     10.3.  OAuth URI Registration . . . . . . . . . . . . . . . . .  17
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  18
   12. Informative References  . . . . . . . . . . . . . . . . . . .  18
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Pushed authorization requests (PAR), defined by this document, enable
   OAuth [RFC6749] clients to push the payload of an authorization
   request directly to the authorization server in exchange for a
   request URI value, which is used as reference to the authorization
   request payload data in a subsequent call to the authorization
   endpoint via the user-agent.

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   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:

   *  There is no cryptographic integrity and authenticity protection.
      An attacker could, for example, modify the scope of access
      requested or swap the context of a payment transaction by changing
      scope values.  Although protocol facilities exist to enable
      clients or users to detect some such changes, preventing
      modifications early in the process is a more robust solution.

   *  There is no mechanism to ensure confidentiality of the request
      parameters.  Although HTTPS is required for the authorization
      endpoint, the request data passes through the user-agent in the
      clear and query string data can inadvertently leak to web server
      logs and to other sites via referer.  The impact of which can be
      significant, if personal identifiable information or other
      regulated data is sent in the authorization request (which might
      well be the case in identity, open banking, and similar
      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 request object,
   which is a signed and optionally encrypted JSON Web Token (JWT)
   [RFC7519].  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 an authorization request directly to the
   authorization server in exchange for a "request_uri" value usable at
   the authorization server in a subsequent authorization request.

   PAR fosters OAuth security by providing clients a simple means for a
   confidential and integrity protected authorization request.  Clients
   requiring an even higher security level, especially cryptographically
   confirmed non-repudiation, are able to use JWT-based request objects
   as defined by [I-D.ietf-oauth-jwsreq] in conduction with a pushed
   authorization request.

   PAR allows the authorization server to authenticate the client before
   any user interaction happens.  The increased confidence in the
   identity of the client during the authorization process allows the

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   authorization server to refuse illegitimate requests much earlier in
   the process, which can prevent attempts to spoof clients or otherwise
   tamper with or misuse an authorization request.

   Note that HTTP "POST" requests to the authorization endpoint via the
   user-agent, 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.  However, it's only
   optional per [RFC6749] and, even when supported, it is a viable
   option for traditional web applications but is prohibitively
   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.  Introductory Example

   A client typically initiates an authorization request by directing
   the user-agent to make an HTTP request like the following to the
   authorization server's authorization endpoint (extra line breaks and
   indentation for display purposes only):

     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 with a "POST" request to the pushed
   authorization request endpoint (extra line breaks for display
   purposes only):

     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:

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     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 by directing the user-agent to make an HTTP
   request to the authorization server's authorization endpoint like the
   following (extra line breaks and indentation for display purposes
   only):

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

1.2.  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", "authorization
   server", "authorization endpoint", "authorization request", "token
   endpoint", 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 HTTP "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 authorization request 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 MAY also support
   sending the set of authorization request parameters as a request
   object according to [I-D.ietf-oauth-jwsreq] and Section 3.

   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
   [RFC7591] 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.  Similarly, the
   "token_endpoint_auth_methods_supported" authorization server metadata
   [RFC8414] parameter lists client authentication methods supported by
   the authorization server when accepting direct requests from clients,
   including requests to the pushed authorization request endpoint.

   Due to historical reasons there is potential ambiguity regarding the
   appropriate audience value to use when employing JWT client assertion
   based authentication (defined in Section 2.2 of [RFC7523] with
   "private_key_jwt" or "client_secret_jwt" authentication method names
   per Section 9 of [OIDC]).  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 sends the parameters that comprise an authorization request
   directly to the pushed authorization request endpoint.  A typical
   parameter set might include: "client_id", "response_type",
   "redirect_uri", "scope", "state", "code_challenge", and
   "code_challenge_method" as shown in the example below.  However, the
   pushed authorization request can be composed of any of the parameters
   applicable for use at authorization endpoint including those defined
   in [RFC6749] as well as all applicable extensions.  The "request_uri"
   authorization request parameter is one exception, which MUST NOT be
   provided.

   The request also includes, as appropriate for the given client, any
   additional parameters necessary for client authentication (e.g.,
   "client_secret", or "client_assertion" and "client_assertion_type").

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   Such parameters are defined and registered for use at the token
   endpoint but are applicable only for client authentication.  When
   present in a pushed authorization request, they are relied upon only
   for client authentication and are not germane to the authorization
   request itself.  Any token endpoint parameters that are not related
   to client authentication have no defined meaning for a pushed
   authorization request.  The "client_id" parameter is defined with the
   same semantics for both authorization requests and requests to the
   token endpoint; as a required authorization request parameter, it is
   similarly required in a pushed authorization request.

   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 its authentication 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 (extra line breaks in
   the message-body for display purposes only):

     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=account-information

   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.

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   The authorization server MAY allow clients with authentication
   credentials to establish per-authorization request redirect URIs with
   every pushed authorization request.  Described in more detail in
   Section 2.4, this is possible since, in contrast to [RFC6749], this
   specification gives the authorization server the the ability to
   authenticate clients and validate client requests before the actual
   authorization request is performed.

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 as a positive integer.  The request URI
      lifetime is at the discretion of the authorization server but will
      typically be relatively short (e.g., between 5 and 600 seconds).

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

   The following is an example of such a response:

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    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

   The authorization server returns an error response with the same
   format as is specified for error responses from the token endpoint in
   Section 5.2 of [RFC6749] using the appropriate error code from
   therein or from Section 4.1.2.1 of [RFC6749].  In those cases where
   Section 4.1.2.1 of [RFC6749] prohibits automatic redirection with an
   error back to the requesting client and hence doesn't define an error
   code, for example when the request fails due to a missing, invalid,
   or mismatching redirection URI, the "invalid_request" error code can
   be used as the default error code.  Error codes defined by OAuth
   extension can also be used when such an extension is involved in the
   initial processing of authorization request that was pushed.  Since
   initial processing of the pushed authorization request does not
   involve resource owner interaction, error codes related to user
   interaction, such as "consent_required" defined by [OIDC], are never
   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 above, the pushed authorization request endpoint
   can also make use of the following HTTP status codes:

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

   *  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.

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   *  429: If the number of requests from a client during a particular
      time period exceeds 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"
     }

2.4.  Management of Client Redirect URIs

   While OAuth 2.0 [RFC6749] allows clients to use unregistered
   "redirect_uri" values in certain circumstances, or for the
   authorization server to apply its own matching semantics to the
   "redirect_uri" value presented by the client at the authorization
   endpoint, the OAuth Security BCP [I-D.ietf-oauth-security-topics] as
   well as OAuth 2.1 [I-D.ietf-oauth-v2-1] require an authorization
   server exactly match the "redirect_uri" parameter against the set of
   redirect URIs previously established for a particular client.  This
   is a means for early detection of client impersonation attempts and
   prevents token leakage and open redirection.  As a downside, this can
   make client management more cumbersome since the redirect URI is
   typically the most volatile part of a client policy.

   The exact matching requirement MAY be relaxed when using pushed
   authorization requests for clients that have established
   authentication credentials with the authorization server.  This is
   possible since, in contrast to a traditional authorization request,
   the authorization server authenticates the client before the
   authorization process starts and thus ensures it is interacting with
   the legitimate client.  The authorization server MAY allow such
   clients to specify "redirect_uri" values that were not previously
   registered with the authorization server.  This will give the client
   more flexibility (e.g. to mint distinct redirect URI values per
   authorization server at runtime) and can simplify client management.
   It is at the discretion of the authorization server to apply
   restrictions on supplied "redirect_uri" values, e.g. the
   authorization server MAY require a certain URI prefix or allow only a
   query parameter to vary at runtime.

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   Note: The ability to set up transaction specific redirect URIs is
   also useful in situations where client ids and corresponding
   credentials and policies are managed by a trusted 3rd party, e.g. via
   client certificates containing client permissions.  Such an
   externally managed client could interact with an authorization server
   trusting the respective 3rd party without the need for an additional
   registration step.

3.  The "request" 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 [RFC8705] 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] (extra line breaks for display purposes
   only):

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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
     &client_id=s6BhdRkqt3

   The authorization server MUST 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.  Validate the request object signature as specified in JAR
       [I-D.ietf-oauth-jwsreq], section 6.2.

   3.  If the client has authentication credentials established with the
       authorization server, reject the request if the authenticated
       "client_id" does not match the "client_id" claim in the request
       object.  Additionally requiring the "iss" claim to match the
       "client_id" is at the discretion of authorization server.

   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 (extra line breaks and indentation 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 (extra line breaks and indentation for display purposes
   only):

     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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   Since parts of the authorization request content, e.g. the
   "code_challenge" parameter value, are unique to a particular
   authorization request, the client MUST only use a "request_uri" value
   once.  Authorization servers SHOULD treat "request_uri" values as
   one-time use but MAY allow for duplicate requests due to a user
   reloading/refreshing their user-agent.

   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 a client can post an
      authorization request in exchange for a "request_uri" value usable
      at the authorization server.

   "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".

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   Note that the presence of "pushed_authorization_request_endpoint" is
   sufficient for a client to determine that it may use the pushed
   authorization request flow.  A "request_uri" value obtained from the
   PAR endpoint is usable at the authorization endpoint regardless of
   other authorization server metadata such as
   "request_uri_parameter_supported" or
   "require_request_uri_registration".

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
      whether the only means of initiating an authorization request the
      client is allowed to use is a pushed authorization request.  If
      omitted, the default value is "false".

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 launch
   other attacks towards the user.  The authorization server MUST only
   accept new redirect URIs in the pushed authorization request from
   authenticated clients.

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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.

7.5.  Request URI Swapping

   An attacker could capture the request URI from one request and then
   substitute it into a different authorization request.  For example,
   in the context of OpenID Connect, an attacker could replace a request
   URI asking for a high level of authentication assurance with one that
   requires a lower level of assurance.  Clients SHOULD make use of
   PKCE, a unique "state" parameter, or the OIDC "nonce" parameter in
   the pushed request object to prevent this attack.

8.  Privacy Considerations

   OAuth 2.0 is a complex and flexible framework with broad ranging
   privacy implications due to the very nature of it having one entity
   intermediate user authorization to data access between two other
   entities.  The privacy considerations of all of OAuth are beyond the
   scope of this document, which only defines an alternative way of
   initiating one message sequence in the larger framework.  Using
   pushed authorization requests, however, may improve privacy by
   reducing the potential for inadvertent information disclosure due to
   passing authorization request data directly between client and
   authorization server over a secure connection in the message-body of
   an HTTP request rather than in the query component of a URL that
   passes through the user-agent in the clear.

9.  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.

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   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,
   Karsten Meyer zu Selhausen, and Takahiko Kawasaki for their valuable
   feedback on this draft.

10.  IANA Considerations

10.1.  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 ]]

10.2.  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 ]]

10.3.  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.

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   Change Controller:  IESG
   Specification Document(s):  Section 2.2 of [[ this document ]]

11.  Normative References

   [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>.

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
              Authorization Framework: JWT Secured Authorization Request
              (JAR)", Work in Progress, Internet-Draft, draft-ietf-
              oauth-jwsreq-30, 10 September 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-30>.

   [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>.

   [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>.

   [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>.

12.  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>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

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   [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>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [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>.

   [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-v2-1]
              Hardt, D., Parecki, A., and T. Lodderstedt, "The OAuth 2.1
              Authorization Framework", Work in Progress, Internet-
              Draft, draft-ietf-oauth-v2-1-00, 30 July 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-v2-1-00>.

   [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-16, 5 October 2020, <https://tools.ietf.org/html/
              draft-ietf-oauth-security-topics-16>.

   [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>.

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Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -07

   *  updated references

   -06

   *  Add a note clarifying that the presence of
      "pushed_authorization_request_endpoint" is sufficient for a client
      to know that it can use the PAR flow

   -05

   *  Mention use of "invalid_request" error code for cases, like a bad
      "redirect_uri", that don't have a more specific one

   -04

   *  Edits to address WGLC comments

   *  Replace I-D.ietf-oauth-mtls reference with now published RFC8705

   *  Moved text about redirect URI management from introduction into
      separate section

   -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

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

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

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   -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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