Internet-Draft OAuth PAR May 2021
Lodderstedt, et al. Expires 15 November 2021 [Page]
Workgroup:
Web Authorization Protocol
Internet-Draft:
draft-ietf-oauth-par-08
Published:
Intended Status:
Standards Track
Expires:
Authors:
T. Lodderstedt
yes.com
B. Campbell
Ping Identity
N. Sakimura
NAT.Consulting
D. Tonge
Moneyhub Financial Technology
F. Skokan
Auth0

OAuth 2.0 Pushed Authorization Requests

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.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 15 November 2021.

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Table of Contents

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.

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:

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 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 with a POST request to the pushed authorization request endpoint as illustrated in the following example (extra line breaks for display purposes only). The client can authenticate (e.g., using the Authorization header as shown) because the the request is made directly to the authorization server.

  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 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.

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). 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 (Section 2.3 of [RFC6749]).
  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 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 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 (see Section 10.10 of [RFC6749] for specifics). 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 request_uri value MUST be bound to the client that posted the authorization request.

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

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.

  • 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.

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):

  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):

 {
   "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

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.

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 required 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.

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 [RFC7636], a unique state parameter [RFC6749], or the OIDC "nonce" parameter [OIDC] 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 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, Karsten Meyer zu Selhausen, Roman Danyliw, 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.
Change Controller:
IESG
Specification Document(s):
Section 2.2 of [[ this document ]]

11. Normative References

[I-D.ietf-oauth-jwsreq]
Sakimura, N., Bradley, J., and M. B. Jones, "The OAuth 2.0 Authorization Framework: JWT Secured Authorization Request (JAR)", Work in Progress, Internet-Draft, draft-ietf-oauth-jwsreq-34, , <https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-34>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8414]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, , <https://www.rfc-editor.org/info/rfc8414>.

12. Informative References

[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-18, , <https://tools.ietf.org/html/draft-ietf-oauth-security-topics-18>.
[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-v2-1-02, , <https://tools.ietf.org/html/draft-ietf-oauth-v2-1-02>.
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters", <http://www.iana.org/assignments/oauth-parameters>.
[OIDC]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and C. Mortimore, "OpenID Connect Core 1.0 incorporating errata set 1", , <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, , <https://www.rfc-editor.org/info/rfc6749>.
[RFC6755]
Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace for OAuth", RFC 6755, DOI 10.17487/RFC6755, , <https://www.rfc-editor.org/info/rfc6755>.
[RFC7517]
Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, , <https://www.rfc-editor.org/info/rfc7517>.
[RFC7519]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, , <https://www.rfc-editor.org/info/rfc7519>.
[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, , <https://www.rfc-editor.org/info/rfc7523>.
[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, , <https://www.rfc-editor.org/info/rfc7591>.
[RFC7636]
Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key for Code Exchange by OAuth Public Clients", RFC 7636, DOI 10.17487/RFC7636, , <https://www.rfc-editor.org/info/rfc7636>.
[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, , <https://www.rfc-editor.org/info/rfc8705>.
[RFC8707]
Campbell, B., Bradley, J., and H. Tschofenig, "Resource Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707, , <https://www.rfc-editor.org/info/rfc8707>.

Appendix A. Document History

[[ To be removed from the final specification ]]

-08

-07

-06

-05

-04

-03

-02

-01

-00 (WG draft)

-01

-00

Authors' Addresses

Torsten Lodderstedt
yes.com
Brian Campbell
Ping Identity
Nat Sakimura
NAT.Consulting
Dave Tonge
Moneyhub Financial Technology
Filip Skokan
Auth0