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Versions: 00

OAuth Working Group                                           J. Ideskog
Internet-Draft                                                T. Spencer
Intended status: Standards Track                               Curity AB
Expires: September 19, 2018                               March 18, 2018


                        OAuth 2.0 Assisted Token
                    draft-ideskog-assisted-token-00

Abstract

   The OAuth 2.0 authorization flow for Single Page Applications (SPAs),
   often referred to as the assisted token flow, enables OAuth clients
   to request user authorization written in scripting languages, like
   JavaScript, with a simplified integration compared to the implicit
   grant type flow.  Communication does not rely on redirection of the
   user agent, but instead leverages HTML's iframe element, child
   windows, and the postMessage interface.  This communication is done
   using an additional endpoint, the assisted token endpoint, which this
   document defines.

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 September 19, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Assisted Token Endpoint . . . . . . . . . . . . . . . . . . .   5
     3.1.  Access Token Scope  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Cross-Origin Support  . . . . . . . . . . . . . . . . . .   7
   4.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Assisted Token Request  . . . . . . . . . . . . . . . . .   7
     4.2.  Assisted Token Response . . . . . . . . . . . . . . . . .   8
     4.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Client Metadata . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Authorization Server Metadata . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  OAuth URI Registration  . . . . . . . . . . . . . . . . .  10
       7.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11
     7.2.  OAuth 2.0 Authorization Server Metadata . . . . . . . . .  11
       7.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     8.1.  Framing . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.2.  Handle Tokens . . . . . . . . . . . . . . . . . . . . . .  12
     8.3.  Warning Against Untrusted Scripts . . . . . . . . . . . .  12
     8.4.  Origin of Event and Authorization Server  . . . . . . . .  13
     8.5.  Token Storage . . . . . . . . . . . . . . . . . . . . . .  13
     8.6.  Visibility of User Agent's Address Bar  . . . . . . . . .  14
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This OAuth 2.0 protocol flow for Single-page Applications (SPA),
   often referred to as the assisted token flow, provides clients
   written in scripting languages, like JavaScript, with a simplified
   integration (compared to the implicit grant type flow) and ensures
   that end users are not redirected away from the current page in order
   to obtain authorization from the resource owner.  The communication
   between the client and the authorization server takes place within an
   HTML iframe element or child window that is only displayed when



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   interactive user interaction is required; this is the case when
   authentication and/or authorization are necessary.  To communicate
   the result from this iframe or child window to the client
   application, HTML's postMessage interface is used instead of the
   redirection endpoint defined in Section 3.1.2 of
   The OAuth 2.0 Authorization Framework.  This difference is important
   for many SPAs which take time to reload and may not be able to
   recreate the same state prior to the user being redirected to the
   authorization server.

   Another goal of the assisted token flow is to simplify integration of
   the client with the authorization server.  Though [RFC6749] resulted
   in a much simpler integration for client applications compared to its
   predecessor, [RFC5849], developers still struggle with the many
   inputs required to perform the various OAuth flows.  For this reason,
   the assisted token flow introduces a new endpoint called the assisted
   token endpoint rather than extending and reusing the token endpoint
   defined in Section 3.2 of [RFC6749].  As a result, client developers
   do not need to specify a response_type parameter in the authorization
   request.  This coupled with the use of HTML's postMessage interface
   for communication between the client and the authorization server
   means that the redirect_uri and state parameters are also
   unnecessary.  Consequently, client developers only need to provide a
   client_id, create a dynamic iframe or open a child window, and handle
   the postMessage that is fired by the authorization server in order to
   implement OAuth.

   This interaction is shown in Figure 1.

   +----------------------------+                      +---------------+
   |           Client           |                      | Authorization |
   | +--------------------------+                      |     Server    |
   | |              +-----------+                      |               |
   | |--(A)-------->|           |--(B)-- Client ------>|               |
   | |              |           |        Identifier    |               |
   | |              |  Hidden   |                      |               |
   | |              |  iframe   |<-(C)-- HTML with ----|               |
   | |<-(D)- Token -|           |        postMessage   |               |
   | |       or     |           |        including     |               |
   | |       error  +-----------+        access token  |               |
   | +--------------------------+        or error      |               |
   |                            |                      |               |
   +----------------------------+                      +---------------+


                       Figure 1: Assisted Token Flow





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   The assisted token flow illustrated in Figure 1 includes the
   following steps:

      (A) The client creates a hidden iframe element using a scripting
      language like JavaScript.  The src attribute of this iframe is the
      URL of the assisted token endpoint of the authorization server.

      (B) The query string of the src attribute on the iframe includes,
      at a minimum, the client identifier.

      (C) If the user is already logged in and has granted consent to
      the client, the authorization server immediately returns an HTML
      document that includes a script that is executing; this fires an
      event which is communicated to the client using an HTML
      window.postMessage.

      (D) The client handles this event -- the payload of which is
      either an access token or an error; the client then closes the
      dynamic iframe without revealing it to the user.

   If the resource owner has not authenticated or has not authorized the
   client, then interaction between the resource owner and the
   authorization server is required to obtain these.  In such a case,
   the HTML in step (C) of Figure 1 will include an error indicating
   that user involvement is required.  This will be handled by the
   client in step (D) and login and/or consent will commence.  This
   process is illustrated in Figure 2.

   +----------------------------+                      +---------------+
   |           Client           |                      | Authorization |
   | +--------------------------+                      |     Server    |
   | |              +-----------+        Client        |               |
   | |--(E)-------->|           |--(F)-- Identifier -->|               |
   | |              |  Visible  |                      |               |
   | |              |  iframe   |--(G)-- User -------->|               |
   | |              |  or child |        authenticates |               |
   | |<-(I)- Token -|  window   |                      |               |
   | |              |           |<-(H)-- HTML with ----|               |
   | |              +-----------+        postMessage   |               |
   | +--------------------------+        including     |               |
   |                            |        access token  |               |
   +----------------------------+                      +---------------+


            Figure 2: Assisted Token Login and/or Consent Flow

   The flow shown in Figure 2 includes the following steps:




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      (E) The client creates a _visible_ iframe or pops open a child
      window after receiving an indication from the authorization server
      that user interaction is required.  As in the previous flow, the
      src attribute value of this iframe or the input to the open method
      of the user agents's window object is the URL of the authorization
      server's assisted token endpoint.

      (F) The query string of this URL includes, at a minimum, the
      client identifier.

      (G) The authorization server prompts the resource owner to
      authenticate and/or authorize the client.

      (H) The authorization server returns an HTML document that
      includes a script that is executed; this fires an event which is
      communicated to the client using an HTML window.postMessage.

      (I) The client handles this event -- the payload of which is an
      access token; the client then closes the iframe or child window
      that it previously opened to facilitate user interaction.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in Key words for use in
   RFCs to Indicate Requirement Levels [RFC2119].

   Handle Token
      An opaque token that refers to internal data of the authorization
      server (e.g., the user ID, scope of the token, etc.) as described
      in Section 3.1 of [RFC6819].

   All other terms used in this document are as defined in [RFC6749].
   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

3.  Assisted Token Endpoint

   The means through which the client obtains the location of the
   assisted token endpoint is either by using the authorization server's
   metadata as set forth in Section 6, the service documentation, or
   some other method that is beyond the scope of this specification.

   The endpoint URI MAY include an "application/x-www-form-urlencoded"
   formatted (per Appendix B of [RFC6749]) query component (Section 3.4
   of [RFC3986]), which MUST be retained when adding additional query
   parameters.  The endpoint URI MUST NOT include a fragment component.



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   Since requests to the assisted token endpoint result in the
   transmission of clear-text credentials (in the HTTP request and
   response), the authorization server MUST require the use of TLS as
   described in Section 1.6 of [RFC6749] when sending requests to the
   assisted token endpoint.

   The client MUST use the HTTP "GET" method when making access token
   requests to this endpoint.

   Parameters sent without a value MAY be treated as if they were
   omitted from the request.  The authorization server MUST ignore
   unrecognized request parameters.  Request and response parameters
   MUST NOT be included more than once.

   After completing its interaction with the resource owner, the
   authorization server will fire an event using the HTML postMessage
   interface.  This message MUST NOT be posted to all origins, denoted
   by "*".  Instead, the authorization server MUST post this message
   only to the client's allowed origin(s) previously established with
   the authorization server during the client registration process.

3.1.  Access Token Scope

   Like the authorization and token endpoints, the assisted token
   endpoint allows the client to specify the scope of the access request
   using the "scope" request parameter.  In turn, the authorization
   server uses the "scope" response parameter to inform the client of
   the scope of the access token issued.  Unlike the typical behavior of
   those endpoints, however, access tokens issued by the authorization
   server using the assisted token endpoint MUST use the client's pre-
   configured scope or the authorization server's pre-defined default if
   none have been configured for the client.

   If the client did not include a "scope" request parameter or if the
   issued access token scope is different from the one requested by the
   client, the authorization server MUST include the "scope" response
   parameter to inform the client of the actual scope granted.  Even
   when the scope of the issued access token is the same as the one
   requested by the client, the authorization server SHOULD include the
   "scope" response parameter.

   The format, constraints, and grammar of the scope parameter value is
   as defined in Section 3.3 of [RFC6749].

   The authorization server SHOULD NOT return an error if a scope has
   not been pre-configured for the client; only if the authorization
   server does not have a pre-defined default scope.




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3.2.  Cross-Origin Support

   The assisted token endpoint MAY support CORS [W3C.WD-cors-20120403].

4.  Protocol

4.1.  Assisted Token Request

   The assisted token request is an HTTP GET request constructed by the
   client with the following parameters provided on the query string:

   client_id  REQUIRED.  The client identifier as described in
           Section 2.2 of [RFC6749].

   for_origin  OPTIONAL.  The origin of the client in case multiple
           allowed origins are configured with the authorization server
           and support for user agents that do not support [CSP-2] but
           only X-Frame-Options [RFC7034].  See Section Section 8.1 for
           details.

   prompt  OPTIONAL.  Space delimited, case sensitive list of ASCII
           [USASCII] string values that can be used to determine the
           login state of the resource owner at the authorization
           server.  The defined values are:

           none    The authorization server MUST NOT display any
                   authentication or consent user interface pages.  An
                   error is returned if the user is not already
                   authenticated or if the client has not received
                   consent (either explicitly by the resource owner or
                   by the authorization server's configuration of the
                   client) or if the authorization server cannot fulfill
                   other conditions for processing.  This can be used as
                   a method to probing for existing authentication and/
                   or consent.

           consent The authorization server SHOULD prompt the user for
                   consent before returning information to the client.
                   If it cannot obtain consent, it MUST return an error.

           Other values may be provided in this list; the authorization
           server MUST ignore them without producing an error if it
           cannot understand them.

   scope   OPTIONAL.  The scope of the access request as described in
           Section 3.1.





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4.2.  Assisted Token Response

   The response from the assisted token endpoint is an HTML document
   that executes a script which invokes the HTML postMessage interface
   to send a message to either the parent window (in the case shown in
   Figure 1) or the opener (in the case illustrated in Figure 2).  The
   origin that this event is posted to is that of the client.  The
   contents of this message is a JSON object with the following fields:

   access_token  REQUIRED.  The access token issued by the authorization
           server.

   expires_in  RECOMMENDED.  The lifetime in seconds of the access
           token.

   scope   REQUIRED.  The scope of the access token as described in
           Section 3.1.

   sub     RECOMMENDED.  A locally unique and never reassigned
           identifier within the authorization server for the user,
           which is intended to be consumed by the client.  The sub
           value is a case sensitive string.

   token_type  REQUIRED.  The type of the token issued as described in
           Section 7.1 of [RFC6749].  Value is case insensitive.

4.3.  Error Response

   As with a successful response, an error is returned to the client
   using an the HTML postMessage interface.  Such an error is returned
   whenever the resource owner denies the access request or whenever the
   request fails for reasons other than the origin of the client being
   disallowed to frame the assisted token endpoint.  The error message
   includes a JSON object with the following fields:

   error   REQUIRED.  A single ASCII [USASCII] error code from the
           following:

           invalid_request  The request is missing a required parameter,
                   includes an invalid parameter value, includes a
                   parameter more than once, or is otherwise malformed.

           unauthorized_client  The client is not authorized to request
                   an access token using this method.

           access_denied  The resource owner or authorization server
                   denied the request.




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           consent_required  The client includes a prompt value of
                   consent, but consent by the resource owner was
                   required.

           interaction_required  The client included a prompt value of
                   none, but either the user needed to authenticate or
                   consent to the client's access or some other
                   requirement of the authorization server prevented it
                   from providing access without some form of user
                   interaction.

           unsupported_response_type  The authorization server does not
                   support obtaining an access token using this method.

           invalid_scope  The requested scope is invalid, unknown, or
                   malformed.

           server_error  The authorization server encountered an
                   unexpected condition that prevented it from
                   fulfilling the request.  (This error code is needed
                   because a 500 Internal Server Error HTTP status code
                   cannot be returned to the client directly in the
                   child frame.)

           temporarily_unavailable  The authorization server is
                   currently unable to handle the request due to a
                   temporary overloading or maintenance of the server.
                   (This error code is needed because a 503 Service
                   Unavailable HTTP status code cannot be returned to
                   the client directly in the child frame.)

           Values for the "error" parameter MUST NOT include characters
           outside the set %x20-21 / %x23-5B / %x5D-7E.

   error_description  OPTIONAL.  Human-readable ASCII [USASCII] text
           providing additional information, used to assist the client
           developer in understanding the error that occurred.  Values
           for the "error_description" parameter MUST NOT include
           characters outside the set %x20-21 / %x23-5B / %x5D-7E.

   error_uri  OPTIONAL.  A URI identifying a human-readable web page
           with information about the error, used to provide the client
           developer with additional information about the error.
           Values for the "error_uri" parameter MUST conform to the URI-
           reference syntax and thus MUST NOT include characters outside
           the set %x21 / %x23-5B / %x5D-7E.





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5.  Client Metadata

   The authorization server MAY allow dynamic clients to request the use
   of the assisted token flow when registering.  Such a client may
   indicate that it will interact with the authorization server using
   the assisted token flow by including the string element
   "urn:ietf:params:oauth:grant-type:assisted_token" in the array
   associated with the "grant_types" metadata field sent to the client
   registration endpoint (as defined in Section 3. of [RFC7591]).  If
   the authorization server allows the client to register with this
   grant type, the "grant_types" included in the response MUST include
   the value "urn:ietf:params:oauth:grant-type:assisted_token".  The
   inclusion of this value in the "grant_types" field is done despite
   the fact that the client will not use this grant type at the token
   endpoint but rather the assisted token endpoint (Section 3).

   The following client metadata field is defined by this specification.
   It MAY be included in a registration request, as set forth in
   Section 2. of [RFC7591].

   allowed_origins
      Array of origin strings for use in sending messages from the
      authorization server to the client using the HTML's postMessage
      interface.

6.  Authorization Server Metadata

   Support for the assisted token flow MUST be declared in the OAuth 2.0
   Authorization Server Metadata [I-D.ietf-oauth-discovery] with the
   following metadata:

   assisted_token_endpoint
      REQUIRED.  URL of the authorization server's assisted token
      endpoint defined in Section 3.

   grant_types_supported
      REQUIRED.  A JSON array specified in Section 2. of
      [I-D.ietf-oauth-discovery] which should contain the value
      "urn:ietf:params:oauth:grant-type:assisted_token" as defined in
      Section 5.

7.  IANA Considerations

7.1.  OAuth URI Registration

   This specification registers the following values in the IANA "OAuth
   URI" registry [IANA.OAuth.Parameters] established by [RFC6755].




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7.1.1.  Registry Contents

   o  URN: urn:ietf:params:oauth:grant-type:assisted_token
   o  Common Name: Assisted token flow grant type for OAuth 2.0
   o  Change controller: IESG
   o  Specification Document:Section 5of [[ this specification ]]

7.2.  OAuth 2.0 Authorization Server Metadata

   This specification registers the following values in the IANA "OAuth
   2.0 Authorization Server Metadata" registry [IANA.OAuth.Parameters]
   established by [I-D.ietf-oauth-discovery].

7.2.1.  Registry Contents

   o  Metadata name: assisted_token_endpoint
   o  Metadata Description: The Assisted Token Endpoint.
   o  Change controller: IESG
   o  Specification Document: Section 6 of [[ this specification ]]

8.  Security Considerations

   In addition to all the security considerations discussed in OAuth 2.0
   [RFC6819], the following security considerations SHOULD be taken into
   account.

8.1.  Framing

   Due to the use of an iframe to host the assisted token endpoint, the
   authorization server MUST take precautions to ensure that only
   trusted origins are allowed to frame it.  The authorization server
   MUST prevent any origin from framing the assisted token endpoint
   except ones that an administrator has explicitly allowed.  It may do
   this in any manner that is available to the application.

   One such mechanism that MAY be deployed is Content Security Policy
   [CSP-2].  This protocol SHOULD be used on the assisted token endpoint
   (and, if applicable, other endpoints used to authenticated the user
   in a specific deployment) to prevent framing from unauthorized
   origins.  Using CSP allows the authorization server to specify
   multiple origins in a single response header field and to constrain
   these using flexible patterns (see [CSP-2] for details).  This
   standard provides a robust mechanism for protecting against click-
   jacking when combining policies that restrict "child-src" with the
   sources of scripts that are allowed to execute by using "script-src"
   policies.





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   Because some user agents do not support [CSP-2], this technique
   SHOULD be combined with others.  In particular, the authorization
   server SHOULD return an "X-Frame-Options" response header on the
   assisted token endpoint (and, if applicable, other endpoints used to
   authenticate the user in a specific deployment).  As defined in
   [RFC7034], this header will cause user agents that support it (but
   not CSP) to block framing from any origin that is not specified in
   this header's value.  Because this header's value can only include
   one origin, the framer should use the "for_origin" parameter (as
   specified in Section 4.1) to include its own origin.

   Some user agents do not support X-Frame-Options [RFC7034] nor
   [CSP-2].  Therefore, the authorization server SHOULD include a frame-
   busting script like that shown in Figure 7 of [FRAME-BUSTING].  Such
   a script would use JavaScript to break out of any unauthorized origin
   that is framing the assisted token endpoint.  The authorization
   server MAY simply break out of all frames in case X-Frame-Options
   [RFC7034] and [CSP-2] are unsupported by the user agent, though this
   would render the assisted token flow non-functional.  The choice of
   whether or not this drastic countermeasure should be employed depends
   on the user agents being targetted in a certain deployment.

8.2.  Handle Tokens

   Because the client applications that use the assisted token flow are
   written in scripting languages like JavaScript and are hosted in Web
   pages, users may keep such applications open in their user agents for
   a prolonged period of time.  During such period, the token issued to
   the client may expire or be revoked.  To ensure that such expired
   tokens left remnant in the user agent are benign, a Handle Token
   SHOULD always be issued by the assisted token endpoint.  This ensures
   that no identity data is exposed (even when the token is not yet
   expired) and that a revoked token does not increase risks.

8.3.  Warning Against Untrusted Scripts

   As admitted in Section 8 of [RFC6454], preventing exfiltration of
   cookies, tokens, and other such credentials in web browsers has
   historically proven difficult to implement.  Instead, the same-origin
   policy has emerged as the cornerstone of security for such user
   agents.  Using this security model, it is not possible to prevent
   access to a token issued to a client if that client includes
   nefarious scripts from untrustworthy sources that have access to the
   Document Object Model (DOM) where the token is stored.  For this
   reason, the authorization server MUST warn client application
   developers who interact with the assisted token endpoint _not_ to use
   untrusted scripts in their applications.  This warning SHOULD at




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   least be conveyed through the documentation but MAY also be provided
   through other mechanisms.

8.4.  Origin of Event and Authorization Server

   As described in Section 4.1, the authorization server will return an
   access token to the client using HTML's postMessage interface.  The
   receiver of this message is provided with an event object that
   contains an origin property.  A client application MUST compare this
   with that of the authorization server before consuming the message.
   Otherwise, it runs the risk of processing messages posted from
   untrusted origins.  An example of a proper message handler is shown
   in the following non-normative, JavaScript listing:


   <script type="text/javascript">
       window.addEventListener("message", function(evt) {
           if (evt.origin !== "https://oauth-server.example.com")
               return; // Ignore event from untrusted source

           ...
       });
   </script>

   Figure 3: Comparing the origin of the postMessage event with that of
                         the authorization server

8.5.  Token Storage

   Most client applications that use the assisted token flow will
   maintain the access token issued by the authorization server in a
   persisted state; this will commonly be an HTTP cookie or local
   storage.  This is necessary, for instance, to create a pleasing user
   experience when a user navigates away from the application in their
   web browser and then returns.  To ensure that the token is stored
   safely, the authorization server MAY provide application developers
   with guidance in the accompanying documentation on how to safely
   persist tokens.  The authorization server MAY also provide script
   libraries that perform this action according to best common
   practices.  The authorization server MAY also store the token in an
   HTTP cookie in its own DNS domain (rather than that of the client)
   using the assisted token endpoint's path.  In some cases, this would
   elevate any storage requirements from the client application
   developer.  Besides simplifying the programming model for developers,
   this technique allows the authorization server to check the validity
   of the token and determine if the token has expired or if the
   associated grant has been revoked in subsequent requests to the
   assisted token endpoint; this will be possible because the requests



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   will include the token in the HTTP Cookie request header.  In such
   cases, the authorization server can take the appropriate action, such
   as authenticating the user anew or request consent, before issuing a
   new token.  If the token is stored by the application, however, this
   kind of verification cannot be performed by the authorization server
   without an explicit request to validate a stored token.

8.6.  Visibility of User Agent's Address Bar

   When the authorization server and client are provided by separate
   parties, it is important that the resource owner is able to
   distinguish the two.  The only safe way of doing so is by examination
   of the user agent's address bar where the validity of the certificate
   and location can be made.  In such situations, whenever user
   interaction is required, the client SHOULD open the assisted token
   endpoint in a new browser window rather than a hidden iframe.  The
   authorization serer MAY take any measures deemed appropriate in a
   deployment to ensure that the client has not framed the user's manual
   interaction; however, the necessity for interactive user
   authentication and/or consent SHOULD be possible for the client to
   determine in a hidden iframe.

9.  Privacy Considerations

   In some deployments, the assisted token endpoint may be served from a
   distinct domain from that of the client.  In such cases, the client
   will be a third-party domain, and the resource owner's user agent may
   prevent the authorization server from storing any third-party
   cookies.  If the authorization server requires state to be persisted
   when performing the assisted token flow, it SHOULD provide a privacy-
   preserving mechanism to store and retrieve its state even when the
   assisted token endpoint is hosted on a distinct domain from that of
   the client.  The technical details of how to accomplish this are
   implementation specific, and are beyond the scope of this
   specification.  If the authorization server does not support clients
   that are hosted from a third-party domain, it MUST indicate this to
   the client through some mechanism (e.g., its associated
   documentation).

10.  References

10.1.  Normative References

   [I-D.ietf-oauth-discovery]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-10 (work in progress), November 2017.




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

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [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>.

   [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>.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

10.2.  Informative References

   [CSP-2]    West, M., Barth, A., and D. Veditz, "Content Security
              Policy Level 2", July 2015, <https://www.w3.org/TR/CSP2>.

   [FRAME-BUSTING]
              Rydstedt, G., Bursztein, E., Boneh, D., and C. Jackson,
              "Busting frame busting: a study of clickjacking
              vulnerabilities at popular sites", July 2010,
              <http://seclab.stanford.edu/websec/framebusting/>.

   [RFC5849]  Hammer-Lahav, E., Ed., "The OAuth 1.0 Protocol", RFC 5849,
              DOI 10.17487/RFC5849, April 2010,
              <https://www.rfc-editor.org/info/rfc5849>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.






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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>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,
              <https://www.rfc-editor.org/info/rfc6819>.

   [RFC7034]  Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
              Options", RFC 7034, DOI 10.17487/RFC7034, October 2013,
              <https://www.rfc-editor.org/info/rfc7034>.

   [W3C.WD-cors-20120403]
              Kesteren, A., "Cross-Origin Resource Sharing", World Wide
              Web Consortium LastCall WD-cors-20120403, April 2012,
              <http://www.w3.org/TR/2012/WD-cors-20120403>.

Appendix A.  Acknowledgements

   The following individuals contributed ideas, feedback, and wording to
   this specification:

   Mark Dobrinic, Karl McGuinness

Appendix B.  Document History

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

   -00

   o  Initial draft.

Authors' Addresses

   Jacob Ideskog
   Curity AB

   Email: jacob@curity.io


   Travis Spencer
   Curity AB

   Email: travis@curity.io
   URI:   http://travisspencer.com/





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