Web Authorization Protocol                                    A. Parecki
Internet-Draft                                                      Okta
Intended status: Best Current Practice                          D. Waite
Expires: 17 March 2023                                     Ping Identity
                                                       13 September 2022


                    OAuth 2.0 for Browser-Based Apps
                 draft-ietf-oauth-browser-based-apps-11

Abstract

   This specification details the security considerations and best
   practices that must be taken into account when developing browser-
   based applications that use OAuth 2.0.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Web Authorization
   Protocol Working Group mailing list (oauth@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/oauth-wg/oauth-browser-based-apps.

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 17 March 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  First-Party Applications  . . . . . . . . . . . . . . . . . .   5
   6.  Application Architecture Patterns . . . . . . . . . . . . . .   6
     6.1.  Single-Domain Browser-Based Apps (not using OAuth)  . . .   6
     6.2.  Backend For Frontend (BFF) Proxy  . . . . . . . . . . . .   7
       6.2.1.  Security considerations . . . . . . . . . . . . . . .   9
     6.3.  Token Mediating Backend . . . . . . . . . . . . . . . . .   9
       6.3.1.  Security Considerations . . . . . . . . . . . . . . .  11
     6.4.  JavaScript Applications obtaining tokens directly . . . .  11
       6.4.1.  Storing Tokens in Local or Session Storage  . . . . .  13
       6.4.2.  Service Worker as the OAuth Client  . . . . . . . . .  13
   7.  Authorization Code Flow . . . . . . . . . . . . . . . . . . .  15
     7.1.  Initiating the Authorization Request from a Browser-Based
           Application . . . . . . . . . . . . . . . . . . . . . . .  15
     7.2.  Handling the Authorization Code Redirect  . . . . . . . .  16
   8.  Refresh Tokens  . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     9.1.  Cross-Site Scripting Attacks (XSS)  . . . . . . . . . . .  17
     9.2.  Reducing the Impact of Token Exfiltration . . . . . . . .  18
     9.3.  Registration of Browser-Based Apps  . . . . . . . . . . .  18
     9.4.  Client Authentication . . . . . . . . . . . . . . . . . .  18
     9.5.  Client Impersonation  . . . . . . . . . . . . . . . . . .  19
     9.6.  Cross-Site Request Forgery Protections  . . . . . . . . .  19
     9.7.  Authorization Server Mix-Up Mitigation  . . . . . . . . .  19
     9.8.  Cross-Domain Requests . . . . . . . . . . . . . . . . . .  20
     9.9.  Content Security Policy . . . . . . . . . . . . . . . . .  20
     9.10. OAuth Implicit Flow . . . . . . . . . . . . . . . . . . .  21
       9.10.1.  Attacks on the Implicit Flow . . . . . . . . . . . .  21
       9.10.2.  Countermeasures  . . . . . . . . . . . . . . . . . .  22
       9.10.3.  Disadvantages of the Implicit Flow . . . . . . . . .  22
       9.10.4.  Historic Note  . . . . . . . . . . . . . . . . . . .  23
     9.11. Additional Security Considerations  . . . . . . . . . . .  24
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24



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     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Server Support Checklist . . . . . . . . . . . . . .  25
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  26
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   This specification describes the current best practices for
   implementing OAuth 2.0 authorization flows in applications executing
   in a browser.

   For native application developers using OAuth 2.0 and OpenID Connect,
   an IETF BCP (best current practice) was published that guides
   integration of these technologies.  This document is formally known
   as [RFC8252] or BCP 212, but nicknamed "AppAuth" after the OpenID
   Foundation-sponsored set of libraries that assist developers in
   adopting these practices.  [RFC8252] makes specific recommendations
   for how to securely implement OAuth in native applications, including
   incorporating additional OAuth extensions where needed.

   OAuth 2.0 for Browser-Based Apps addresses the similarities between
   implementing OAuth for native apps and browser-based apps, and
   includes additional considerations when running in a browser.  This
   is primarily focused on OAuth, except where OpenID Connect provides
   additional considerations.

   Many of these recommendations are derived from the OAuth 2.0 Security
   Best Current Practice [oauth-security-topics] and browser-based apps
   are expected to follow those recommendations as well.  This draft
   expands on and further restricts various recommendations in
   [oauth-security-topics].

2.  Notational Conventions

   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
   [RFC2119].

3.  Terminology

   In addition to the terms defined in referenced specifications, this
   document uses the following terms:

   "OAuth":  In this document, "OAuth" refers to OAuth 2.0, [RFC6749]
      and [RFC6750].



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   "Browser-based application":  An application that is dynamically
      downloaded and executed in a web browser, usually written in
      JavaScript.  Also sometimes referred to as a "single-page
      application", or "SPA".

4.  Overview

   At the time that OAuth 2.0 [RFC6749] and [RFC6750] were created,
   browser-based JavaScript applications needed a solution that strictly
   complied with the same-origin policy.  Common deployments of OAuth
   2.0 involved an application running on a different domain than the
   authorization server, so it was historically not possible to use the
   Authorization Code flow which would require a cross-origin POST
   request.  This was one of the motivations for the definition of the
   Implicit flow, which returns the access token in the front channel
   via the fragment part of the URL, bypassing the need for a cross-
   origin POST request.

   However, there are several drawbacks to the Implicit flow, generally
   involving vulnerabilities associated with the exposure of the access
   token in the URL.  See Section 9.10 for an analysis of these attacks
   and the drawbacks of using the Implicit flow in browsers.  Additional
   attacks and security considerations can be found in
   [oauth-security-topics].

   In recent years, widespread adoption of Cross-Origin Resource Sharing
   (CORS), which enables exceptions to the same-origin policy, allows
   browser-based apps to use the OAuth 2.0 Authorization Code flow and
   make a POST request to exchange the authorization code for an access
   token at the token endpoint.  In this flow, the access token is never
   exposed in the less-secure front channel.  Furthermore, adding PKCE
   to the flow ensures that even if an authorization code is
   intercepted, it is unusable by an attacker.

   For this reason, and from other lessons learned, the current best
   practice for browser-based applications is to use the OAuth 2.0
   Authorization Code flow with PKCE.

   Browser-based applications:

   *  MUST use the OAuth 2.0 Authorization Code flow with the PKCE
      extension when obtaining an access token

   *  MUST Protect themselves against CSRF attacks by either:

      -  ensuring the authorization server supports PKCE, or





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      -  by using the OAuth 2.0 "state" parameter or the OpenID Connect
         "nonce" parameter to carry one-time use CSRF tokens

   *  MUST Register one or more redirect URIs, and use only exact
      registered redirect URIs in authorization requests

   OAuth 2.0 authorization servers supporting browser-based
   applications:

   *  MUST Require exact matching of registered redirect URIs

   *  MUST Support the PKCE extension

   *  MUST NOT issue access tokens in the authorization response

   *  If issuing refresh tokens to browser-based applications, then:

      -  MUST rotate refresh tokens on each use or use sender-
         constrained refresh tokens, and

      -  MUST set a maximum lifetime on refresh tokens or expire if they
         are not used in some amount of time

5.  First-Party Applications

   While OAuth was initially created to allow third-party applications
   to access an API on behalf of a user, it has proven to be useful in a
   first-party scenario as well.  First-party apps are applications
   where the same organization provides both the API and the
   application.

   Examples of first-party applications are a web email client provided
   by the operator of the email account, or a mobile banking application
   created by bank itself.  (Note that there is no requirement that the
   application actually be developed by the same company; a mobile
   banking application developed by a contractor that is branded as the
   bank's application is still considered a first-party application.)
   The first-party app consideration is about the user's relationship to
   the application and the service.

   To conform to this best practice, first-party applications using
   OAuth or OpenID Connect MUST use a redirect-based flow (such as the
   OAuth Authorization Code flow) as described later in this document.

   The resource owner password credentials grant MUST NOT be used, as
   described in [oauth-security-topics] Section 2.4.  Instead, by using
   the Authorization Code flow and redirecting the user to the
   authorization server, this provides the authorization server the



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   opportunity to prompt the user for multi-factor authentication
   options, take advantage of single sign-on sessions, or use third-
   party identity providers.  In contrast, the resource owner password
   credentials grant does not provide any built-in mechanism for these,
   and would instead be extended with custom code.

6.  Application Architecture Patterns

   Here are the main architectural patterns available when building
   browser-based applications.

   *  single-domain, not using OAuth

   *  a JavaScript application with a stateful backend component

      -  storing tokens and proxying all requests (BFF Proxy)

      -  obtaining tokens and passing them to the frontend (Token
         Mediating Backend)

   *  a JavaScript application obtaining access tokens

      -  via JavaScript code executed in the DOM

      -  through a service worker

   These architectures have different use cases and considerations.

6.1.  Single-Domain Browser-Based Apps (not using OAuth)

   For simple system architectures, such as when the JavaScript
   application is served from a domain that can share cookies with the
   domain of the API (resource server) and the authorization server,
   OAuth adds additional attack vectors that could be avoided with a
   different solution.

   In particular, using any redirect-based mechanism of obtaining an
   access token enables the redirect-based attacks described in
   [oauth-security-topics] Section 4, but if the application,
   authorization server and resource server share a domain, then it is
   unnecessary to use a redirect mechanism to communicate between them.

   An additional concern with handling access tokens in a browser is
   that in case of successful cross-site scripting (XSS) attack, tokens
   could be read and further used or transmitted by the injected code if
   no secure storage mechanism is in place.





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   As such, it could be considered to use an HTTP-only cookie between
   the JavaScript application and API so that the JavaScript code can't
   access the cookie value itself.  The Secure cookie attribute should
   be used to ensure the cookie is not included in unencrypted HTTP
   requests.  Additionally, the SameSite cookie attribute can be used to
   counter some CSRF attacks, but should not be considered the extent of
   the CSRF protection, as described in [draft-ietf-httpbis-rfc6265bis]

   OAuth was originally created for third-party or federated access to
   APIs, so it may not be the best solution in a common-domain
   deployment.  That said, there are still some advantages in using
   OAuth even in a common-domain architecture:

   *  Allows more flexibility in the future, such as if you were to
      later add a new domain to the system.  With OAuth already in
      place, adding a new domain wouldn't require any additional
      rearchitecting.

   *  Being able to take advantage of existing library support rather
      than writing bespoke code for the integration.

   *  Centralizing login and multifactor support, account management,
      and recovery at the OAuth server, rather than making it part of
      the application logic.

   *  Splitting of responsibilities between authenticating a user and
      serving resources

   Using OAuth for browser-based apps in a first-party same-domain
   scenario provides these advantages, and can be accomplished by any of
   the architectural patterns described below.

6.2.  Backend For Frontend (BFF) Proxy


















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   +-------------+  +--------------+ +---------------+
   |             |  |              | |               |
   |Authorization|  |    Token     | |   Resource    |
   |  Endpoint   |  |   Endpoint   | |    Server     |
   |             |  |              | |               |
   +-------------+  +--------------+ +---------------+

          ^                ^                   ^
          |             (D)|                (G)|
          |                v                   v
          |
          |         +--------------------------------------+
          |         |                                      |
          |         |   Backend for Frontend Proxy (BFF)   |
       (B)|         |                                      |
          |         +--------------------------------------+
          |
          |           ^     ^     +          ^    +
          |        (A)|  (C)|  (E)|       (F)|    |(H)
          v           v     +     v          +    v

   +-------------------------------------------------+
   |                                                 |
   |                   Browser                       |
   |                                                 |
   +-------------------------------------------------+

   In this architecture, commonly referred to as "backend for frontend"
   or "BFF", the JavaScript code is loaded from a dynamic BFF Proxy (A)
   that has the ability to execute code and handle the full
   authentication flow itself.  This enables the ability to keep the
   call to actually get an access token outside the JavaScript
   application.

   Note that this BFF Proxy is not the Resource Server, it is the OAuth
   client and would be accessing data at a separate resource server.

   In this case, the BFF Proxy initiates the OAuth flow itself, by
   redirecting the browser to the authorization endpoint (B).  When the
   user is redirected back, the browser delivers the authorization code
   to the BFF Proxy (C), where it can then exchange it for an access
   token at the token endpoint (D) using its client secret and PKCE code
   verifier.  The BFF Proxy then keeps the access token and refresh
   token stored internally, and creates a separate session with the
   browser-based app via a traditional browser cookie (E).






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   When the JavaScript application in the browser wants to make a
   request to the Resource Server, it instead makes the request to the
   BFF Proxy (F), and the BFF Proxy will make the request with the
   access token to the Resource Server (G), and forward the response (H)
   back to the browser.

   (Common examples of this architecture are an Angular front-end with a
   .NET backend, or a React front-end with a Spring Boot backend.)

   The BFF Proxy SHOULD be considered a confidential client, and issued
   its own client secret.  The BFF Proxy SHOULD use the OAuth 2.0
   Authorization Code grant with PKCE to initiate a request for an
   access token.  Detailed recommendations for confidential clients can
   be found in [oauth-security-topics] Section 2.1.1.

   In this scenario, the connection between the browser and BFF Proxy
   SHOULD be a session cookie provided by the BFF Proxy.

6.2.1.  Security considerations

   Security of the connection between code running in the browser and
   this BFF Proxy is assumed to utilize browser-level protection
   mechanisms.  Details are out of scope of this document, but many
   recommendations can be found in the OWASP Cheat Sheet series
   (https://cheatsheetseries.owasp.org/), such as setting an HTTP-only
   and Secure cookie to authenticate the session between the browser and
   BFF Proxy.  Additionally, cookies MUST be protected from leakage by
   other means, such as logs.

   In this architecture, tokens are never sent to the front-end and are
   never accessible by any JavaScript code, so it fully protects against
   XSS attackers stealing tokens.  However, an XSS attacker may still be
   able to make authenticated requests to the BFF Proxy which will in
   turn make requests to the resource server including the user's
   legitimate token.  While the attacker is unable to extract and use
   the access token elsewhere, they can still effectively make
   authenticated requests to the resource server.

6.3.  Token Mediating Backend

   An alternative to a full BFF where all resource requests go through
   the backend is to use a token mediating backend which obtains the
   tokens and then forwards the tokens to the browser.








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   +-------------+  +--------------+ +---------------+
   |             |  |              | |               |
   |Authorization|  |    Token     | |   Resource    |
   |  Endpoint   |  |   Endpoint   | |    Server     |
   |             |  |              | |               |
   +-------------+  +--------------+ +---------------+

          ^                ^                      ^
          |             (D)|                      |
          |                v                      |
          |                                       |
          |    +-------------------------+        |
          |    |                         |        |
          |    | Token Mediating Backend |        |
       (B)|    |                         |        |
          |    +-------------------------+        |
          |                                       |
          |           ^     ^     +               |
          |        (A)|  (C)|  (E)|            (F)|
          v           v     +     v               +

   +-------------------------------------------------+
   |                                                 |
   |                   Browser                       |
   |                                                 |
   +-------------------------------------------------+

   The frontend code makes a request to the Token Mediating Backend (A),
   and the backend initiates the OAuth flow itself, by redirecting the
   browser to the authorization endpoint (B).  When the user is
   redirected back, the browser delivers the authorization code to the
   application server (C), where it can then exchange it for an access
   token at the token endpoint (D) using its client secret and PKCE code
   verifier.  The backend delivers the tokens to the browser (E), which
   stores them for later use.  The browser makes requests to the
   resource server directly (F) including the token it has stored.

   The main advantage this architecture provides over the full BFF
   architecture previously described is that the backend service is only
   involved in the acquisition of tokens, and doesn't have to proxy
   every request in the future.  Routing every API call through a
   backend can be expensive in terms of performance and latency, and can
   create challenges in deploying the application across many regions.
   Instead, routing only the token acquisition through a backend means
   fewer requests are made to the backend.  This improves the
   performance and reduces the latency of requests from the frontend,
   and reduces the amount of infrastructure needed in the backend.




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   Similar to the previously described BFF Proxy pattern, The Token
   Mediating Backend SHOULD be considered a confidential client, and
   issued its own client secret.  The Token Mediating Backend SHOULD use
   the OAuth 2.0 Authorization Code grant with PKCE to initiate a
   request for an access token.  Detailed recommendations for
   confidential clients can be found in [oauth-security-topics]
   Section 2.1.1.

   In this scenario, the connection between the browser and Token
   Mediating Backend SHOULD be a session cookie provided by the backend.

   The Token Mediating Backend SHOULD cache tokens it obtains from the
   authorization server such that when the frontend needs to obtain new
   tokens, it can do so without the additional round trip to the
   authorization server if the tokens are still valid.

   The frontend SHOULD NOT persist tokens in local storage or similar
   mechanisms; instead, the frontend SHOULD store tokens only in memory,
   and make a new request to the backend if no tokens exist.  This
   provides fewer attack vectors for token exfiltration should an XSS
   attack be successful.

   Editor's Note: A method of implementing this architecture is
   described by the [tmi-bff] draft, although it is currently an expired
   individual draft and has not been proposed for adoption to the OAuth
   Working Group.

6.3.1.  Security Considerations

   If the backend caches tokens from the authorization server, it
   presents scopes elevation risks if applied indiscriminately.  If the
   token cached by the authorization server features a superset of the
   scopes requested by the frontend, the backend SHOULD NOT return it to
   the frontend; instead it SHOULD perform a new request with the
   smaller set of scopes to the authorization server.

   In the case of a successful XSS attack, the attacker may be able to
   access the tokens if the tokens are persisted in the frontend, but is
   less likely to be able to access the tokens if they are stored only
   in memory.  However, a successful XSS attack will also allow the
   attacker to call the Token Mediating Backend itself to retrieve the
   cached token or start a new OAuth flow.

6.4.  JavaScript Applications obtaining tokens directly

   This section describes the architecture of a JavaScript application
   obtaining tokens from the authorization itself, with no intermediate
   proxy server.



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                         +---------------+           +--------------+
                         |               |           |              |
                         | Authorization |           |   Resource   |
                         |    Server     |           |    Server    |
                         |               |           |              |
                         +---------------+           +--------------+

                                ^     ^                 ^     +
                                |     |                 |     |
                                |(B)  |(C)              |(D)  |(E)
                                |     |                 |     |
                                |     |                 |     |
                                +     v                 +     v

   +-----------------+         +-------------------------------+
   |                 |   (A)   |                               |
   | Static Web Host | +-----> |           Browser             |
   |                 |         |                               |
   +-----------------+         +-------------------------------+

   In this architecture, the JavaScript code is first loaded from a
   static web host into the browser (A), and the application then runs
   in the browser.  This application is considered a public client,
   since there is no way to issue it a client secret in this model.

   The code in the browser initiates the Authorization Code flow with
   the PKCE extension (described in Section 7) (B) above, and obtains an
   access token via a POST request (C).

   The application is then responsible for storing the access token (and
   optional refresh token) as securely as possible using appropriate
   browser APIs.

   When the JavaScript application in the browser wants to make a
   request to the Resource Server, it can interact with the Resource
   Server directly.  It includes the access token in the request (D) and
   receives the Resource Server's response (E).

   In this scenario, the Authorization Server and Resource Server MUST
   support the necessary CORS headers to enable the JavaScript code to
   make these POST requests from the domain on which the script is
   executing.  (See Section 9.8 for additional details.)









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   Besides the general risks of XSS, if tokens are stored or handled by
   the browser, XSS poses an additional risk of token exfiltration.  In
   this architecture, the JavaScript application is storing the access
   token so that it can make requests directly to the resource server.
   There are two primary methods by which the application can store the
   token, with different security considerations of each.

6.4.1.  Storing Tokens in Local or Session Storage

   If the JavaScript in the DOM will be making requests directly to the
   resource server, the simplest mechanism is to store the tokens
   somewhere accessible to the DOM.

   In case of a successful XSS attack, the injected code will have full
   access to the stored tokens and can exfiltrate them to the attacker.

6.4.2.  Service Worker as the OAuth Client

   In this scenario, a Service Worker (https://developer.mozilla.org/en-
   US/docs/Web/API/Service_Worker_API) is responsible for obtaining
   tokens from the authorization server and making requests to the
   resource server.

   Service workers are run in a separate context from the DOM, have no
   access to the DOM, and the DOM has no access to the service worker.
   This makes service workers the most secure place to store tokens, as
   an XSS attack is unable to exfiltrate the tokens.

   In this architecture, a service worker intercepts calls from the
   frontend to the resource server.  As such, it completely isolates
   calls to the authorization server from XSS attack surface, as all
   tokens are safely kept in the service worker context without any
   access from other JavaScript contexts.  The service worker is then
   solely responsible for adding the token in the authorization header
   to calls to the resource server.
















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                                                                 Resource               Authorization
  User       Application        Service Worker                    server                   server
   |   browse     |                   |                              |                        |
   | ------------>|                   |                              |                        |
   |              |------------------->                              |           /authorize   |
   |              |                   -------------------------------------------------------->
   |              |                   |                 redirect w/ authorization code        |
   |              |                   < - - - - - - - - - - - - - - - - - - - - - - - - - - - |
   |              |                   |                              |                        |
   |              |                   |  token request w/ auth code  |               /token   |
   |              |                   | ------------------------------------------------------>
   |              |                   | <- - - - - - - - - - - - - - - - - - - - - - - - - - -|
   |              |                   |                              |                        |
   |              | resource request  |                              |                        |
   |              |-------------------> resource request with token  |                        |
   |              |                   | ---------------------------->|                        |
   |              |                   |                              |                        |
  User       Application        Service Worker                   Resource               Authorization
                                                                  server                   server

6.4.2.1.  Implementation Guidelines

   *  The service worker MUST initiate the OAuth 2.0 Authorization Code
      grant with PKCE itself.

   *  The service worker MUST intercept the authorization code when the
      _authorization server_ redirects to the application.

   *  The service worker implementation MUST then initiate the token
      request itself.

   *  The service worker MUST not transmit tokens, authorization codes
      or PKCE secrets (e.g. code verifier) to the frontend application.

   *  The service worker MUST block authorization requests and token
      requests initiating from the frontend application in order to
      avoid any front-end side-channel for getting credentials: the only
      way of starting the authorization flow is through the service
      worker.  This protects against re-authorization from XSS-injected
      code.

   *  The application MUST register the Service Worker before running
      any code interacting with the user.








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6.4.2.2.  Security Considerations

   A successful XSS attack on an application using this Service Worker
   pattern would be unable to exfiltrate existing tokens stored by the
   application.  However, an XSS attacker may still be able to cause the
   Service Worker to make authenticated requests to the resource server
   including the user's legitimate token.

   In case of a vulnerability leading to the Service Worker not being
   registered, an XSS attack would result in the attacker being able to
   initiate a new OAuth flow to obtain new tokens itself.

   To prevent the Service Worker from being unregistered, the Service
   Worker registration must happen as first step of the application
   start, and before any user interaction.  Starting the Service worker
   before the rest of the application, and the fact that there is no way
   to remove a Service Worker from an active application
   (https://www.w3.org/TR/service-workers/#navigator-service-worker-
   unregister), reduces the risk of an XSS attack being able to prevent
   the Service Worker from being registered.

7.  Authorization Code Flow

   Browser-based applications that are public clients and use the
   Authorization Code grant type described in Section 4.1 of OAuth 2.0
   [RFC6749] MUST also follow these additional requirements described in
   this section.

7.1.  Initiating the Authorization Request from a Browser-Based
      Application

   Browser-based applications that are public clients MUST implement the
   Proof Key for Code Exchange (PKCE [RFC7636]) extension when obtaining
   an access token, and authorization servers MUST support and enforce
   PKCE for such clients.

   The PKCE extension prevents an attack where the authorization code is
   intercepted and exchanged for an access token by a malicious client,
   by providing the authorization server with a way to verify the client
   instance that exchanges the authorization code is the same one that
   initiated the flow.

   Browser-based applications MUST prevent CSRF attacks against their
   redirect URI.  This can be accomplished by any of the below:

   *  using PKCE, and confirming that the authorization server supports
      PKCE




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   *  using a unique value for the OAuth 2.0 "state" parameter

   *  if the application is using OpenID Connect, by using the OpenID
      Connect "nonce" parameter

7.2.  Handling the Authorization Code Redirect

   Authorization servers MUST require an exact match of a registered
   redirect URI.  As described in [oauth-security-topics] Section 4.1.1.
   this helps to prevent attacks targeting the authorization code.

8.  Refresh Tokens

   Refresh tokens provide a way for applications to obtain a new access
   token when the initial access token expires.  With public clients,
   the risk of a leaked refresh token is greater than leaked access
   tokens, since an attacker may be able to continue using the stolen
   refresh token to obtain new access tokens potentially without being
   detectable by the authorization server.

   Javascript-accessible storage mechanisms like _Local Storage_ provide
   an attacker with several opportunities by which a refresh token can
   be leaked, just as with access tokens.  As such, these mechanisms are
   considered a higher risk for handling refresh tokens.

   Authorization servers may choose whether or not to issue refresh
   tokens to browser-based applications. [oauth-security-topics]
   describes some additional requirements around refresh tokens on top
   of the recommendations of [RFC6749].  Applications and authorization
   servers conforming to this BCP MUST also follow the recommendations
   in [oauth-security-topics] around refresh tokens if refresh tokens
   are issued to browser-based applications.

   In particular, authorization servers:

   *  MUST either rotate refresh tokens on each use OR use sender-
      constrained refresh tokens as described in [oauth-security-topics]
      Section 4.13.2

   *  MUST either set a maximum lifetime on refresh tokens OR expire if
      the refresh token has not been used within some amount of time

   *  MUST NOT extend the lifetime of the new refresh token beyond the
      lifetime of the initial refresh token







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   *  upon issuing a rotated refresh token, MUST NOT extend the lifetime
      of the new refresh token beyond the lifetime of the initial
      refresh token if the refresh token has a preestablished expiration
      time

   For example:

   *  A user authorizes an application, issuing an access token that
      lasts 1 hour, and a refresh token that lasts 24 hours

   *  After 1 hour, the initial access token expires, so the application
      uses the refresh token to get a new access token

   *  The authorization server returns a new access token that lasts 1
      hour, and a new refresh token that lasts 23 hours

   *  This continues until 24 hours pass from the initial authorization

   *  At this point, when the application attempts to use the refresh
      token after 24 hours, the request will fail and the application
      will have to involve the user in a new authorization request

   By limiting the overall refresh token lifetime to the lifetime of the
   initial refresh token, this ensures a stolen refresh token cannot be
   used indefinitely.

   Authorization servers MAY set different policies around refresh token
   issuance, lifetime and expiration for browser-based applications
   compared to other public clients.

9.  Security Considerations

9.1.  Cross-Site Scripting Attacks (XSS)

   For all known architectures, all precautions MUST be taken to prevent
   cross-site scripting (XSS) attacks.  In general, XSS attacks are a
   huge risk, and can lead to full compromise of the application.

   If tokens are handled or accessible by the browser, there is a risk
   that a XSS attack can lead to token exfiltration.

   Even if tokens are never sent to the frontend and are never
   accessible by any JavaScript code, an XSS attacker may still be able
   to make authenticated requests to the resource server by mimicking
   legitimate code in the DOM.  For example, the attacker may make a
   request to the BFF Proxy which will in turn make requests to the
   resource server including the user's legitimate token.  In the
   Service Worker example, the attacker may make an API call to the



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   Service Worker which will then turn around and make a request to the
   resource server with the legitimate token.  While the attacker is
   unable to extract and use the access token elsewhere, they can still
   effectively make authenticated requests to the resource server to
   steal or modify data.

9.2.  Reducing the Impact of Token Exfiltration

   If tokens are ever accessible to the browser or to any JavaScript
   code, there is always a risk of token exfiltration.  The particular
   risk may change depending on the architecture chosen.  Regardless of
   the particular architecture chosen, these additional security
   considerations limit the impact of token exfiltration:

   *  The authorization server SHOULD restrict access tokens to strictly
      needed resources, to avoid escalating the scope of the attack.

   *  To avoid information disclosure from ID Tokens, the authorization
      server SHOULD NOT include any ID token claims that aren't used by
      the frontend.

   *  Refresh tokens should be used in accordance with the guidance in
      Section 8.

9.3.  Registration of Browser-Based Apps

   Browser-based applications (with no backend) are considered public
   clients as defined by Section 2.1 of OAuth 2.0 [RFC6749], and MUST be
   registered with the authorization server as such.  Authorization
   servers MUST record the client type in the client registration
   details in order to identify and process requests accordingly.

   Authorization servers MUST require that browser-based applications
   register one or more redirect URIs.

9.4.  Client Authentication

   Since a browser-based application's source code is delivered to the
   end-user's browser, it cannot contain provisioned secrets.  As such,
   a browser-based app with native OAuth support is considered a public
   client as defined by Section 2.1 of OAuth 2.0 [RFC6749].










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   Secrets that are statically included as part of an app distributed to
   multiple users should not be treated as confidential secrets, as one
   user may inspect their copy and learn the shared secret.  For this
   reason, and those stated in Section 5.3.1 of [RFC6819], it is NOT
   RECOMMENDED for authorization servers to require client
   authentication of browser-based applications using a shared secret,
   as this serves little value beyond client identification which is
   already provided by the client_id request parameter.

   Authorization servers that still require a statically included shared
   secret for SPA clients MUST treat the client as a public client, and
   not accept the secret as proof of the client's identity.  Without
   additional measures, such clients are subject to client impersonation
   (see Section 9.5 below).

9.5.  Client Impersonation

   As stated in Section 10.2 of OAuth 2.0 [RFC6749], the authorization
   server SHOULD NOT process authorization requests automatically
   without user consent or interaction, except when the identity of the
   client can be assured.

   If authorization servers restrict redirect URIs to a fixed set of
   absolute HTTPS URIs, preventing the use of wildcard domains, wildcard
   paths, or wildcard query string components, this exact match of
   registered absolute HTTPS URIs MAY be accepted by authorization
   servers as proof of identity of the client for the purpose of
   deciding whether to automatically process an authorization request
   when a previous request for the client_id has already been approved.

9.6.  Cross-Site Request Forgery Protections

   Clients MUST prevent Cross-Site Request Forgery (CSRF) attacks
   against their redirect URI.  Clients can accomplish this by either
   ensuring the authorization server supports PKCE and relying on the
   CSRF protection that PKCE provides, or if the client is also an
   OpenID Connect client, using the OpenID Connect "nonce" parameter, or
   by using the "state" parameter to carry one-time-use CSRF tokens as
   described in Section 7.1.

   See Section 2.1 of [oauth-security-topics] for additional details.

9.7.  Authorization Server Mix-Up Mitigation

   Authorization server mix-up attacks mark a severe threat to every
   client that supports at least two authorization servers.  To conform
   to this BCP such clients MUST apply countermeasures to defend against
   mix-up attacks.



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   It is RECOMMENDED to defend against mix-up attacks by identifying and
   validating the issuer of the authorization response.  This can be
   achieved either by using the "iss" response parameter, as defined in
   [oauth-iss-auth-resp], or by using the "iss" Claim of the ID token
   when OpenID Connect is used.

   Alternative countermeasures, such as using distinct redirect URIs for
   each issuer, SHOULD only be used if identifying the issuer as
   described is not possible.

   Section 4.4 of [oauth-security-topics] provides additional details
   about mix-up attacks and the countermeasures mentioned above.

9.8.  Cross-Domain Requests

   To complete the Authorization Code flow, the browser-based
   application will need to exchange the authorization code for an
   access token at the token endpoint.  If the authorization server
   provides additional endpoints to the application, such as metadata
   URLs, dynamic client registration, revocation, introspection,
   discovery or user info endpoints, these endpoints may also be
   accessed by the browser-based app.  Since these requests will be made
   from a browser, authorization servers MUST support the necessary CORS
   headers (defined in [Fetch]) to allow the browser to make the
   request.

   This specification does not include guidelines for deciding whether a
   CORS policy for the token endpoint should be a wildcard origin or
   more restrictive.  Note, however, that the browser will attempt to
   GET or POST to the API endpoint before knowing any CORS policy; it
   simply hides the succeeding or failing result from JavaScript if the
   policy does not allow sharing.

9.9.  Content Security Policy

   A browser-based application that wishes to use either long-lived
   refresh tokens or privileged scopes SHOULD restrict its JavaScript
   execution to a set of statically hosted scripts via a Content
   Security Policy ([CSP2]) or similar mechanism.  A strong Content
   Security Policy can limit the potential attack vectors for malicious
   JavaScript to be executed on the page.










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9.10.  OAuth Implicit Flow

   The OAuth 2.0 Implicit flow (defined in Section 4.2 of OAuth 2.0
   [RFC6749]) works by the authorization server issuing an access token
   in the authorization response (front channel) without the code
   exchange step.  In this case, the access token is returned in the
   fragment part of the redirect URI, providing an attacker with several
   opportunities to intercept and steal the access token.

   Authorization servers MUST NOT issue access tokens in the
   authorization response, and MUST issue access tokens only from the
   token endpoint.

9.10.1.  Attacks on the Implicit Flow

   Many attacks on the Implicit flow described by [RFC6819] and
   Section 4.1.2 of [oauth-security-topics] do not have sufficient
   mitigation strategies.  The following sections describe the specific
   attacks that cannot be mitigated while continuing to use the Implicit
   flow.

9.10.1.1.  Threat: Manipulation of the Redirect URI

   If an attacker is able to cause the authorization response to be sent
   to a URI under their control, they will directly get access to the
   authorization response including the access token.  Several methods
   of performing this attack are described in detail in
   [oauth-security-topics].

9.10.1.2.  Threat: Access Token Leak in Browser History

   An attacker could obtain the access token from the browser's history.
   The countermeasures recommended by [RFC6819] are limited to using
   short expiration times for tokens, and indicating that browsers
   should not cache the response.  Neither of these fully prevent this
   attack, they only reduce the potential damage.

   Additionally, many browsers now also sync browser history to cloud
   services and to multiple devices, providing an even wider attack
   surface to extract access tokens out of the URL.

   This is discussed in more detail in Section 4.3.2 of
   [oauth-security-topics].








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9.10.1.3.  Threat: Manipulation of Scripts

   An attacker could modify the page or inject scripts into the browser
   through various means, including when the browser's HTTPS connection
   is being intercepted by, for example, a corporate network.  While
   man-in-the-middle attacks are typically out of scope of basic
   security recommendations to prevent, in the case of browser-based
   apps they are much easier to perform.  An injected script can enable
   an attacker to have access to everything on the page.

   The risk of a malicious script running on the page may be amplified
   when the application uses a known standard way of obtaining access
   tokens, namely that the attacker can always look at the
   window.location variable to find an access token.  This threat
   profile is different from an attacker specifically targeting an
   individual application by knowing where or how an access token
   obtained via the Authorization Code flow may end up being stored.

9.10.1.4.  Threat: Access Token Leak to Third-Party Scripts

   It is relatively common to use third-party scripts in browser-based
   apps, such as analytics tools, crash reporting, and even things like
   a Facebook or Twitter "like" button.  In these situations, the author
   of the application may not be able to be fully aware of the entirety
   of the code running in the application.  When an access token is
   returned in the fragment, it is visible to any third-party scripts on
   the page.

9.10.2.  Countermeasures

   In addition to the countermeasures described by [RFC6819] and
   [oauth-security-topics], using the Authorization Code flow with PKCE
   extension prevents the attacks described above by avoiding returning
   the access token in the redirect response at all.

   When PKCE is used, if an authorization code is stolen in transport,
   the attacker is unable to do anything with the authorization code.

9.10.3.  Disadvantages of the Implicit Flow

   There are several additional reasons the Implicit flow is
   disadvantageous compared to using the standard Authorization Code
   flow.








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   *  OAuth 2.0 provides no mechanism for a client to verify that a
      particular access token was intended for that client, which could
      lead to misuse and possible impersonation attacks if a malicious
      party hands off an access token it retrieved through some other
      means to the client.

   *  Returning an access token in the front-channel redirect gives the
      authorization server no assurance that the access token will
      actually end up at the application, since there are many ways this
      redirect may fail or be intercepted.

   *  Supporting the Implicit flow requires additional code, more upkeep
      and understanding of the related security considerations, while
      limiting the authorization server to just the Authorization Code
      flow reduces the attack surface of the implementation.

   *  If the JavaScript application gets wrapped into a native app, then
      [RFC8252] also requires the use of the Authorization Code flow
      with PKCE anyway.

   In OpenID Connect, the ID Token is sent in a known format (as a JWT),
   and digitally signed.  Returning an ID token using the Implicit flow
   (response_type=id_token) requires the client validate the JWT
   signature, as malicious parties could otherwise craft and supply
   fraudulent ID tokens.  Performing OpenID Connect using the
   Authorization Code flow provides the benefit of the client not
   needing to verify the JWT signature, as the ID token will have been
   fetched over an HTTPS connection directly from the authorization
   server.  Additionally, in many cases an application will request both
   an ID token and an access token, so it is simplier and provides fewer
   attack vectors to obtain both via the Authorization Code flow.

9.10.4.  Historic Note

   Historically, the Implicit flow provided an advantage to browser-
   based apps since JavaScript could always arbitrarily read and
   manipulate the fragment portion of the URL without triggering a page
   reload.  This was necessary in order to remove the access token from
   the URL after it was obtained by the app.

   Modern browsers now have the Session History API (described in
   "Session history and navigation" of [HTML]), which provides a
   mechanism to modify the path and query string component of the URL
   without triggering a page reload.  This means modern browser-based
   apps can use the unmodified OAuth 2.0 Authorization Code flow, since
   they have the ability to remove the authorization code from the query
   string without triggering a page reload thanks to the Session History
   API.



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9.11.  Additional Security Considerations

   The OWASP Foundation (https://www.owasp.org/) maintains a set of
   security recommendations and best practices for web applications, and
   it is RECOMMENDED to follow these best practices when creating an
   OAuth 2.0 Browser-Based application.

10.  IANA Considerations

   This document does not require any IANA actions.

11.  References

11.1.  Normative References

   [CSP2]     West, M., "Content Security Policy", October 2018.

   [draft-ietf-httpbis-rfc6265bis]
              Chen, L., Englehardt, S., West, M., and J. Wilander,
              "Cookies: HTTP State Management Mechanism", October 2021.

   [Fetch]    whatwg, "Fetch", 2018.

   [oauth-iss-auth-resp]
              Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0
              Authorization Server Issuer Identifier in Authorization
              Response", January 2021.

   [oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", April 2021.

   [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>.

   [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>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.






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   [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>.

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

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
              <https://www.rfc-editor.org/info/rfc8252>.

11.2.  Informative References

   [HTML]     whatwg, "HTML", 2020.

   [tmi-bff]  Bertocci, V. and B. Cambpell, "Token Mediating and session
              Information Backend For Frontend", April 2021.

Appendix A.  Server Support Checklist

   OAuth authorization servers that support browser-based apps MUST:

   1.  Require "https" scheme redirect URIs.

   2.  Require exact matching of registered redirect URIs.

   3.  Support PKCE [RFC7636].  Required to protect authorization code
       grants sent to public clients.  See Section 7.1

   4.  Support cross-domain requests at the token endpoint in order to
       allow browsers to make the authorization code exchange request.
       See Section 9.8

   5.  Not assume that browser-based clients can keep a secret, and
       SHOULD NOT issue secrets to applications of this type.

   6.  Not support the Resource Owner Password grant for browser-based
       clients.

   7.  Follow the [oauth-security-topics] recommendations on refresh
       tokens, as well as the additional requirements described in
       Section 8.






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Appendix B.  Document History

   [[ To be removed from the final specification ]]

   -11

   *  Added a new architecture pattern: Token Mediating Backend

   *  Revised and added clarifications for the Service Worker pattern

   *  Editorial improvements in descriptions of the different
      architectures

   *  Rephrased headers

   -10

   *  Revised the names of the architectural patterns

   *  Added a new pattern using a service worker as the OAuth client to
      manage tokens

   *  Added some considerations when storing tokens in Local or Session
      Storage

   -09

   *  Provide additional context for the same-domain architecture
      pattern

   *  Added reference to draft-ietf-httpbis-rfc6265bis to clarify that
      SameSite is not the only CSRF protection measure needed

   *  Editorial improvements

   -08

   *  Added a note to use the "Secure" cookie attribute in addition to
      SameSite etc

   *  Updates to bring this draft in sync with the latest Security BCP

   *  Updated text for mix-up countermeasures to reference the new "iss"
      extension

   *  Changed "SHOULD" for refresh token rotation to MUST either use
      rotation or sender-constraining to match the Security BCP




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   *  Fixed references to other specs and extensions

   *  Editorial improvements in descriptions of the different
      architectures

   -07

   *  Clarify PKCE requirements apply only to issuing access tokens

   *  Change "MUST" to "SHOULD" for refresh token rotation

   *  Editorial clarifications

   -06

   *  Added refresh token requirements to AS summary

   *  Editorial clarifications

   -05

   *  Incorporated editorial and substantive feedback from Mike Jones

   *  Added references to "nonce" as another way to prevent CSRF attacks

   *  Updated headers in the Implicit Flow section to better represent
      the relationship between the paragraphs

   -04

   *  Disallow the use of the Password Grant

   *  Add PKCE support to summary list for authorization server
      requirements

   *  Rewrote refresh token section to allow refresh tokens if they are
      time-limited, rotated on each use, and requiring that the rotated
      refresh token lifetimes do not extend past the lifetime of the
      initial refresh token, and to bring it in line with the Security
      BCP

   *  Updated recommendations on using state to reflect the Security BCP

   *  Updated server support checklist to reflect latest changes

   *  Updated the same-domain JS architecture section to emphasize the
      architecture rather than domain




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   *  Editorial clarifications in the section that talks about OpenID
      Connect ID tokens

   -03

   *  Updated the historic note about the fragment URL clarifying that
      the Session History API means browsers can use the unmodified
      authorization code flow

   *  Rephrased "Authorization Code Flow" intro paragraph to better lead
      into the next two sections

   *  Softened "is likely a better decision to avoid using OAuth
      entirely" to "it may be..." for common-domain deployments

   *  Updated abstract to not be limited to public clients, since the
      later sections talk about confidential clients

   *  Removed references to avoiding OpenID Connect for same-domain
      architectures

   *  Updated headers to better describe architectures (Apps Served from
      a Static Web Server -> JavaScript Applications without a Backend)

   *  Expanded "same-domain architecture" section to better explain the
      problems that OAuth has in this scenario

   *  Referenced Security BCP in implicit flow attacks where possible

   *  Minor typo corrections

   -02

   *  Rewrote overview section incorporating feedback from Leo Tohill

   *  Updated summary recommendation bullet points to split out
      application and server requirements

   *  Removed the allowance on hostname-only redirect URI matching, now
      requiring exact redirect URI matching

   *  Updated Section 6.2 to drop reference of SPA with a backend
      component being a public client

   *  Expanded the architecture section to explicitly mention three
      architectural patterns available to JS apps

   -01



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   *  Incorporated feedback from Torsten Lodderstedt

   *  Updated abstract

   *  Clarified the definition of browser-based apps to not exclude
      applications cached in the browser, e.g. via Service Workers

   *  Clarified use of the state parameter for CSRF protection

   *  Added background information about the original reason the
      implicit flow was created due to lack of CORS support

   *  Clarified the same-domain use case where the SPA and API share a
      cookie domain

   *  Moved historic note about the fragment URL into the Overview

Appendix C.  Acknowledgements

   The authors would like to acknowledge the work of William Denniss and
   John Bradley, whose recommendation for native apps informed many of
   the best practices for browser-based applications.  The authors would
   also like to thank Hannes Tschofenig and Torsten Lodderstedt, the
   attendees of the Internet Identity Workshop 27 session at which this
   BCP was originally proposed, and the following individuals who
   contributed ideas, feedback, and wording that shaped and formed the
   final specification:

   Annabelle Backman, Brian Campbell, Brock Allen, Christian Mainka,
   Daniel Fett, George Fletcher, Hannes Tschofenig, Janak Amarasena,
   John Bradley, Joseph Heenan, Justin Richer, Karl McGuinness, Karsten
   Meyer zu Selhausen, Leo Tohill, Mike Jones, Philippe De Ryck, Tomek
   Stojecki, Torsten Lodderstedt, Vittorio Bertocci and Yannick Majoros.

Authors' Addresses

   Aaron Parecki
   Okta
   Email: aaron@parecki.com
   URI:   https://aaronparecki.com


   David Waite
   Ping Identity
   Email: david@alkaline-solutions.com






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