OAuth 2.0 Web Message Response Mode for Popup- and Iframe-based Authorization Flows
draft-meyerzuselha-oauth-web-message-response-mode-00

Web Authorization Protocol                         K. Meyer zu Selhausen
Internet-Draft                                                 Hackmanit
Intended status: Standards Track                              L. Jannett
Expires: 26 May 2024                              Ruhr University Bochum
                                                               C. Mainka
                                                               Hackmanit
                                                        23 November 2023

    OAuth 2.0 Web Message Response Mode for Popup- and Iframe-based
                          Authorization Flows
         draft-meyerzuselha-oauth-web-message-response-mode-00

Abstract

   This specification defines the web message response mode that
   authorization servers use for transmitting authorization response
   parameters via the user-agent's postMessage API to the client.  This
   mode is intended for authorization flows that use secondary windows,
   which are well-suited for browser-based applications.

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 26 May 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Web Message Response Mode . . . . . . . . . . . . . . . . . .   3
   3.  Popup-Based Authorization Flow Using the Web Message Response
           Mode  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Iframe-Based Authorization Flow Using the Web Message Response
           Mode  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Secure Iframe Integration . . . . . . . . . . . . . . . . . .   7
     5.1.  User-Session in Iframes . . . . . . . . . . . . . . . . .   7
   6.  Authorization Server Metadata . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     7.1.  Receiver Origin Validation  . . . . . . . . . . . . . . .   8
     7.2.  Initiator Origin Validation and Cross-Site Request Forgery
           Protection  . . . . . . . . . . . . . . . . . . . . . . .   8
     7.3.  Data Validation . . . . . . . . . . . . . . . . . . . . .   8
     7.4.  Cross-Site Leak Protections on the Authorization
           Server  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.5.  Redirection URI vs. Receiver Origin . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   10. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   OAuth [RFC6749] uses HTTP redirects to transfer authorization
   response parameters from the authorization server via the user-agent
   to the client's redirection endpoint.  In this case, the
   authorization response parameters are encoded in the query string
   (response_mode=query) or in the fragment (response_mode=fragment) of
   the redirect_uri [oauth.encoding] (Section 2.1).  [RFC6749]
   (Section 1.7) allows other mechanisms available via the user-agent to
   accomplish this redirection, such as response_mode=form_post
   [oauth.post].

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   The standardized query, fragment, and form post response modes are
   designed for single-window authorization but not for multi-window
   authorization flows.  A common example is a popup-based authorization
   flow, where the client's primary window opens the authorization
   server in a secondary window.  The secondary window cannot use HTTP
   redirects to transfer response parameters back to the client in the
   primary window.

   This specification defines the web message response mode that uses
   the user-agent's postMessage API [whatwg.postmessage] to exchange
   messages between two different browser windows.  Clients inform the
   authorization server to use this response mode for returning the
   authorization response by setting the response_mode parameter in the
   authorization request to web_message.  This response mode facilitates
   popup-based and iframe-based authorization flows, in which the
   authorization server is called in a secondary window or embedded in a
   frame on the client.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the terms "client", "user-agent",
   "authorization server", "authorization endpoint", "redirection
   endpoint", "redirection URI", "authorization request", and
   "authorization response" defined by the OAuth 2.0 Authorization
   Framework [RFC6749].  It further uses the term "response mode"
   defined by the OAuth 2.0 Multiple Response Type Encoding Practices
   [oauth.encoding].

   This specification defines the following additional terminology.

   primary window  This is the top-level browsing window that initially
      holds the client's website.  This window has no parent windows and
      it was not opened by any other window.
   secondary window  This is the window in which the client in the
      primary window opens the authorization server.

2.  Web Message Response Mode

   This specification defines the web message response mode, which is
   described with the following response_mode parameter value in the
   authorization request [oauth.encoding].

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   web_message  In the web message response mode, the authorization
      server in the secondary window encodes the authorization response
      parameters in a JSON dictionary and transmits it via the user-
      agent's postMessage API to the client in the primary window.

   If the authorization request includes the value web_message for the
   response_mode parameter, the authorization server:

   *  MUST use the user-agent's postMessage API to return the
      authorization response to the client.
   *  MUST encode the response paramters as key-value string pairs in a
      JSON dictionary.
   *  MUST follow [RFC6749] and the security best practices
      [I-D.ietf-oauth-security-topics] when validating the redirection
      URI.
   *  MUST use the full redirection URI (the exact same URI also used in
      other response modes such as query, fragment, or form post) as the
      postMessage's receiver origin.  The user-agent's postMessage API
      inherently parses the redirection URI and extracts its origin.
   *  MUST NOT parse the URI itself to reduce the attack surface
      concerning parsing issues.

   The client's redirection URI MUST serve as the postMessage's receiver
   origin to protect the authorization response from being leaked to
   malicious origins.

   This example illustrates how an authorization server (identified by
   the issuer https://as.example) in a secondary window returns the
   authorization response to the client (whose registered redirection
   URI is https://client.example/cb) in the primary window using the
   postMessage API:

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   const primaryWindowRef = window.opener // popup-based auth. flow
   const primaryWindowRef = window.parent // iframe-based auth. flow
   primaryWindowRef.postMessage({"code": "XXXXXXXX","state":"XXXXXXXX",\
   "iss": "https://as.example"},"https://client.example/cb")

3.  Popup-Based Authorization Flow Using the Web Message Response Mode

   In a popup-based authorization flow, the client opens the
   authorization endpoint in a secondary window.  The authorization
   server uses the user-agent's postMessage API to return the
   authorization response parameters from the secondary window back to
   the client running in the primary window.  The flow is depicted in
   Figure 1 and described in the following.

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   +------------------------------+
   |        Primary Window        |
   |    (client.example/login)    |    +-----------------------+
   |                              |    |   Secondary Window    |
   |1) Register message event     |    |  (as.example/authz)   |
   |   handler                    |    |                       |
   |                              |    |3) Authenticate user   |
   |2) Open authz request with    |    |   and authorize       |
   |   web message response mode  |    |   access              |
   |   parameter in popup window  +--->|                       |
   |                              |    |4) Send postMessage    |
   |5) Validate and process authz |<---+   with authz response |
   |   response in event handler  |    |   to opener window    |
   |                              |    |                       |
   +------------------------------+    +-----------------------+

       Figure 1: Overview of popup-based authorization flow using the
                         web message response mode.

   1.  The client registers a message event handler that receives the
       authorization response from the authorization server.  The client
       MUST validate the message's origin with an exact string matching
       the authorization server's origin.  As the authorization response
       is meant for one-time use, the client MUST remove the message
       event handler after receiving the authorization response to
       prevent any further authorization attempts.

   Example of the registration of a message event handler:

   const callback = (e) => {
     if (e.origin === "https://as.example") {
     // further validation and processing of the authorization response
       process(e.data)
       window.removeEventListener("message", callback)
     }
   }
   window.addEventListener("message", callback)

   2.  The client MUST open the authorization request in a secondary
       window.  Therefore, the client MAY use JavaScript and the user-
       agent's window.open API, MAY use an anchor HTML element with a
       target attribute, or MAY use any other mechanism suitable for
       opening secondary windows.  The authorization request MUST
       include the response_mode parameter value web_message.
       Additional authorization request parameters and techniques, such
       as PAR [RFC9126] and RAR [RFC9396], are unaffected by this
       specification and might be used with the web message response
       mode.

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   Example of using the window.open API to open the authorization
   request in a secondary window:

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   window.open("https://as.example/auth?...&response_mode=web_message",\
   "_blank","left=100,top=100,width=320,height=320")

   Example of using an anchor tag with a target attribute to open the
   authorization request in a secondary window:

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   <a href="https://as.example/auth?...&response_mode=web_message" \
   target="_blank">

   3.  The authorization server receives the authorization request,
       validates its parameters, and proceeds with the end-user
       authentication and authorization, which is outside of the scope
       of this specification.

   4.  If the authorization request includes the response_mode parameter
       value web_message, the authorization server MUST follow the web
       message response mode as described in Section 2 and use the user-
       agent's postMessage API to return the authorization response
       parameters from the secondary to the secondary window's opener
       window.  The receiver window MUST be referenced by the secondary
       window's opener property.

   Example of an authorization server using the postMessage API in a
   secondary window to return the authorization response to the client
   in the primary window:

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   window.opener.postMessage({"code": "XXXXXXXX", "state": "XXXXXXXX", \
   "iss": "https://as.example"}, "https://client.example/cb")

   5.  The client's message event handler receives the postMessage that
       contains the authorization response parameters.  The further
       processing and validation of all parameters is out of the scope
       of this specification and MUST be compliant to [RFC6749] and the
       security best practices [I-D.ietf-oauth-security-topics].

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4.  Iframe-Based Authorization Flow Using the Web Message Response Mode

   In addition to secondary windows being popups, iframes can be used.
   Iframes enable a seamless authorization flow where the end-user only
   sees a single browser tab, without ever leaving the actual client.
   The exchange of tokens works technically similar to popups.  However,
   iframes are more difficult to implement on the authorization server
   side.

5.  Secure Iframe Integration

   An authorization endpoint and consent-page embedded in an iframe MUST
   be protected against Clickjacking attacks
   [I-D.ietf-oauth-security-topics] (Section 4.16).

   If the user's browser supports the Observer v2 API [w3c.observerv2],
   the authorization server MAY use it to allow the client to embed the
   authorization endpoint and consent-page.  Otherwise, the
   authorization server MUST implement Clickjacking countermeasures
   according to [I-D.ietf-oauth-security-topics] (Section 4.16).  The
   authorization server MUST prevent framing the authorization endpoint
   and consent-page except from origins deemed trustworthy.

5.1.  User-Session in Iframes

   Modern browsers have started to disable the support for third-party
   cookies.  Thereby, iframes do not send authentication cookies along
   with requests in sub resource requests, such as iframes.  Using
   iframes as secondary windows therefore requires special exceptions to
   bypass this restriction, such as the Storage Access API
   [mozilla.storageaccessapi].

6.  Authorization Server Metadata

   Authorization servers MUST announce their support for the web message
   response mode defined in Section 2 by adding web_message to the
   response_modes_supported list in their authorization server metadata
   as specified in [RFC8414] (Section 2).

7.  Security Considerations

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7.1.  Receiver Origin Validation

   Authorization servers MUST follow Section 2 and the security best
   practices [I-D.ietf-oauth-security-topics] (Section 4.18.2) when
   validating the postMessage receiver origin.  Otherwise, the
   authorization response may leak to an attacker, as described in
   [I-D.ietf-oauth-security-topics] (Section 4.18.1.1) and
   [I-D.ietf-oauth-security-topics] (Section 4.18.1.2).

7.2.  Initiator Origin Validation and Cross-Site Request Forgery
      Protection

   In redirect-based authorization flows, there is no inherent mechanism
   available that enables a client to verify that the trusted
   authorization server initiates a redirection.  Instead, [RFC6749]
   introduces the state parameter to counter so-called Cross-Site
   Request Forgery (CSRF) attacks [I-D.ietf-oauth-security-topics]
   (Section 4.7) against the client's redirection endpoint by
   maintaining a state between the authorization request and response.

   The postMessage API provides an inherent mechanism to verify the
   initiator of a postMessage.  The client MUST use this mechanism as
   described in Section 2 and the security best practices
   [I-D.ietf-oauth-security-topics] (Section 4.18.2) to verify that the
   trusted authorization server is the initiator of the postMessage.
   Otherwise, an attacker can inject a maliciously crafted authorization
   response to the client [I-D.ietf-oauth-security-topics]
   (Section 4.18.1.3).  This verification prevents some variants of CSRF
   attacks, where the attacker wants to log in a victim to the attacker
   account.

   However, attackers can also use CSRF attacks to log in a victim to
   the victim's own account.  This attack variant cannot be mitigated
   with the checks mentioned above.  Instead, a proper CSRF
   countermeasure, as described in [I-D.ietf-oauth-security-topics]
   (Section 4.7.1) MUST be used.

7.3.  Data Validation

   Even after the initiator origin of the postMessage is validated, the
   client MUST check that the postMessage has the expected format
   [whatwg.postmessage] (Section 9.3.2.1).  In specific, the postMessage
   MUST NOT be processed in unsafe JavaScript sinks like eval or
   innerHTML to prevent cross-site scripting (XSS) flaws and other
   potentially malicious injections.  Otherwise, if the authorization
   server has been attacked using an XSS flaw, further unchecked
   processing of the postMessage could result in the attack being
   propagated into the client.

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7.4.  Cross-Site Leak Protections on the Authorization Server

   The authorization server is opened in a secondary window that needs
   access to its opener window via its opener property to send
   postMessages to that referenced window.  Thus, the authorization
   server cannot use cross-site leak protections like the cross-origin
   opener policy [whatwg.coop] to force the creation of a new top-level
   browsing context and cross-origin isolate their sites.

   However, browsers are working on preventing cross-site leaks without
   breaking the popup-based authorization flows with the restrict-
   properties directive being added to the cross-origin opener policy
   [google.restrictprops].  With this directive, properties that can be
   used for cross-site leaks are not available but postMessage
   communication between cross-origin windows is still allowed.
   Authorization servers MAY set the Cross-Origin-Opener-Policy:
   restrict-properties header to protect against cross-site leaks.

7.5.  Redirection URI vs. Receiver Origin

   In redirect-based authorization flows, the confidentiality of the
   authorization response is scoped to the redirection URI that contains
   a path.  The path separates the authorization response from other,
   potentially vulnerable paths within the same web application.  In
   flows using the web message response mode, the confidentiality of the
   authorization response is scoped to the postMessage's receiver origin
   that does not contain a path.  Thus, cross-site scripting (XSS)
   vulnerabilities on _any_ path within the web application's origin
   will leak the authorization response to the attacker.

8.  IANA Considerations

   This draft makes no requests to IANA.

9.  Normative 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-24, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-24>.

   [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>.

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   [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>.

   [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>.

   [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>.

   [oauth.encoding]
              de Medeiros, B., Scurtescu, M., Tarjan, P., and M. Jones,
              "OAuth 2.0 Multiple Response Type Encoding Practices",
              February 2014, <https://openid.net/specs/oauth-v2-
              multiple-response-types-1_0.html>.

   [whatwg.postmessage]
              "HTML Living Standard: Cross-document messaging",
              <https://html.spec.whatwg.org/multipage/web-
              messaging.html#web-messaging>.

10.  Informative References

   [I-D.sakimura-oauth-wmrm]
              Yamaguchi, T., Sakimura, N., and N. Matake, "OAuth 2.0 Web
              Message Response Mode", Work in Progress, Internet-Draft,
              draft-sakimura-oauth-wmrm-01, 8 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-sakimura-
              oauth-wmrm-01>.

   [RFC9126]  Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
              and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
              RFC 9126, DOI 10.17487/RFC9126, September 2021,
              <https://www.rfc-editor.org/info/rfc9126>.

   [RFC9396]  Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", RFC 9396,
              DOI 10.17487/RFC9396, May 2023,
              <https://www.rfc-editor.org/info/rfc9396>.

   [google.restrictprops]
              and , "Secure popup interactions with restrict-
              properties", 9 August 2023,
              <https://developer.chrome.com/blog/coop-restrict-
              properties/>.

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   [mozilla.storageaccessapi]
              "Storage Access API", 18 October 2023,
              <https://developer.mozilla.org/en-US/docs/Web/API/
              Storage_Access_API>.

   [oauth.post]
              Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
              Mode", 27 April 2015, <http://openid.net/specs/oauth-v2-
              form-post-response-mode-1_0.html>.

   [w3c.observerv2]
              and , "Intersection Observer", 4 November 2019,
              <https://w3c.github.io/IntersectionObserver/>.

   [whatwg.coop]
              "HTML Living Standard: Cross-origin opener policies",
              <https://html.spec.whatwg.org/multipage/
              browsers.html#cross-origin-opener-policies>.

Appendix A.  Acknowledgements

   We would like to acknowledge the prior work of Toru Yamaguchi, Nat
   Sakimura, and Nov Matake in [I-D.sakimura-oauth-wmrm] which tried to
   define a response mode with similarities to this specification.  In
   contrast, this specification is not focused on iframes and does not
   include the use of the OAuth Implicit Grant.

   We would like to thank Vladislav Mladenov, ...

   for their valuable feedback on this document.

Appendix B.  Document History

   [[ To be removed from the final specification ]]

   -00 * initial draft

Authors' Addresses

   Karsten Meyer zu Selhausen
   Hackmanit
   Email: karsten.meyerzuselhausen@hackmanit.de

   Louis Jannett
   Ruhr University Bochum
   Email: louis.jannett@rub.de

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   Christian Mainka
   Hackmanit
   Email: christian.mainka@hackmanit.de

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