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

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Active".
	Authors	Aaron Parecki , David Waite
	Last updated	2022-09-13 (Latest revision 2022-09-06)
	Replaces	draft-parecki-oauth-browser-based-apps
	RFC stream	Internet Engineering Task Force (IETF)
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	Associated WG milestone	Oct 2021 Submit "OAuth 2.0 for Browser-Based Apps" to IES
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draft-ietf-oauth-browser-based-apps-11

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.

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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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^ ^ ^
| (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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^ ^ ^
| (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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^ ^ ^ +
| | | |
|(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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