Open Authentication Protocol A. Parecki
Internet-Draft Okta
Intended status: Best Current Practice D. Waite
Expires: March 25, 2020 Ping Identity
September 22, 2019
OAuth 2.0 for Browser-Based Apps
draft-ietf-oauth-browser-based-apps-04
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.
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 25, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. First-Party Applications . . . . . . . . . . . . . . . . . . 4
6. Application Architecture Patterns . . . . . . . . . . . . . . 5
6.1. Browser-Based Apps that Can Share Data with the Resource
Server . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.2. JavaScript Applications with a Backend . . . . . . . . . 6
6.3. JavaScript Applications without a Backend . . . . . . . . 8
7. Authorization Code Flow . . . . . . . . . . . . . . . . . . . 9
7.1. Initiating the Authorization Request from a Browser-Based
Application . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Handling the Authorization Code Redirect . . . . . . . . 9
8. Refresh Tokens . . . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9.1. Registration of Browser-Based Apps . . . . . . . . . . . 11
9.2. Client Authentication . . . . . . . . . . . . . . . . . . 11
9.3. Client Impersonation . . . . . . . . . . . . . . . . . . 11
9.4. Cross-Site Request Forgery Protections . . . . . . . . . 12
9.5. Authorization Server Mix-Up Mitigation . . . . . . . . . 12
9.6. Cross-Domain Requests . . . . . . . . . . . . . . . . . . 12
9.7. Content-Security Policy . . . . . . . . . . . . . . . . . 13
9.8. OAuth Implicit Grant Authorization Flow . . . . . . . . . 13
9.8.1. Threat: Interception of the Redirect URI . . . . . . 13
9.8.2. Threat: Access Token Leak in Browser History . . . . 13
9.8.3. Threat: Manipulation of Scripts . . . . . . . . . . . 14
9.8.4. Threat: Access Token Leak to Third Party Scripts . . 14
9.8.5. Countermeasures . . . . . . . . . . . . . . . . . . . 14
9.8.6. Disadvantages of the Implicit Flow . . . . . . . . . 14
9.8.7. Historic Note . . . . . . . . . . . . . . . . . . . . 15
9.9. Additional Security Considerations . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Server Support Checklist . . . . . . . . . . . . . . 17
Appendix B. Document History . . . . . . . . . . . . . . . . . . 17
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
This specification describes the current best practices for
implementing OAuth 2.0 authorization flows in applications running
entirely in a browser.
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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 as well as 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.
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].
"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 RFC 6749 was 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
the principal motivation 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.
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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 9.8 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 assures 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:
o Use the OAuth 2.0 authorization code flow with the PKCE extension
o Protect themselves against CSRF attacks by using the OAuth 2.0
state parameter to carry one-time use CSRF tokens, or by ensuring
the authorization server supports PKCE
o Register one or more redirect URIs, and not vary the redirect URI
per authorization request
OAuth 2.0 authorization servers MUST:
o Require exact matching of registered redirect URIs
o Support the PKCE extension
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.
For example, 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
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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 the OAuth Authorization Code flow as
described later in this document.
The Resource Owner Password Grant MUST NOT be used, as described in
[oauth-security-topics] section 3.4.
By using the Authorization Code flow and redirecting the user to the
authorization server, this provides the authorization server the
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 Password grant does not
provide any built-in mechanism for these, and would instead be
extended with custom code.
6. Application Architecture Patterns
There are three primary architectural patterns available when
building browser-based applications.
o a JavaScript application with no backend, accessing resource
servers directly
o a JavaScript application with a backend
o a JavaScript application that has methods of sharing data with
resource servers, such as using common-domain cookies
These three architectures have different use cases and
considerations.
6.1. Browser-Based Apps that Can Share Data with the Resource Server
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), 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], but if the application, AS and API share a
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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 there is no secure storage mechanism where JavaScript code can
keep the access token to be later used in an API request. Using an
OAuth flow results in the JavaScript code getting an access token,
needing to store it somewhere, and then retrieve it to make an API
request. Instead, a more secure design is to use an HTTP-only cookie
between the JavaScript application and API so that the JavaScript
code can't access the cookie value itself.
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, using OAuth even in a common-domain
architecture does mean you can more easily rearchitect things later,
such as if you were to later add a new domain to the system.
6.2. JavaScript Applications with a Backend
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+-------------+
| |
|Authorization|
| Server |
| |
+-------------+
^ +
|(A) |(B)
| |
+ v
+-------------+ +--------------+
| | +---------> | |
| Application | (C) | Resource |
| Server | | Server |
| | <---------+ | |
+-------------+ (D) +--------------+
^ +
| |
| | browser
| | cookie
| |
+ v
+-------------+
| |
| Browser |
| |
+-------------+
In this architecture, the JavaScript code is loaded from a dynamic
Application Server that also has the ability to execute code itself.
This enables the ability to keep all of the steps involved in
obtaining an access token outside of the JavaScript application.
(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 Application Server SHOULD be considered a confidential client,
and issued its own client secret. The Application Server SHOULD use
the OAuth 2.0 authorization code grant to initiate a request for an
access token. Upon handling the redirect from the Authorization
Server, the Application Server will request an access token using the
authorization code returned (A), which will be returned to the
Application Server (B). The Application Server utilizes its own
session with the browser to store the access token.
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When the JavaScript application in the browser wants to make a
request to the Resource Server, it MUST instead make the request to
the Application Server, and the Application Server will make the
request with the access token to the Resource Server (C), and forward
the response (D) back to the browser.
Security of the connection between code running in the browser and
this Application Server is assumed to utilize browser-level
protection mechanisms. Details are out of scope of this document,
but many recommendations can be found at the OWASP Foundation
(https://www.owasp.org/), such as setting an HTTP-only and Secure
cookie to authenticate the session between the browser and
Application Server.
In this scenario, the session between the browser and Application
Server MAY be either a session cookie provided by the Application
Server, OR the access token itself. Note that if the access token is
used as the session identifier, this exposes the access token to the
end user even if it is not available to the JavaScript application,
so some authorization servers may wish to limit the capabilities of
these clients to mitigate risk.
6.3. JavaScript Applications without a Backend
+---------------+ +--------------+
| | | |
| 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). The application then runs in
the browser, and is considered a public client since it has no
ability to be issued a client secret.
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The code in the browser then 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 JavaScript app
is then responsible for storing the access token securely using
appropriate browser APIs.
When the JavaScript application in the browser wants to make a
request to the Resource Server, it can include the access token in
the request (D) and make the request directly.
In this scenario, the Authorization Server and Resource Server MUST
support the necessary CORS headers to enable the JavaScript code to
make this POST request from the domain on which the script is
executing. (See Section 9.6 for additional details.)
7. Authorization Code Flow
Public browser-based apps that 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
Public browser-based apps MUST implement the Proof Key for Code
Exchange (PKCE [RFC7636]) extension to OAuth, and authorization
servers MUST support 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 same
client instance that exchanges the authorization code is the same one
that initiated the flow.
Browser-based apps MUST use a unique value for the the OAuth 2.0
"state" parameter on each request, and MUST verify the returned state
in the authorization response matches the original state the app
created.
Browser-based apps MUST follow the recommendations in
[oauth-security-topics] section 3.1 to protect themselves during
redirect flows.
7.2. Handling the Authorization Code Redirect
Authorization servers MUST require an exact match of a registered
redirect URI.
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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.
Browser-based applications provide an attacker with several
opportunities by which a refresh token can be leaked, just as with
access tokens. As such, these applications are considered a higher
risk for handling refresh tokens.
[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.
In particular, authorization servers:
o MUST rotate refresh tokens on each use, in order to be able to
detect a stolen refresh token if one is replayed (described in
[oauth-security-topics] section 4.12)
o MUST either set a maximum lifetime on refresh tokens OR expire if
the refresh token has not been used within some amount of time
o 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:
o A user authorizes an application, issuing an access token that
lasts 1 hour, and a refresh token that lasts 24 hours
o After 1 hour, the initial access token expires, so the application
uses the refresh token to get a new access token
o The authorization server returns a new access token that lasts 1
hour, and a new refresh token that lasts 23 hours
o This continues until 24 hours pass from the initial authorization
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o 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.
9. Security Considerations
9.1. Registration of Browser-Based Apps
Browser-based applications 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.
9.2. 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].
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 9.3 below).
9.3. 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
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client can be assured. Even when the user has previously approved an
authorization request for a given client_id, the request SHOULD be
processed as if no previous request had been approved, unless the
identity of the client can be proven.
If authorization servers restrict redirect URIs to a fixed set of
absolute HTTPS URIs without wildcard domains, paths, or 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.
9.4. Cross-Site Request Forgery Protections
Section 5.3.5 of [RFC6819] recommends using the "state" parameter to
link client requests and responses to prevent CSRF (Cross-Site
Request Forgery) attacks. To conform to this best practice, use of
the "state" parameter is REQUIRED, as described in Section 7.1,
unless the application has a method of ensuring the authorization
server supports PKCE, since PKCE also prevents CSRF attacks.
9.5. Authorization Server Mix-Up Mitigation
The security considerations around the authorization server mix-up
that are referenced in Section 8.10 of [RFC8252] also apply to
browser-based apps.
Clients MUST use a unique redirect URI for each authorization server
used by the application. The client MUST store the redirect URI
along with the session data (e.g. along with "state") and MUST verify
that the URI on which the authorization response was received exactly
matches.
9.6. 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.
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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.
9.7. 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.
9.8. OAuth Implicit Grant Authorization Flow
The OAuth 2.0 Implicit grant authorization flow (defined in
Section 4.2 of OAuth 2.0 [RFC6749]) works by receiving an access
token in the HTTP redirect (front-channel) immediately 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.
Several attacks on the implicit flow are described by [RFC6819] and
[oauth-security-topics], not all of which have sufficient mitigation
strategies.
9.8.1. Threat: Interception of the Redirect URI
If an attacker is able to cause the authorization response to be sent
to a URI under his control, he will directly get access to the
fragment carrying the access token. A method of performing this
attack is described in detail in [oauth-security-topics].
9.8.2. 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.
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This is discussed in more detail in Section 4.3.2 of
[oauth-security-topics].
9.8.3. Threat: Manipulation of Scripts
An attacker could modify the page or inject scripts into the browser
via various means, including when the browser's HTTPS connection is
being man-in-the-middled by for example a corporate network. While
this type of attack is typically out of scope of basic security
recommendations to prevent, in the case of browser-based apps it is
much easier to perform this kind of attack, where an injected script
can suddenly have access to everything on the page.
The risk of a malicious script running on the page is far greater
when the application uses a known standard way of obtaining access
tokens, namely that the attacker can always look at the
window.location to find an access token. This threat profile is very
different compared to 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.
9.8.4. 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.
9.8.5. Countermeasures
In addition to the countermeasures described by [RFC6819] and
[oauth-security-topics], using the authorization code with PKCE
avoids these attacks.
When PKCE is used, if an authorization code is stolen in transport,
the attacker is unable to do anything with the authorization code.
9.8.6. Disadvantages of the Implicit Flow
There are several additional reasons the Implicit flow is
disadvantageous compared to using the standard Authorization Code
flow.
o OAuth 2.0 provides no mechanism for a client to verify that an
access token was issued to it, which could lead to misuse and
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possible impersonation attacks if a malicious party hands off an
access token it retrieved through some other means to the client.
o 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.
o 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.
o 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.
9.8.7. Historic Note
Historically, the Implicit flow provided an advantage to single-page
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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9.9. 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., Barth, A., and D. Veditz, "Content Security
Policy", December 2016.
[Fetch] whatwg, "Fetch", 2018.
[oauth-security-topics]
Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"OAuth 2.0 Security Best Current Practice", July 2019.
[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>.
[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>.
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11.2. Informative References
[HTML] whatwg, "HTML", 2018.
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 9.6
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.
Appendix B. Document History
[[ To be removed from the final specification ]]
-04
o Disallow the use of the Password Grant
o Add PKCE support to summary list for authorization server
requirements
o 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
o Updated recommendations on using state to reflect the Security BCP
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o Updated server support checklist to reflect latest changes
o Updated the same-domain JS architecture section to emphasize the
architecture rather than domain
o Editorial clarifications in the section that talks about OpenID
Connect ID tokens
-03
o Updated the historic note about the fragment URL clarifying that
the Session History API means browsers can use the unmodified
authorization code flow
o Rephrased "Authorization Code Flow" intro paragraph to better lead
into the next two sections
o Softened "is likely a better decision to avoid using OAuth
entirely" to "it may be..." for common-domain deployments
o Updated abstract to not be limited to public clients, since the
later sections talk about confidential clients
o Removed references to avoiding OpenID Connect for same-domain
architectures
o Updated headers to better describe architectures (Apps Served from
a Static Web Server -> JavaScript Applications without a Backend)
o Expanded "same-domain architecture" section to better explain the
problems that OAuth has in this scenario
o Referenced Security BCP in implicit flow attacks where possible
o Minor typo corrections
-02
o Rewrote overview section incorporating feedback from Leo Tohill
o Updated summary recommendation bullet points to split out
application and server requirements
o Removed the allowance on hostname-only redirect URI matching, now
requiring exact redirect URI matching
o Updated section 6.2 to drop reference of SPA with a backend
component being a public client
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o Expanded the architecture section to explicitly mention three
architectural patterns available to JS apps
-01
o Incorporated feedback from Torsten Lodderstedt
o Updated abstract
o Clarified the definition of browser-based apps to not exclude
applications cached in the browser, e.g. via Service Workers
o Clarified use of the state parameter for CSRF protection
o Added background information about the original reason the
implicit flow was created due to lack of CORS support
o Clarified the same-domain use case where the SPA and API share a
cookie domain
o 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, Leo
Tohill, Tomek Stojecki, Torsten Lodderstedt, and Vittorio Bertocci.
Authors' Addresses
Aaron Parecki
Okta
Email: aaron@parecki.com
URI: https://aaronparecki.com
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David Waite
Ping Identity
Email: david@alkaline-solutions.com
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