Internet Engineering Task Force (IETF) T. Lodderstedt
Request for Comments: 9700 SPRIND
BCP: 240 J. Bradley
Updates: 6749, 6750, 6819 Yubico
Category: Best Current Practice A. Labunets
ISSN: 2070-1721 Independent Researcher
D. Fett
Authlete
November 2024
OAuth 2.0 Security
Best Current Practice for OAuth 2.0 Security
Abstract
This document describes best current security practice for OAuth 2.0.
It updates and extends the threat model and security advice given in
RFCs 6749, 6750, and 6819 to incorporate practical experiences
gathered since OAuth 2.0 was published and covers new threats
relevant due to the broader application of OAuth 2.0. Further, it
deprecates some modes of operation that are deemed less secure or
even insecure.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9700.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Structure
1.2. Conventions and Terminology
2. Best Practices
2.1. Protecting Redirect-Based Flows
2.1.1. Authorization Code Grant
2.1.2. Implicit Grant
2.2. Token Replay Prevention
2.2.1. Access Tokens
2.2.2. Refresh Tokens
2.3. Access Token Privilege Restriction
2.4. Resource Owner Password Credentials Grant
2.5. Client Authentication
2.6. Other Recommendations
3. The Updated OAuth 2.0 Attacker Model
4. Attacks and Mitigations
4.1. Insufficient Redirect Redirection URI Validation
4.1.1. Redirect URI Validation Attacks on Authorization Code
Grant
4.1.2. Redirect URI Validation Attacks on Implicit Grant
4.1.3. Countermeasures
4.2. Credential Leakage via Referer Headers
4.2.1. Leakage from the OAuth Client
4.2.2. Leakage from the Authorization Server
4.2.3. Consequences
4.2.4. Countermeasures
4.3. Credential Leakage via Browser History
4.3.1. Authorization Code in Browser History
4.3.2. Access Token in Browser History
4.4. Mix-Up Attacks
4.4.1. Attack Description
4.4.2. Countermeasures
4.4.2.1. Mix-Up Defense via Issuer Identification
4.4.2.2. Mix-Up Defense via Distinct Redirect URIs
4.5. Authorization Code Injection
4.5.1. Attack Description
4.5.2. Discussion
4.5.3. Countermeasures
4.5.3.1. PKCE
4.5.3.2. Nonce
4.5.3.3. Other Solutions
4.5.4. Limitations
4.6. Access Token Injection
4.6.1. Countermeasures
4.7. Cross-Site Request Forgery
4.7.1. Countermeasures
4.8. PKCE Downgrade Attack
4.8.1. Attack Description
4.8.2. Countermeasures
4.9. Access Token Leakage at the Resource Server
4.9.1. Access Token Phishing by Counterfeit Resource Server
4.9.2. Compromised Resource Server
4.9.3. Countermeasures
4.10. Misuse of Stolen Access Tokens
4.10.1. Sender-Constrained Access Tokens
4.10.2. Audience-Restricted Access Tokens
4.10.3. Discussion: Preventing Leakage via Metadata
4.11. Open Redirection
4.11.1. Client as Open Redirector
4.11.2. Authorization Server as Open Redirector
4.12. 307 Redirect
4.13. TLS Terminating Reverse Proxies
4.14. Refresh Token Protection
4.14.1. Discussion
4.14.2. Recommendations
4.15. Client Impersonating Resource Owner
4.15.1. Countermeasures
4.16. Clickjacking
4.17. Attacks on In-Browser Communication Flows
4.17.1. Examples
4.17.1.1. Insufficient Limitation of Receiver Origins
4.17.1.2. Insufficient URI Validation
4.17.1.3. Injection after Insufficient Validation of Sender
Origin
4.17.2. Recommendations
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A.
Acknowledgements
Authors' Addresses
1. Introduction
Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 (referred
to as simply "OAuth" in this document) has gained massive traction in
the market and became the standard for API protection and the basis
for federated login using OpenID Connect [OpenID.Core]. While OAuth
is used in a variety of scenarios and different kinds of deployments,
the following challenges can be observed:
* OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (i.e., well-known
patterns that are considered insecure). Although most of these
threats are discussed in the OAuth 2.0 Threat Model and Security
Considerations [RFC6819], continued exploitation demonstrates a
need for more specific recommendations, easier to implement
mitigations, and more defense in depth.
* OAuth is being used in environments with higher security
requirements than considered initially, such as open banking,
eHealth, eGovernment, and electronic signatures. Those use cases
call for stricter guidelines and additional protection.
* OAuth is being used in much more dynamic setups than originally
anticipated, creating new challenges with respect to security.
Those challenges go beyond the original scope of [RFC6749],
[RFC6750], and [RFC6819].
OAuth initially assumed static relationships between clients,
authorization servers, and resource servers. The URLs of the
servers were known to the client at deployment time and built an
anchor for the trust relationships among those parties. The
validation of whether the client is talking to a legitimate server
was based on TLS server authentication (see Section 4.5.4 of
[RFC6819]). With the increasing adoption of OAuth, this simple
model dissolved and, in several scenarios, was replaced by a
dynamic establishment of the relationship between clients on one
side and the authorization and resource servers of a particular
deployment on the other side. This way, the same client could be
used to access services of different providers (in case of
standard APIs, such as email or OpenID Connect) or serve as a
front end to a particular tenant in a multi-tenant environment.
Extensions of OAuth, such as the OAuth 2.0 Dynamic Client
Registration Protocol [RFC7591] and OAuth 2.0 Authorization Server
Metadata [RFC8414] were developed to support the use of OAuth in
dynamic scenarios.
* Technology has changed. For example, the way browsers treat
fragments when redirecting requests has changed, and with it, the
implicit grant's underlying security model.
This document provides updated security recommendations to address
these challenges. It introduces new requirements beyond those
defined in existing specifications such as OAuth 2.0 [RFC6749] and
OpenID Connect [OpenID.Core] and deprecates some modes of operation
that are deemed less secure or even insecure. However, this document
does not supplant the security advice given in [RFC6749], [RFC6750],
and [RFC6819], but complements those documents.
Naturally, not all existing ecosystems and implementations are
compatible with the new requirements, and following the best
practices described in this document may break interoperability.
Nonetheless, it is RECOMMENDED that implementers upgrade their
implementations and ecosystems as soon as feasible.
OAuth 2.1, under development as [OAUTH-V2.1], will incorporate
security recommendations from this document.
1.1. Structure
The remainder of this document is organized as follows: Section 2
summarizes the most important best practices for every OAuth
implementor.
implementer. Section 3 presents the updated OAuth attacker model.
Section 4 is a detailed analysis of the threats and implementation
issues that can be found in the wild (at the time of writing) along
with a discussion of potential countermeasures.
1.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the terms "access token", "authorization
endpoint", "authorization grant", "authorization server", "client",
"client identifier" (client ID), "protected resource", "refresh
token", "resource owner", "resource server", and "token endpoint"
defined by OAuth 2.0 [RFC6749].
An "open redirector" is an endpoint on a web server that forwards a
user's browser to an arbitrary URI obtained from a query parameter.
2. Best Practices
This section describes the core set of security mechanisms and
measures that are considered to be best practices at the time of
writing. Details about these security mechanisms and measures
(including detailed attack descriptions) and requirements for less
commonly used options are provided in Section 4.
2.1. Protecting Redirect-Based Flows
When comparing client redirect redirection URIs against pre-registered URIs,
authorization servers MUST utilize exact string matching except for
port numbers in localhost redirection URIs of native apps (see
Section 4.1.3). This measure contributes to the prevention of
leakage of authorization codes and access tokens (see Section 4.1).
It can also help to detect mix-up attacks (see Section 4.4).
Clients and authorization servers MUST NOT expose URLs that forward
the user's browser to arbitrary URIs obtained from a query parameter
(open redirectors) as described in Section 4.11. Open redirectors
can enable exfiltration of authorization codes and access tokens.
Clients MUST prevent Cross-Site Request Forgery (CSRF). In this
context, CSRF refers to requests to the redirection endpoint that do
not originate at the authorization server, but at a malicious third
party (see Section 4.4.1.8 of [RFC6819] for details). Clients that
have ensured that the authorization server supports Proof Key for
Code Exchange (PKCE) [RFC7636] MAY rely on the CSRF protection
provided by PKCE. In OpenID Connect flows, the nonce parameter
provides CSRF protection. Otherwise, one-time use CSRF tokens
carried in the state parameter that are securely bound to the user
agent MUST be used for CSRF protection (see Section 4.7.1).
When an OAuth client can interact with more than one authorization
server, a defense against mix-up attacks (see Section 4.4) is
REQUIRED. To this end, clients SHOULD
* use the iss parameter as a countermeasure according to [RFC9207],
or
* use an alternative countermeasure based on an iss value in the
authorization response (such as the iss claim in the ID Token in
[OpenID.Core] or in [OpenID.JARM] responses), processing it that
value as described in [RFC9207].
In the absence of these options, clients MAY instead use distinct
redirect
redirection URIs to identify authorization endpoints and token
endpoints, as described in Section 4.4.2.
An authorization server that redirects a request potentially
containing user credentials MUST avoid forwarding these user
credentials accidentally (see Section 4.12 for details).
2.1.1. Authorization Code Grant
Clients MUST prevent authorization code injection attacks (see
Section 4.5) and misuse of authorization codes using one of the
following options:
* Public clients MUST use PKCE [RFC7636] to this end, as motivated
in Section 4.5.3.1.
* For confidential clients, the use of PKCE [RFC7636] is
RECOMMENDED, as it provides strong protection against misuse and
injection of authorization codes as described in Section 4.5.3.1.
Also, as a side effect, it prevents CSRF even in the presence of
strong attackers as described in Section 4.7.1.
* With additional precautions, described in Section 4.5.3.2,
confidential OpenID Connect [OpenID.Core] clients MAY use the
nonce parameter and the respective Claim in the ID Token instead.
In any case, the PKCE challenge or OpenID Connect nonce MUST be
transaction-specific and securely bound to the client and the user
agent in which the transaction was started. Authorization servers
are encouraged to make a reasonable effort at detecting and
preventing the use of constant values for the PKCE challenge or
OpenID Connect nonce.
Note: Although PKCE was designed as a mechanism to protect native
apps, this advice applies to all kinds of OAuth clients, including
web applications.
When using PKCE, clients SHOULD use PKCE code challenge methods that
do not expose the PKCE verifier in the authorization request.
Otherwise, attackers that can read the authorization request (cf.
attacker
Attacker (A4) in Section 3) can break the security provided by PKCE.
Currently, S256 is the only such method.
Authorization servers MUST support PKCE [RFC7636].
If a client sends a valid PKCE code_challenge parameter in the
authorization request, the authorization server MUST enforce the
correct usage of code_verifier at the token endpoint.
Authorization servers MUST mitigate PKCE downgrade attacks by
ensuring that a token request containing a code_verifier parameter is
accepted only if a code_challenge parameter was present in the
authorization request; see Section 4.8.2 for details.
Authorization servers MUST provide a way to detect their support for
PKCE. It is RECOMMENDED for authorization servers to publish the
element code_challenge_methods_supported in their Authorization
Server Metadata [RFC8414] containing the supported PKCE challenge
methods (which can be used by the client to detect PKCE support).
Authorization servers MAY instead provide a deployment-specific way
to ensure or determine PKCE support by the authorization server.
2.1.2. Implicit Grant
The implicit grant (response type "token") token) and other response types
causing the authorization server to issue access tokens in the
authorization response are vulnerable to access token leakage and
access token replay as described in Sections 4.1, 4.2, 4.3, and 4.6.
Moreover, no standardized method for sender-constraining exists to
bind access tokens to a specific client (as recommended in
Section 2.2) when the access tokens are issued in the authorization
response. This means that an attacker can use the leaked or stolen
access token at a resource endpoint.
In order to avoid these issues, clients SHOULD NOT use the implicit
grant (response type "token") token) or other response types issuing access
tokens in the authorization response, unless access token injection
in the authorization response is prevented and the aforementioned
token leakage vectors are mitigated.
Clients SHOULD instead use the response type code (i.e.,
authorization code grant type) as specified in Section 2.1.1 or any
other response type that causes the authorization server to issue
access tokens in the token response, such as the code id_token
response type. This allows the authorization server to detect replay
attempts by attackers and generally reduces the attack surface since
access tokens are not exposed in URLs. It also allows the
authorization server to sender-constrain the issued tokens (see
Section 2.2.
2.2. Token Replay Prevention
2.2.1. Access Tokens
A sender-constrained access token scopes the applicability of an
access token to a certain sender. This sender is obliged to
demonstrate knowledge of a certain secret as a prerequisite for the
acceptance of that token at the recipient (e.g., a resource server).
Authorization and resource servers SHOULD use mechanisms for sender-
constraining access tokens, such as Mutual mutual TLS for OAuth 2.0
[RFC8705] or OAuth 2.0 Demonstrating Proof of Possession (DPoP)
[RFC9449] (see Section 4.10.1), to prevent misuse of stolen and
leaked access tokens.
2.2.2. Refresh Tokens
Refresh tokens for public clients MUST be sender-constrained or use
refresh token rotation as described in Section 4.14. [RFC6749]
already mandates that refresh tokens for confidential clients can
only be used by the client for which they were issued.
2.3. Access Token Privilege Restriction
The privileges associated with an access token SHOULD be restricted
to the minimum required for the particular application or use case.
This prevents clients from exceeding the privileges authorized by the
resource owner. It also prevents users from exceeding their
privileges authorized by the respective security policy. Privilege
restrictions also help to reduce the impact of access token leakage.
In particular, access tokens SHOULD be audience-restricted to a
specific resource server or, if that is not feasible, to a small set
of resource servers. To put this into effect, the authorization
server associates the access token with certain resource servers, and
every resource server is obliged to verify, for every request,
whether the access token sent with that request was meant to be used
for that particular resource server. If it was not, the resource
server MUST refuse to serve the respective request. The aud claim as
defined in [RFC9068] MAY be used to audience-restrict access tokens.
Clients and authorization servers MAY utilize the parameters scope or
resource as specified in [RFC6749] and [RFC8707], respectively, to
determine the resource server they want to access.
Additionally, access tokens SHOULD be restricted to certain resources
and actions on resource servers or resources. To put this into
effect, the authorization server associates the access token with the
respective resource and actions and every resource server is obliged
to verify, for every request, whether the access token sent with that
request was meant to be used for that particular action on the
particular resource. If not, the resource server must refuse to
serve the respective request. Clients and authorization servers MAY
utilize the parameter scope as specified in [RFC6749] and
authorization_details as specified in [RFC9396] to determine those
resources and/or actions.
2.4. Resource Owner Password Credentials Grant
The resource owner password credentials grant [RFC6749] MUST NOT be
used. This grant type insecurely exposes the credentials of the
resource owner to the client. Even if the client is benign, usage of
this grant results in an increased attack surface (credentials (i.e., credentials
can leak in more places than just the authorization server) and in
training users are trained to enter their credentials in places other than the
authorization server.
Furthermore, the resource owner password credentials grant is not
designed to work with two-factor authentication and authentication
processes that require multiple user interaction steps.
Authentication with cryptographic credentials (cf. WebCrypto
[W3C.WebCrypto], WebAuthn [W3C.WebAuthn]) may be impossible to
implement with this grant type, as it is usually bound to a specific
web origin.
2.5. Client Authentication
Authorization servers SHOULD enforce client authentication if it is
feasible, in the particular deployment, to establish a process for
issuance/registration of credentials for clients and ensuring the
confidentiality of those credentials.
It is RECOMMENDED to use asymmetric cryptography for client
authentication, such as mutual TLS (mTLS) for OAuth 2.0 [RFC8705] or signed
JWTs ("Private Key JWT") in accordance with [RFC7521] and [RFC7523] (in
[OpenID.Core] [RFC7523].
The latter is defined in [OpenID.Core] as the client authentication
method private_key_jwt). When asymmetric cryptography for client
authentication is used, authorization servers do not need to store
sensitive symmetric keys, making these methods more robust against
leakage of keys.
2.6. Other Recommendations
The use of OAuth Authorization Server Metadata [RFC8414] can help to
improve the security of OAuth deployments:
* It ensures that security features and other new OAuth features can
be enabled automatically by compliant software libraries.
* It reduces chances for misconfigurations -- for example,
misconfigured endpoint URLs (that might belong to an attacker) or
misconfigured security features.
* It can help to facilitate rotation of cryptographic keys and to
ensure cryptographic agility.
It is therefore RECOMMENDED that authorization servers publish OAuth
Authorization Server Metadata according to [RFC8414] and that clients
make use of this Authorization Server Metadata (when available) to
configure themselves.
Under the conditions described in Section 4.15.1, authorization
servers SHOULD NOT allow clients to influence their client_id or any
other claim that could cause confusion with a genuine resource owner.
It is RECOMMENDED to use end-to-end TLS according to [BCP195] between
the client and the resource server. If TLS traffic needs to be
terminated at an intermediary, refer to Section 4.13 for further
security advice.
Authorization responses MUST NOT be transmitted over unencrypted
network connections. To this end, authorization servers MUST NOT
allow redirect redirection URIs that use the http scheme except for native
clients that use loopback interface redirection as described in
Section 7.3 of [RFC8252].
If the authorization response is sent with in-browser communication
techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
redirects, both the initiator and receiver of the in-browser message
MUST be strictly verified as described in Section 4.17.
To support browser-based clients, endpoints directly accessed by such
clients including the Token Endpoint, Authorization Server Metadata
Endpoint, jwks_uri Endpoint, and Dynamic Client Registration Endpoint
MAY support the use of Cross-Origin Resource Sharing (CORS)
[WHATWG.CORS]. However, CORS MUST NOT be supported at the
authorization endpoint, as the client does not access this endpoint
directly; instead, the client redirects the user agent to it.
3. The Updated OAuth 2.0 Attacker Model
In [RFC6819], a threat model is laid out that describes the threats
against which OAuth deployments must be protected. While doing so,
[RFC6819] makes certain assumptions about attackers and their
capabilities, i.e., it implicitly establishes an attacker model. In
the following, this attacker model is made explicit and is updated
and expanded to account for the potentially dynamic relationships
involving multiple parties (as described in Section 1), to include
new types of attackers, and to define the attacker model more
clearly.
The goal of this document is to ensure that the authorization of a
resource owner (with a user agent) at an authorization server and the
subsequent usage of the access token at a resource server is
protected, as well as practically possible, at least against the
following attackers.
(A1) Web attackers that can set up and operate an arbitrary number
of network endpoints (besides the "honest" ones) including
browsers and servers. Web attackers may set up websites that
are visited by the resource owner, operate their own user
agents, and participate in the protocol.
In particular, web attackers may operate OAuth clients that are
registered at the authorization server, and they may operate
their own authorization and resource servers that can be used
(in parallel to the "honest" ones) by the resource owner and
other resource owners.
It must also be assumed that web attackers can lure the user to
navigate their browser to arbitrary attacker-chosen URIs at any
time. In practice, this can be achieved in many ways, for
example, by injecting malicious advertisements into
advertisement networks or by sending legitimate-looking emails.
Web attackers can use their own user credentials to create new
messages as well as any secrets they learned previously. For
example, if a web attacker learns an authorization code of a
user through a misconfigured redirect redirection URI, the web attacker
can then try to redeem that code for an access token.
They cannot, however, read or manipulate messages that are not
targeted towards them (e.g., sent to a URL controlled by a non-
attacker-controlled of an authorization server).
server not under control of an attacker).
(A2) Network attackers that additionally have full control over the
network over which protocol participants communicate. They can
eavesdrop on, manipulate, and spoof messages, except when these
are properly protected by cryptographic methods (e.g., TLS).
Network attackers can also block arbitrary messages.
While an example for a web attacker would be a customer of an
internet service provider, network attackers could be the internet
service provider itself, an attacker in a public (Wi-Fi) network
using ARP spoofing, or a state-sponsored attacker with access to
internet exchange points, for instance.
The aforementioned attackers (A1) and (A2) conform to the attacker
model that was used in formal analysis efforts for OAuth
[arXiv.1601.01229]. This is a minimal attacker model. Implementers
MUST take into account all possible types of attackers in the
environment of their OAuth implementations. For example, in
[arXiv.1901.11520], a very strong attacker model is used that
includes attackers that have full control over the token endpoint.
This models effects of a possible misconfiguration of endpoints in
the ecosystem, which can be avoided by using authorization server
metadata as described in Section 2.6. Such an attacker is therefore
not listed here.
However, previous attacks on OAuth have shown that the following
types of attackers are relevant in particular:
(A3) Attackers that can read, but not modify, the contents of the
authorization response (i.e., the authorization response can
leak to an attacker).
Examples of such attacks include open redirector attacks; attacks and
mix-up attacks (see Section 4.4), where the client is tricked
into sending credentials to an attacker-controlled
authorization server.
Also, this includes attacks that take advantage of:
* insufficient checking of redirect URIs (see Section 4.1);
* problems existing on mobile operating systems (where systems, where
different apps can register themselves on the same URI); mix-up attacks
(see Section 4.4), where the client is tricked into sending
credentials to an attacker-controlled authorization server; URI; and
the fact that
* URLs are often stored/logged by browsers (history), proxy servers, and
operating systems.
(A4) Attackers that can read, but not modify, the contents of the
authorization request (i.e., the authorization request can
leak, in the same manner as above, to an attacker).
(A5) Attackers that can acquire an access token issued by an
authorization server. For example, a resource server may be
compromised by an attacker, an access token may be sent to an
attacker-controlled resource server due to a misconfiguration,
or social engineering may be used to get a resource owner to
use an attacker-controlled resource server. Also see
Section 4.9.2.
(A3), (A4), and (A5) typically occur together with either (A1) or
(A2). Attackers can collaborate to reach a common goal.
Note that an attacker Attacker (A1) or (A2) can be a resource owner or act as
one. For example, such an attacker can use their own browser to
replay tokens or authorization codes obtained by any of the attacks
described above at the client or resource server.
This document focuses on threats resulting from attackers Attackers (A1) to
(A5).
4. Attacks and Mitigations
This section gives a detailed description of attacks on OAuth
implementations, along with potential countermeasures. Attacks and
mitigations already covered in [RFC6819] are not listed here, except
where new recommendations are made.
This section further defines additional requirements (beyond those
defined in Section 2) for certain cases and protocol options.
4.1. Insufficient Redirect Redirection URI Validation
Some authorization servers allow clients to register redirect redirection URI
patterns instead of complete redirect redirection URIs. The authorization
servers then match the redirect redirection URI parameter value at the
authorization endpoint against the registered patterns at runtime.
This approach allows clients to encode transaction state into
additional redirect URI parameters or to register a single pattern
for multiple redirect redirection URIs.
This approach turned out to be more complex to implement and more
error-prone to manage than exact redirect redirection URI matching. Several
successful attacks exploiting flaws in the pattern-matching
implementation or concrete configurations have been observed in the
wild (see, e.g., [research.rub2]). Insufficient validation of the
redirect
redirection URI effectively breaks client identification or
authentication (depending on grant and client type) and allows the
attacker to obtain an authorization code or access token, either
* by directly sending the user agent to a URI under the attacker's
control, or
* by exposing the OAuth credentials to an attacker by utilizing an
open redirector at the client in conjunction with the way user
agents handle URL fragments.
These attacks are shown in detail in the following subsections.
4.1.1. Redirect URI Validation Attacks on Authorization Code Grant
For a client using the grant type code, an attack may work as
follows:
Assume the redirect redirection URL pattern https://*.somesite.example/* is
registered for the client with the client ID s6BhdRkqt3. The
intention is to allow any subdomain of somesite.example to be a valid
redirect
redirection URI for the client, for example,
https://app1.somesite.example/redirect. However, a naive
implementation on the authorization server might interpret the
wildcard * as "any character" and not "any character valid for a
domain name". The authorization server, therefore, might permit
https://attacker.example/.somesite.example as a redirect redirection URI,
although attacker.example is a different domain potentially
controlled by a malicious party.
The attack can then be conducted as follows:
To begin, the attacker needs to trick the user into opening a
tampered URL in their browser that launches a page under the
attacker's control, say, https://www.evil.example (see attacker A1 in
Section 3).
This URL initiates the following authorization request with the
client ID of a legitimate client to the authorization endpoint (line
breaks for display only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
&redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example
HTTP/1.1
Host: server.somesite.example
The authorization server validates the redirect redirection URI and compares
it to the registered redirect redirection URL patterns for the client
s6BhdRkqt3. The authorization request is processed and presented to
the user.
If the user does not see the redirect redirection URI or does not recognize
the attack, the code is issued and immediately sent to the attacker's
domain. If an automatic approval of the authorization is enabled
(which is not recommended for public clients according to [RFC6749]),
the attack can be performed even without user interaction.
If the attacker impersonates a public client, the attacker can
exchange the code for tokens at the respective token endpoint.
This attack will not work as easily for confidential clients, since
the code exchange requires authentication with the legitimate
client's secret. However, the attacker can use the legitimate
confidential client to redeem the code by performing an authorization
code injection attack; see Section 4.5.
It is important to note that redirect redirection URI validation
vulnerabilities can also exist if the authorization server handles
wildcards properly. For example, assume that the client registers
the redirect redirection URL pattern https://*.somesite.example/* and the
authorization server interprets this as "allow redirect redirection URIs
pointing to any host residing in the domain somesite.example". If an
attacker manages to establish a host or subdomain in
somesite.example, the attacker can impersonate the legitimate client.
For example, this could be caused by a subdomain takeover attack
[research.udel], where an outdated CNAME record (say, external-service.somesite.example) external-
service.somesite.example) points to an external DNS name that no
longer exists (say, customer-abc.service.example) and can be taken
over by an attacker (e.g., by registering as customer-abc with the
external service).
4.1.2. Redirect URI Validation Attacks on Implicit Grant
The attack described above works for the implicit grant as well. If
the attacker is able to send the authorization response to an
attacker-controlled URI, the attacker will directly get access to the
fragment carrying the access token.
Additionally, implicit grants (and also other grants when using
response_mode=fragment as defined in [OAuth.Responses]) can be
subject to a further kind of attack. The attack utilizes the fact
that user agents reattach fragments to the destination URL of a
redirect if the location header does not contain a fragment (see
Section 17.11 of [RFC9110]). The attack described here combines this
behavior with the client as an open redirector (see Section 4.11.1)
in order to obtain access tokens. This allows circumvention even of
very narrow redirect redirection URI patterns, but not of strict URL matching.
Assume the registered URL pattern for client s6BhdRkqt3 is
https://client.somesite.example/cb?*, i.e., any parameter is allowed
for redirects to https://client.somesite.example/cb. Unfortunately,
the client exposes an open redirector. This endpoint supports a
parameter redirect_to which takes a target URL and will send the
browser to this URL using an HTTP Location header redirect 303.
The attack can now be conducted as follows:
To begin, as above, the attacker needs to trick the user into opening
a tampered URL in their browser that launches a page under the
attacker's control, say, https://www.evil.example.
Afterwards, the website initiates an authorization request that is
very similar to the one in the attack on the code flow. Different to
above, it utilizes the open redirector by encoding
redirect_to=https://attacker.example into the parameters of the
redirect
redirection URI, and it uses the response type "token" token (line breaks for
display only):
GET /authorize?response_type=token&state=9ad67f13
&client_id=s6BhdRkqt3
&redirect_uri=https%3A%2F%2Fclient.somesite.example
%2Fcb%26redirect_to%253Dhttps%253A%252F
%252Fattacker.example%252F HTTP/1.1
Host: server.somesite.example
Then, since the redirect redirection URI matches the registered pattern, the
authorization server permits the request and sends the resulting
access token in a 303 redirect (some response parameters omitted for
readability):
HTTP/1.1 303 See Other
Location: https://client.somesite.example/cb?
redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&...
At client.somesite.example, the request arrives at the open
redirector. The endpoint will read the redirect parameter and will
issue an HTTP 303 Location header redirect to the URL
https://attacker.example/.
HTTP/1.1 303 See Other
Location: https://attacker.example/
Since the redirector at client.somesite.example does not include a
fragment in the Location header, the user agent will reattach the
original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the
URL and will navigate to the following URL:
https://attacker.example/#access_token=2YotnFZFEjr1z...
The attacker's page at attacker.example can then access the fragment
and obtain the access token.
4.1.3. Countermeasures
The complexity of implementing and managing pattern matching
correctly obviously causes security issues. This document therefore
advises simplifying the required logic and configuration by using
exact redirect redirection URI matching. This means the authorization server
MUST ensure that the two URIs are equal; see Section 6.2.1 of
[RFC3986], Simple String Comparison, for details. The only exception
is native apps using a localhost URI: In this case, the authorization
server MUST allow variable port numbers as described in Section 7.3
of [RFC8252].
Additional recommendations:
* Web servers on which redirect redirection URIs are hosted MUST NOT expose
open redirectors (see Section 4.11).
* Browsers reattach URL fragments to Location redirection URLs only
if the URL in the Location header does not already contain a
fragment. Therefore, servers MAY prevent browsers from
reattaching fragments to redirection URLs by attaching an
arbitrary fragment identifier, for example #_, to URLs in Location
headers.
* Clients SHOULD use the authorization code response type instead of
response types that cause access token issuance at the
authorization endpoint. This offers countermeasures against the
reuse of leaked credentials through the exchange process with the
authorization server and against token replay through sender-
constraining of the access tokens.
If the origin and integrity of the authorization request containing
the redirect redirection URI can be verified, for example, when using
[RFC9101] or [RFC9126] with client authentication, the authorization
server MAY trust the redirect redirection URI without further checks.
4.2. Credential Leakage via Referer Headers
The contents of the authorization request URI or the authorization
response URI can unintentionally be disclosed to attackers through
the Referer HTTP header (see Section 10.1.3 of [RFC9110]), by leaking
from either the authorization server's or the client's website,
respectively. Most importantly, authorization codes or state values
can be disclosed in this way. Although specified otherwise in
Section 10.1.3 of [RFC9110], the same may happen to access tokens
conveyed in URI fragments due to browser implementation issues, as
illustrated by a (now fixed) issue in the Chromium project
[bug.chromium].
4.2.1. Leakage from the OAuth Client
Leakage from the OAuth client requires that the client, as a result
of a successful authorization request, renders a page that
* contains links to other pages under the attacker's control and a
user clicks on such a link, or
* includes third-party content (advertisements in iframes, images,
etc.), for example, if the page contains user-generated content
(blog).
As soon as the browser navigates to the attacker's page or loads the
third-party content, the attacker receives the authorization response
URL and can extract code or state (and potentially access_token).
4.2.2. Leakage from the Authorization Server
In a similar way, an attacker can learn state from the authorization
request if the authorization endpoint at the authorization server
contains links or third-party content as above.
4.2.3. Consequences
An attacker that learns a valid code or access token through a
Referer header can perform the attacks as described in Sections
4.1.1, 4.5 and 4.6. If the attacker learns state, the CSRF
protection achieved by using state is lost, resulting in CSRF attacks
as described in Section 4.4.1.8 of [RFC6819].
4.2.4. Countermeasures
The page rendered as a result of the OAuth authorization response and
the authorization endpoint SHOULD NOT include third-party resources
or links to external sites.
The following measures further reduce the chances of a successful
attack:
* Suppress the Referer header by applying an appropriate Referrer
Policy [W3C.webappsec-referrer-policy] to the document (either as
part of the "referrer" meta attribute or by setting a Referrer-
Policy header). For example, the header Referrer-Policy: no-
referrer in the response completely suppresses the Referer header
in all requests originating from the resulting document.
* Use authorization code instead of response types causing access
token issuance from the authorization endpoint.
* Bind the authorization code to a confidential client or PKCE
challenge. In this case, the attacker lacks the secret to request
the code exchange.
* As described in Section 4.1.2 of [RFC6749], authorization codes
MUST be invalidated by the authorization server after their first
use at the token endpoint. For example, if an authorization
server invalidated the code after the legitimate client redeemed
it, the attacker would fail to exchange this code later.
This does not mitigate the attack if the attacker manages to
exchange the code for a token before the legitimate client does
so. Therefore, [RFC6749] further recommends that, when an attempt
is made to redeem a code twice, the authorization server SHOULD
revoke all tokens issued previously based on that code.
* The state value SHOULD be invalidated by the client after its
first use at the redirection endpoint. If this is implemented,
and an attacker receives a token through the Referer header from
the client's website, the state was already used, invalidated by
the client and cannot be used again by the attacker. (This does
not help if the state leaks from the authorization server's
website, since then the state has not been used at the redirection
endpoint at the client yet.)
* Use the form post response mode instead of a redirect for the
authorization response (see [OAuth.Post]).
4.3. Credential Leakage via Browser History
Authorization codes and access tokens can end up in the browser's
history of visited URLs, enabling the attacks described in the
following.
4.3.1. Authorization Code in Browser History
When a browser navigates to client.example/
redirection_endpoint?code=abcd as a result of a redirect from a
provider's authorization endpoint, the URL including the
authorization code may end up in the browser's history. An attacker
with access to the device could obtain the code and try to replay it.
Countermeasures:
* Authorization code replay prevention as described in
Section 4.4.1.1 of [RFC6819], and Section 4.5.
* Use the form post response mode instead of redirect for the
authorization response (see [OAuth.Post]).
4.3.2. Access Token in Browser History
An access token may end up in the browser history if a client or a
website that already has a token deliberately navigates to a page
like provider.com/get_user_profile?access_token=abcdef. [RFC6750]
discourages this practice and advises transferring tokens via a
header, but in practice websites often pass access tokens in query
parameters.
In the case of implicit grant, a URL like client.example/
redirection_endpoint#access_token=abcdef may also end up in the
browser history as a result of a redirect from a provider's
authorization endpoint.
Countermeasures:
* Clients MUST NOT pass access tokens in a URI query parameter in
the way described in Section 2.3 of [RFC6750]. The authorization
code grant or alternative OAuth response modes like the form post
response mode [OAuth.Post] can be used to this end.
4.4. Mix-Up Attacks
Mix-up attacks are can occur in scenarios where an OAuth client interacts
with two or more authorization servers and at least one authorization
server is under the control of the attacker. This can be the case,
for example, if the attacker uses dynamic registration to register
the client at their own authorization server or if an authorization
server becomes compromised.
The goal of the attack is to obtain an authorization code or an
access token for an uncompromised authorization server. This is
achieved by tricking the client into sending those credentials to the
compromised authorization server (the attacker) instead of using them
at the respective endpoint of the uncompromised authorization/
resource server.
4.4.1. Attack Description
The description here follows [arXiv.1601.01229], with variants of the
attack outlined below.
Preconditions: For this variant of the attack to work, it is assumed
that
* the implicit or authorization code grant is used with multiple
authorization servers of which one is considered "honest" (H-AS)
and one is operated by the attacker (A-AS), and
* the client stores the authorization server chosen by the user in a
session bound to the user's browser and uses the same redirection
endpoint URI for each authorization server.
In the following, it is further assumed that the client is registered
with H-AS (URI: https://honest.as.example, client ID: 7ZGZldHQ) and
with A-AS (URI: https://attacker.example, client ID: 666RVZJTA).
URLs shown in the following example are shortened for presentation to
include only parameters relevant to the attack.
Attack on the authorization code grant:
1. The user selects to start the grant using A-AS (e.g., by clicking
on a button on the client's website).
2. The client stores in the user's session that the user selected
"A-AS" and redirects the user to A-AS's authorization endpoint
with a Location header containing the URL
https://attacker.example/
authorize?response_type=code&client_id=666RVZJTA.
3. When the user's browser navigates to the attacker's authorization
endpoint, the attacker immediately redirects the browser to the
authorization endpoint of H-AS. In the authorization request,
the attacker replaces the client ID of the client at A-AS with
the client's ID at H-AS. Therefore, the browser receives a
redirection (303 See Other) with a Location header pointing to
https://honest.as.example/
authorize?response_type=code&client_id=7ZGZldHQ
4. The user authorizes the client to access their resources at H-AS.
(Note that a vigilant user might at this point detect that they
intended to use A-AS instead of H-AS. The first attack variant
listed below avoids this.) does not have this limitation.) H-AS issues a code and
sends it (via the browser) back to the client.
5. Since the client still assumes that the code was issued by A-AS,
it will try to redeem the code at A-AS's token endpoint.
6. The attacker therefore obtains code and can either exchange the
code for an access token (for public clients) or perform an
authorization code injection attack as described in Section 4.5.
Variants:
* Mix-Up with Interception: This variant works only if the attacker
can intercept and manipulate the first request/response pair from
a user's browser to the client (in which the user selects a
certain authorization server and is then redirected by the client
to that authorization server), as in Attacker (A2) (see
Section 3). This capability can, for example, be the result of a
attacker-in-the-middle attack on the user's connection to the
client. In the attack, the user starts the flow with H-AS. The
attacker intercepts this request and changes the user's selection
to A-AS. The rest of the attack proceeds as in Step 2 and
following above.
* Implicit Grant: In the implicit grant, the attacker receives an
access token instead of the code in Step 4. The attacker's
authorization server receives the access token when the client
makes either a request to the A-AS userinfo endpoint or a request
to the attacker's resource server (since the client believes it
has completed the flow with A-AS).
* Per-AS Redirect URIs: If clients use different redirect redirection URIs
for different authorization servers, clients do not store the
selected authorization server in the user's session, and
authorization servers do not check the redirect redirection URIs properly,
attackers can mount an attack called "Cross-Social Network Request
Forgery". These attacks have been observed in practice. Refer to
[research.jcs_14] for details.
* OpenID Connect: Some variants can be used to attack OpenID
Connect. In these attacks, the attacker misuses features of the
OpenID Connect Discovery [OpenID.Discovery] mechanism or replays
access tokens or ID Tokens to conduct a mix-up attack. The
attacks are described in detail in Appendix A of
[arXiv.1704.08539] and Section 6 of [arXiv.1508.04324v2]
("Malicious Endpoints Attacks").
4.4.2. Countermeasures
When an OAuth client can only interact with one authorization server,
a mix-up defense is not required. In scenarios where an OAuth client
interacts with two or more authorization servers, however, clients
MUST prevent mix-up attacks. Two different methods are discussed
below.
For both defenses, clients MUST store, for each authorization
request, the issuer they sent the authorization request to and bind
this information to the user agent. The issuer serves, via the
associated metadata, as an abstract identifier for the combination of
the authorization endpoint and token endpoint that are to be used in
the flow. If an issuer identifier is not available (for example, if
neither OAuth Authorization Server Metadata [RFC8414] nor OpenID
Connect Discovery [OpenID.Discovery] is used), a different unique
identifier for this tuple or the tuple itself can be used instead.
For brevity of presentation, such a deployment-specific identifier
will be subsumed under the issuer (or issuer identifier) in the
following.
It is important to note that just storing the authorization server
URL is not sufficient to identify mix-up attacks. An attacker might
declare an uncompromised authorization server's authorization
endpoint URL as "their" authorization server URL, but declare a token
endpoint under their own control.
4.4.2.1. Mix-Up Defense via Issuer Identification
This defense requires that the authorization server sends its issuer
identifier in the authorization response to the client. When
receiving the authorization response, the client MUST compare the
received issuer identifier to the stored issuer identifier. If there
is a mismatch, the client MUST abort the interaction.
There are different ways this issuer identifier can be transported to
the client:
* The issuer information can be transported, for example, via a
separate response parameter iss, defined in [RFC9207].
* When OpenID Connect is used and an ID Token is returned in the
authorization response, the client can evaluate the iss claim in
the ID Token.
In both cases, the iss value MUST be evaluated according to
[RFC9207].
While this defense may require deploying new OAuth features to
transport the issuer information, it is a robust and relatively
simple defense against mix-up.
4.4.2.2. Mix-Up Defense via Distinct Redirect URIs
For this defense, clients MUST use a distinct redirect redirection URI for
each issuer they interact with.
Clients MUST check that the authorization response was received from
the correct issuer by comparing the distinct redirect redirection URI for the
issuer to the URI where the authorization response was received on.
If there is a mismatch, the client MUST abort the flow.
While this defense builds upon existing OAuth functionality, it
cannot be used in scenarios where clients only register once for the
use of many different issuers (as in some open banking schemes) and
due to the tight integration with the client registration, it is
harder to deploy automatically.
Furthermore, an attacker might be able to circumvent the protection
offered by this defense by registering a new client with the "honest"
authorization server using the redirect URI that the client assigned
to the attacker's authorization server. The attacker could then run
the attack as described above, replacing the client ID with the
client ID of their newly created client.
This defense SHOULD therefore only be used if other options are not
available.
4.5. Authorization Code Injection
An attacker who has gained access to an authorization code contained
in an authorization response (see Attacker (A3) in Section 3) can try
to redeem the authorization code for an access token or otherwise
make use of the authorization code.
In the case that the authorization code was created for a public
client, the attacker can send the authorization code to the token
endpoint of the authorization server and thereby get an access token.
This attack was described in Section 4.4.1.1. 4.4.1.1 of [RFC6819].
For confidential clients, or in some special situations, the attacker
can execute an authorization code injection attack, as described in
the following.
In an authorization code injection attack, the attacker attempts to
inject a stolen authorization code into the attacker's own session
with the client. The aim is to associate the attacker's session at
the client with the victim's resources or identity, thereby giving
the attacker at least limited access to the victim's resources.
Besides circumventing the client authentication of confidential
clients, other use cases for this attack include:
* The attacker wants to access certain functions in this particular
client. As an example, the attacker wants to impersonate their
victim in a certain app or on a certain website.
* The authorization or resource servers are limited to certain
networks that the attacker is unable to access directly.
Except in these special cases, authorization code injection is
usually not interesting when the code is created for a public client,
as sending the code to the token endpoint is a simpler and more
powerful attack, as described above.
4.5.1. Attack Description
The authorization code injection attack works as follows:
1. The attacker obtains an authorization code (see Attacker (A3) in
Section 3). For the rest of the attack, only the capabilities of
a web attacker (A1) are required.
2. From the attacker's device, the attacker starts a regular OAuth
authorization process with the legitimate client.
3. In the response of the authorization server to the legitimate
client, the attacker replaces the newly created authorization
code with the stolen authorization code. Since this response is
passing through the attacker's device, the attacker can use any
tool that can intercept and manipulate the authorization response
to this end. The attacker does not need to control the network.
4. The legitimate client sends the code to the authorization
server's token endpoint, along with the redirect_uri and the
client's client ID and client secret (or other means of client
authentication).
5. The authorization server checks the client secret, whether the
code was issued to the particular client, and whether the actual
redirect
redirection URI matches the redirect_uri parameter (see
[RFC6749]).
6. All checks succeed and the authorization server issues access and
other tokens to the client. The attacker has now associated
their session with the legitimate client with the victim's
resources and/or identity.
4.5.2. Discussion
Obviously, the check-in step (Step 5) will fail if the code was
issued to another client ID, e.g., a client set up by the attacker.
The check will also fail if the authorization code was already
redeemed by the legitimate user and was one-time use only.
An attempt to inject a code obtained via a manipulated redirect redirection
URI should also be detected if the authorization server stored the
complete redirect redirection URI used in the authorization request and
compares it with the redirect_uri parameter.
Section 4.1.3 of [RFC6749] requires the authorization server to
| ensure that the "redirect_uri" parameter is present if the
| "redirect_uri" parameter was included in the initial authorization
| request as described in Section 4.1.1, and if included ensure that
| their values are identical.
In the attack scenario described in Section 4.5.1, the legitimate
client would use the correct redirect redirection URI it always uses for
authorization requests. But this URI would not match the tampered
redirect
redirection URI used by the attacker (otherwise, the redirect would
not land at the attacker's page). So, the authorization server would
detect the attack and refuse to exchange the code.
This check could also detect attempts to inject an authorization code
that had been obtained from another instance of the same client on
another device if certain conditions are fulfilled:
* the redirect redirection URI itself contains a nonce or another kind of one-
time
one-time use, secret data and
* the client has bound this data to this particular instance of the
client.
But, this approach conflicts with the idea of enforcing exact
redirect URI matching at the authorization endpoint. Moreover, it
has been observed that providers very often ignore the redirect_uri
check requirement at this stage, maybe because it doesn't seem to be
security-critical from reading the specification.
Other providers just pattern match the redirect_uri parameter against
the registered redirect redirection URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the
respective authorization code for every transaction. However, this
kind of check obviously does not fulfill the intent of the
specification, since the tampered redirect redirection URI is not considered.
So, any attempt to inject an authorization code obtained using the
client_id of a legitimate client or by utilizing the legitimate
client on another device will not be detected in the respective
deployments.
It is also assumed that the requirements defined in Section 4.1.3 of
[RFC6749] increase client implementation complexity as clients need
to store or reconstruct the correct redirect redirection URI for the call to
the token endpoint.
Asymmetric methods for client authentication do not stop this attack,
as the legitimate client authenticates at the token endpoint.
This document therefore recommends instead binding every
authorization code to a certain client instance on a certain device
(or in a certain user agent) in the context of a certain transaction
using one of the mechanisms described next.
4.5.3. Countermeasures
There are two good technical solutions to binding authorization codes
to client instances, as follows.
4.5.3.1. PKCE
The PKCE mechanism specified in [RFC7636] can be used as a
countermeasure (even though it was originally designed to secure
native apps). When the attacker attempts to inject an authorization
code, the check of the code_verifier fails: the client uses its
correct verifier, but the code is associated with a code_challenge
that does not match this verifier.
PKCE not only protects against the authorization code injection
attack but also protects authorization codes created for public
clients: PKCE ensures that an attacker cannot redeem a stolen
authorization code at the token endpoint of the authorization server
without knowledge of the code_verifier.
4.5.3.2. Nonce
OpenID Connect's existing nonce parameter can protect against
authorization code injection attacks. The nonce value is one-time
use and is created by the client. The client is supposed to bind it
to the user agent session and send it with the initial request to the
OpenID Provider (OP). The OP puts the received nonce value into the
ID Token that is issued as part of the code exchange at the token
endpoint. If an attacker injects an authorization code in the
authorization response, the nonce value in the client session and the
nonce value in the ID Token received from the token endpoint will not
match, and the attack is detected. The assumption is that an
attacker cannot get hold of the user agent state on the victim's
device (from which the attacker has stolen the respective
authorization code).
It is important to note that this countermeasure only works if the
client properly checks the nonce parameter in the ID Token obtained
from the token endpoint and does not use any issued token until this
check has succeeded. More precisely, a client protecting itself
against code injection using the nonce parameter
1. MUST validate the nonce in the ID Token obtained from the token
endpoint, even if another ID Token was obtained from the
authorization response (e.g., response_type=code+id_token), and
2. MUST ensure that, unless and until that check succeeds, all
tokens (ID Tokens and the access token) are disregarded and not
used for any other purpose.
It is important to note that nonce does not protect authorization
codes of public clients, as an attacker does not need to execute an
authorization code injection attack. Instead, an attacker can
directly call the token endpoint with the stolen authorization code.
4.5.3.3. Other Solutions
Other solutions like binding state to the code, sender-constraining
the code using cryptographic means, or per-instance client
credentials are conceivable, but lack support and bring new security
requirements.
PKCE is the most obvious solution for OAuth clients, as it is
available at the time of writing, while nonce is appropriate for
OpenID Connect clients.
4.5.4. Limitations
An attacker can circumvent the countermeasures described above if
they can modify the nonce or code_challenge values that are used in
the victim's authorization request. The attacker can modify these
values to be the same ones as those chosen by the client in their own
session in Step 2 of the attack above. (This requires that the
victim's session with the client begins after the attacker started
their session with the client.) If the attacker is then able to
capture the authorization code from the victim, the attacker will be
able to inject the stolen code in Step 3 even if PKCE or nonce are
used.
This attack is complex and requires a close interaction between the
attacker and the victim's session. Nonetheless, measures to prevent
attackers from reading the contents of the authorization response
still need to be taken, as described in Sections 4.1, 4.2, 4.3, 4.4,
and 4.11.
4.6. Access Token Injection
In an access token injection attack, the attacker attempts to inject
a stolen access token into a legitimate client (that is not under the
attacker's control). This will typically happen if the attacker
wants to utilize a leaked access token to impersonate a user in a
certain client.
To conduct the attack, the attacker starts an OAuth flow with the
client using the implicit grant and modifies the authorization
response by replacing the access token issued by the authorization
server or directly making up an authorization server response
including the leaked access token. Since the response includes the
state value generated by the client for this particular transaction,
the client does not treat the response as a CSRF attack and uses the
access token injected by the attacker.
4.6.1. Countermeasures
There is no way to detect such an injection attack in pure-OAuth
flows since the token is issued without any binding to the
transaction or the particular user agent.
In OpenID Connect, the attack can be mitigated, as the authorization
response additionally contains an ID Token containing the at_hash
claim. The attacker therefore needs to replace both the access token
as well as the ID Token in the response. The attacker cannot forge
the ID Token, as it is signed or encrypted with authentication. The
attacker also cannot inject a leaked ID Token matching the stolen
access token, as the nonce claim in the leaked ID Token will contain
(with a very high probability) a different value than the one
expected in the authorization response.
Note that further protection, like sender-constrained access tokens,
is still required to prevent attackers from using the access token at
the resource endpoint directly.
The recommendations in Section 2.1.2 follow from this.
4.7. Cross-Site Request Forgery
An attacker might attempt to inject a request to the redirect redirection URI
of the legitimate client on the victim's device, e.g., to cause the
client to access resources under the attacker's control. This is a
variant of an attack known as Cross-Site Request Forgery (CSRF).
4.7.1. Countermeasures
The long-established countermeasure is that clients pass a random
value, also known as a CSRF Token, in the state parameter that links
the request to the redirect redirection URI to the user agent session as
described. This countermeasure is described in detail in
Section 5.3.5 of [RFC6819]. The same protection is provided by PKCE
or the OpenID Connect nonce value.
When using PKCE instead of state or nonce for CSRF protection, it is
important to note that:
* Clients MUST ensure that the authorization server supports PKCE
before using PKCE for CSRF protection. If an authorization server
does not support PKCE, state or nonce MUST be used for CSRF
protection.
* If state is used for carrying application state, and the integrity
of its contents is a concern, clients MUST protect state against
tampering and swapping. This can be achieved by binding the
contents of state to the browser session and/or signed/encrypted by signing/
encrypting state values. One example of this is discussed in the
expired Internet-Draft [JWT-ENCODED-STATE].
The authorization server therefore MUST provide a way to detect their
support for PKCE. Using Authorization Server Metadata according to
[RFC8414] is RECOMMENDED, but authorization servers MAY instead
provide a deployment-specific way to ensure or determine PKCE
support.
PKCE provides robust protection against CSRF attacks even in the
presence of an attacker that can read the authorization response (see
Attacker (A3) in Section 3). When state is used or an ID Token is
returned in the authorization response (e.g.,
response_type=code+id_token), the attacker either learns the state
value and can replay it into the forged authorization response, or
can extract the nonce from the ID Token and use it in a new request
to the authorization server to mint an ID Token with the same nonce.
The new ID Token can then be used for the CSRF attack.
4.8. PKCE Downgrade Attack
An authorization server that supports PKCE but does not make its use
mandatory for all flows can be susceptible to a PKCE downgrade
attack.
The first prerequisite for this attack is that there is an attacker-
controllable flag in the authorization request that enables or
disables PKCE for the particular flow. The presence or absence of
the code_challenge parameter lends itself for this purpose, i.e., the
authorization server enables and enforces PKCE if this parameter is
present in the authorization request, but it does not enforce PKCE if
the parameter is missing.
The second prerequisite for this attack is that the client is not
using state at all (e.g., because the client relies on PKCE for CSRF
prevention) or that the client is not checking state correctly.
Roughly speaking, this attack is a variant of a CSRF attack. The
attacker achieves the same goal as in the attack described in
Section 4.7: The attacker injects an authorization code (and with
that, an access token) that is bound to the attacker's resources into
a session between their victim and the client.
4.8.1. Attack Description
1. The user has started an OAuth session using some client at an
authorization server. In the authorization request, the client
has set the parameter code_challenge=hash(abc) as the PKCE code
challenge (with the hash function and parameter encoding as
defined in [RFC7636]). The client is now waiting to receive the
authorization response from the user's browser.
2. To conduct the attack, the attacker uses their own device to
start an authorization flow with the targeted client. The client
now uses another PKCE code challenge, say,
code_challenge=hash(xyz), in the authorization request. The
attacker intercepts the request and removes the entire
code_challenge parameter from the request. Since this step is
performed on the attacker's device, the attacker has full access
to the request contents, for example, using browser debug tools.
3. If the authorization server allows for flows without PKCE, it
will create a code that is not bound to any PKCE code challenge.
4. The attacker now redirects the user's browser to an authorization
response URL that contains the code for the attacker's session
with the authorization server.
5. The user's browser sends the authorization code to the client,
which will now try to redeem the code for an access token at the
authorization server. The client will send code_verifier=abc as
the PKCE code verifier in the token request.
6. Since the authorization server sees that this code is not bound
to any PKCE code challenge, it will not check the presence or
contents of the code_verifier parameter. It will issue an access
token (which belongs to the attacker's resource) to the client
under the user's control.
4.8.2. Countermeasures
Using state properly would prevent this attack. However, practice
has shown that many OAuth clients do not use or check state properly.
Therefore, authorization servers MUST mitigate this attack.
Note that from the view of the authorization server, in the attack
described above, a code_verifier parameter is received at the token
endpoint although no code_challenge parameter was present in the
authorization request for the OAuth flow in which the authorization
code was issued.
This fact can be used to mitigate this attack. [RFC7636] already
mandates that
* an authorization server that supports PKCE MUST check whether a
code challenge is contained in the authorization request and bind
this information to the code that is issued; and
* when a code arrives at the token endpoint, and there was a
code_challenge in the authorization request for which this code
was issued, there must be a valid code_verifier in the token
request.
Beyond this, to prevent PKCE downgrade attacks, the authorization
server MUST ensure that if there was no code_challenge in the
authorization request, a request to the token endpoint containing a
code_verifier is rejected.
Authorization servers that mandate the use of PKCE (in general or for
particular clients) implicitly implement this security measure.
4.9. Access Token Leakage at the Resource Server
Access tokens can leak from a resource server under certain
circumstances.
4.9.1. Access Token Phishing by Counterfeit Resource Server
An attacker may set up their own resource server and trick a client
into sending access tokens to it that are valid for other resource
servers (see Attackers (A1) and (A5) in Section 3). If the client
sends a valid access token to this counterfeit resource server, the
attacker in turn may use that token to access other services on
behalf of the resource owner.
This attack assumes the client is not bound to one specific resource
server (and its URL) at development time, but client instances are
provided with the resource server URL at runtime. This kind of late
binding is typical in situations where the client uses a service
implementing a standardized API (e.g., for email, calendar, health, calendaring,
eHealth, or open banking) and where the client is configured by a
user or
administrator for a service that this user or company uses. administrator.
4.9.2. Compromised Resource Server
An attacker may compromise a resource server to gain access to the
resources of the respective deployment. Such a compromise may range
from partial access to the system, e.g., its log files, to full
control over the respective server, in which case all controls can be
circumvented and all resources can be accessed. The attacker would
also be able to obtain other access tokens held on the compromised
system that would potentially be valid to access other resource
servers.
Preventing server breaches by hardening and monitoring server systems
is considered a standard operational procedure and, therefore, out of
the scope of this document. This section Section 4.9 focuses on the impact of
OAuth-related breaches and the replaying of captured access tokens.
4.9.3. Countermeasures
The following measures should be taken into account by implementers
in order to cope with access token replay by malicious actors:
* Sender-constrained access tokens, as described in Section 4.10.1,
SHOULD be used to prevent the attacker from replaying the access
tokens on other resource servers. If an attacker has only partial
access to the compromised system, like a read-only access to web
server logs, sender-constrained access tokens may also prevent
replay on the compromised system.
* Audience restriction as described in Section 4.10.2 SHOULD be used
to prevent replay of captured access tokens on other resource
servers.
* The resource server MUST treat access tokens like other sensitive
secrets and not store or transfer them in plaintext.
The first and second recommendations also apply to other scenarios
where access tokens leak (see Attacker (A5) in Section 3).
4.10. Misuse of Stolen Access Tokens
Access tokens can be stolen by an attacker in various ways, for
example, via the attacks described in Sections 4.1, 4.2, 4.3, 4.4,
and 4.9. Some of these attacks can be mitigated by specific security
measures, as described in the respective sections. However, in some
cases, these measures are not sufficient or are not implemented
correctly. Authorization servers therefore SHOULD ensure that access
tokens are sender-constrained and audience-restricted as described in
the following. Architecture and performance reasons may prevent the
use of these measures in some deployments.
4.10.1. Sender-Constrained Access Tokens
As the name suggests, sender-constrained access tokens scope the
applicability of an access token to a certain sender. This sender is
obliged to demonstrate knowledge of a certain secret as a
prerequisite for the acceptance of that token at a resource server.
A typical flow looks like this:
1. The authorization server associates data with the access token
that binds this particular token to a certain client. The
binding can utilize the client's identity, but in most cases, the
authorization server utilizes key material (or data derived from
the key material) known to the client.
2. This key material must be distributed somehow. Either the key
material already exists before the authorization server creates
the binding or the authorization server creates ephemeral keys.
The way preexisting key material is distributed varies among the
different approaches. For example, X.509 certificates can be
used, in which case the distribution happens explicitly during
the enrollment process. Or, the key material is created and
distributed at the TLS layer, in which case it might
automatically happen during the setup of a TLS connection.
3. The resource server must implement the actual proof-of-possession
check. This is typically done on the application level, often
tied to specific material provided by the transport layer (e.g.,
TLS). The resource server must also ensure that a replay of the
proof of possession is not possible.
Two methods for sender-constrained access tokens using proof of
possession have been defined by the OAuth working group and are in
use in practice:
* "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
Access Tokens" [RFC8705]: The approach specified in this) document
allows the use of mutual TLS (mTLS) for both client authentication and
sender-constrained access tokens. For the purpose of sender-
constrained access tokens, the client is identified towards the
resource server by the fingerprint of its public key. During the
processing of an access token request, the authorization server
obtains the client's public key from the TLS stack and associates
its fingerprint with the respective access tokens. The resource
server in the same way obtains the public key from the TLS stack
and compares its fingerprint with the fingerprint associated with
the access token.
* "OAuth 2.0 Demonstrating Proof of Possession (DPoP)" [RFC9449]:
DPoP outlines an application-level sender-constraining mechanism for sender-
constraining access and refresh tokens. It uses proof-of-possession proof-of-
possession based on a public/private key pair and application-level application-
level signing. DPoP can be used with public clients and, in the
case of confidential clients, can be combined with any client
authentication method.
Note that the security of sender-constrained tokens is undermined
when an attacker gets access to the token and the key material. This
is, in particular, the case for corrupted client software and cross-
site scripting attacks (when the client is running in the browser).
If the key material is protected in a hardware or software security
module or only indirectly accessible (like in a TLS stack), sender-
constrained tokens at least protect against the use of the token when
the client is offline, i.e., when the security module or interface is
not available to the attacker. This applies to access tokens as well
as to refresh tokens (see Section 4.14).
4.10.2. Audience-Restricted Access Tokens
Audience restriction essentially restricts access tokens to a
particular resource server. The authorization server associates the
access token with the particular resource server, and the resource
server is then supposed to verify the intended audience. If the
access token fails the intended audience validation, the resource
server refuses to serve the respective request.
In general, audience restriction limits the impact of token leakage.
In the case of a counterfeit resource server, it may (as described
below) also prevent abuse of the phished access token at the
legitimate resource server.
The audience can be expressed using logical names or physical
addresses (like URLs). To prevent phishing, it is necessary to use
the actual URL the client will send requests to. In the phishing
case, this URL will point to the counterfeit resource server. If the
attacker tries to use the access token at the legitimate resource
server (which has a different URL), the resource server will detect
the mismatch (wrong audience) and refuse to serve the request.
In deployments where the authorization server knows the URLs of all
resource servers, the authorization server may just refuse to issue
access tokens for unknown resource server URLs.
For this to work, the client needs to tell the authorization server
the intended resource server. The mechanism in [RFC8707] can be used
for this or the information can be encoded in the scope value
(Section 3.3 of [RFC6749]).
Instead of the URL, it is also possible to utilize the fingerprint of
the resource server's X.509 certificate as the audience value. This
variant would also allow detection of an attempt to spoof the
legitimate resource server's URL by using a valid TLS certificate
obtained from a different CA. It might also be considered a privacy
benefit to hide the resource server URL from the authorization
server.
Audience restriction may seem easier to use since it does not require
any cryptography on the client side. Still, since every access token
is bound to a specific resource server, the client also needs to
obtain a single resource server-specific access token when accessing
several resource servers. (Resource indicators, as specified in
[RFC8707], can help to achieve this.) [TOKEN-BINDING] had the same
property since different token-binding IDs must be associated with
the access token. Using Mutual mutual TLS for OAuth 2.0 [RFC8705], on the
other hand, allows a client to use the access token at multiple
resource servers.
It should be noted that audience restrictions -- or, generally
speaking, an indication by the client to the authorization server
where it wants to use the access token -- have additional benefits
beyond the scope of token leakage prevention. They allow the
authorization server to create a different access token whose format
and content are specifically minted for the respective server. This
has huge functional and privacy advantages in deployments using
structured access tokens.
4.10.3. Discussion: Preventing Leakage via Metadata
An authorization server could provide the client with additional
information about the locations where it is safe to use its access
tokens. This approach, and why it is not recommended, is discussed
in the following.
In the simplest form, this would require the authorization server to
publish a list of its known resource servers, illustrated in the
following example using a non-standard Authorization Server Metadata
parameter resource_servers:
HTTP/1.1 200 OK
Content-Type: application/json
{
"issuer":"https://server.somesite.example",
"authorization_endpoint":
"https://server.somesite.example/authorize",
"resource_servers":[
"email.somesite.example",
"storage.somesite.example",
"video.somesite.example"
]
...
}
The authorization server could also return the URL(s) an access token
is good for in the token response, illustrated by the example and
non-standard return parameter access_token_resource_server:
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"access_token_resource_server":
"https://hostedresource.somesite.example/path1",
...
}
This mitigation strategy would rely on the client to enforce the
security policy and to only send access tokens to legitimate
destinations. Results of OAuth-related security research (see, for
example, [research.ubc] and [research.cmu]) indicate a large portion
of client implementations do not or fail to properly implement
security controls, like state checks. So, relying on clients to
prevent access token phishing is likely to fail as well. Moreover,
given the ratio of clients to authorization and resource servers, it
is considered the more viable approach to move as much as possible
security-related logic to those entities. servers. Clearly, the client has to
contribute to the overall security. However, there are alternative
countermeasures, as described before, in Sections 4.10.1 and 4.10.2, that
provide a better balance between the involved parties.
4.11. Open Redirection
The following attacks can occur when an authorization server or
client has an open redirector. Such endpoints are sometimes
implemented, for example, to show a message before a user is then
redirected to an external website, or to redirect users back to a URL
they were intending to visit before being interrupted, e.g., by a
login prompt.
4.11.1. Client as Open Redirector
Clients MUST NOT expose open redirectors. Attackers may use open
redirectors to produce URLs pointing to the client and utilize them
to exfiltrate authorization codes and access tokens, as described in
Section 4.1.2. Another abuse case is to produce URLs that appear to
point to the client. This might trick users into trusting the URL
and following it in their browser. This can be abused for phishing.
In order to prevent open redirection, clients should only redirect if
the target URLs are allowed or if the origin and integrity of a
request can be authenticated. Countermeasures against open
redirection are described by OWASP [owasp.redir].
4.11.2. Authorization Server as Open Redirector
Just as with clients, attackers could try to utilize a user's trust
in the authorization server (and its URL in particular) for
performing phishing attacks. OAuth authorization servers regularly
redirect users to other websites (the clients), but they must do so
safely.
Section 4.1.2.1 of [RFC6749] already prevents open redirects by
stating that the authorization server MUST NOT automatically redirect
the user agent in case of an invalid combination of client_id and
redirect_uri.
However, an attacker could also utilize a correctly registered
redirect
redirection URI to perform phishing attacks. The attacker could, for
example, register a client via dynamic client registration [RFC7591]
and execute one of the following attacks:
1. Intentionally send an erroneous authorization request, e.g., by
using an invalid scope value, thus instructing the authorization
server to redirect the user agent to its phishing site.
2. Intentionally send a valid authorization request with client_id
and redirect_uri controlled by the attacker. After the user
authenticates, the authorization server prompts the user to
provide consent to the request. If the user notices an issue
with the request and declines the request, the authorization
server still redirects the user agent to the phishing site. In
this case, the user agent will be redirected to the phishing site
regardless of the action taken by the user.
3. Intentionally send a valid silent authentication request
(prompt=none) with client_id and redirect_uri controlled by the
attacker. In this case, the authorization server will
automatically redirect the user agent to the phishing site.
The authorization server MUST take precautions to prevent these
threats. The authorization server MUST always authenticate the user
first and, with the exception of the silent authentication use case,
prompt the user for credentials when needed, before redirecting the
user. Based on its risk assessment, the authorization server needs
to decide whether or not it can trust the redirect redirection URI. It could
take into account URI analytics done internally or through some
external service to evaluate the credibility and trustworthiness of
content behind the URI, and the source of the redirect redirection URI and
other client data.
The authorization server SHOULD only automatically redirect the user
agent if it trusts the redirect redirection URI. If the URI is not trusted,
the authorization server MAY inform the user and rely on the user to
make the correct decision.
4.12. 307 Redirect
At the authorization endpoint, a typical protocol flow is that the
authorization server prompts the user to enter their credentials in a
form that is then submitted (using the HTTP POST method) back to the
authorization server. The authorization server checks the
credentials and, if successful, redirects the user agent to the
client's redirection endpoint.
In [RFC6749], the HTTP status code 302 (Found) is used for this
purpose, but "any other method available via the user-agent to
accomplish this redirection is allowed". When the status code 307 is
used for redirection instead, the user agent will send the user's
credentials via HTTP POST to the client.
This discloses the sensitive credentials to the client. If the
client is malicious, it can use the credentials to impersonate the
user at the authorization server.
The behavior might be unexpected for developers but is defined in
Section 15.4.8 of [RFC9110]. This status code (307) does not require
the user agent to rewrite the POST request to a GET request and
thereby drop the form data in the POST request body.
In the HTTP standard [RFC9110], only the status code 303
unambiguously enforces rewriting the HTTP POST request to an HTTP GET
request. For all other status codes, including the popular 302, user
agents can opt not to rewrite POST to GET requests and therefore to
reveal requests, thereby causing
the user's credentials to be revealed to the client. (In practice,
however, most user agents will only show this behavior for 307
redirects.)
Authorization servers that redirect a request that potentially
contains the user's credentials therefore MUST NOT use the HTTP 307
status code for redirection. If an HTTP redirection (and not, for
example, JavaScript) is used for such a request, the authorization
server SHOULD use HTTP status code 303 (See Other).
4.13. TLS Terminating Reverse Proxies
A common deployment architecture for HTTP applications is to hide the
application server behind a reverse proxy that terminates the TLS
connection and dispatches the incoming requests to the respective
application server nodes.
This section highlights some attack angles of this deployment
architecture with relevance to OAuth and gives recommendations for
security controls.
In some situations, the reverse proxy needs to pass security-related
data to the upstream application servers for further processing.
Examples include the IP address of the request originator, token-
binding IDs, and authenticated TLS client certificates. This data is
usually passed in HTTP headers added to the upstream request. While
the headers are often custom, application-specific headers,
standardized header fields for client certificates and client
certificate chains are defined in [RFC9440].
If the reverse proxy passes through any header sent from the outside,
an attacker could try to directly send the faked header values
through the proxy to the application server in order to circumvent
security controls that way. For example, it is standard practice of
reverse proxies to accept X-Forwarded-For headers and just add the
origin of the inbound request (making it a list). Depending on the
logic performed in the application server, the attacker could simply
add an allowed IP address to the header and render the protection
useless.
A reverse proxy MUST therefore sanitize any inbound requests to
ensure the authenticity and integrity of all header values relevant
for the security of the application servers.
If an attacker were able to get access to the internal network
between the proxy and application server, the attacker could also try
to circumvent security controls in place. Therefore, it is essential
to ensure the authenticity of the communicating entities.
Furthermore, the communication link between the reverse proxy and
application server MUST be protected against eavesdropping,
injection, and replay of messages.
4.14. Refresh Token Protection
Refresh tokens are a convenient and user-friendly way to obtain new
access tokens. They also add to the security of OAuth, since they
allow the authorization server to issue access tokens with a short
lifetime and reduced scope, thus reducing the potential impact of
access token leakage.
4.14.1. Discussion
Refresh tokens are an attractive target for attackers since because they
represent the full scope of grant a resource owner delegated access granted to a certain client client, and
they are not further constrained to a specific resource. If an
attacker is able to exfiltrate and successfully replay a refresh
token, the attacker will be able to mint access tokens and use them
to access resource servers on behalf of the resource owner.
[RFC6749] already provides robust baseline protection by requiring
* confidentiality of the refresh tokens in transit and storage,
* the transmission of refresh tokens over TLS-protected connections
between authorization server and client,
* the authorization server to maintain and check the binding of a
refresh token to a certain client and authentication of this
client during token refresh, if possible, and
* that refresh tokens cannot be generated, modified, or guessed.
[RFC6749] also lays the foundation for further (implementation-
specific) security measures, such as refresh token expiration and
revocation as well as refresh token rotation by defining respective
error codes and response behaviors.
This specification gives recommendations beyond the scope of
[RFC6749] and clarifications.
4.14.2. Recommendations
Authorization servers MUST determine, based on a risk assessment,
whether to issue refresh tokens to a certain client. If the
authorization server decides not to issue refresh tokens, the client
MAY obtain a new access token by utilizing other grant types, such as
the authorization code grant type. In such a case, the authorization
server may utilize cookies and persistent grants to optimize the user
experience.
If refresh tokens are issued, those refresh tokens MUST be bound to
the scope and resource servers as consented by the resource owner.
This is to prevent privilege escalation by the legitimate client and
reduce the impact of refresh token leakage.
For confidential clients, [RFC6749] already requires that refresh
tokens can only be used by the client for which they were issued.
Authorization servers MUST utilize one of these methods to detect
refresh token replay by malicious actors for public clients:
* *Sender-constrained refresh tokens:* the authorization server
cryptographically binds the refresh token to a certain client
instance, e.g., by utilizing [RFC8705] or [RFC9449].
* *Refresh token rotation:* the authorization server issues a new
refresh token with every access token refresh response. The
previous refresh token is invalidated, but information about the
relationship is retained by the authorization server. If a
refresh token is compromised and subsequently used by both the
attacker and the legitimate client, one of them will present an
invalidated refresh token, which will inform the authorization
server of the breach. The authorization server cannot determine
which party submitted the invalid refresh token, but it will
revoke the active refresh token. This stops the attack at the
cost of forcing the legitimate client to obtain a fresh
authorization grant.
Implementation note: The grant to which a refresh token belongs
may be encoded into the refresh token itself. This can enable an
authorization server to efficiently determine the grant to which a
refresh token belongs, and by extension, all refresh tokens that
need to be revoked. Authorization servers MUST ensure the
integrity of the refresh token value in this case, for example,
using signatures.
Authorization servers MAY revoke refresh tokens automatically in case
of a security event, such as:
* password change or
* logout at the authorization server.
Refresh tokens SHOULD expire if the client has been inactive for some
time, i.e., the refresh token has not been used to obtain fresh
access tokens for some time. The expiration time is at the
discretion of the authorization server. It might be a global value
or determined based on the client policy or the grant associated with
the refresh token (and its sensitivity).
4.15. Client Impersonating Resource Owner
Resource servers may make access control decisions based on the
identity of a resource owner for which an access token was issued, or
based on the identity of a client in the client credentials grant.
For example, [RFC9068] (JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens) describes a data structure for access tokens
containing a sub claim defined as follows:
| In cases of access tokens obtained through grants where a resource
| owner is involved, such as the authorization code grant, the value
| of "sub" SHOULD correspond to the subject identifier of the
| resource owner. In cases of access tokens obtained through grants
| where no resource owner is involved, such as the client
| credentials grant, the value of "sub" SHOULD correspond to an
| identifier the authorization server uses to indicate the client
| application.
If both options are possible, a resource server may mistake a
client's identity for the identity of a resource owner. For example,
if a client is able to choose its own client_id during registration
with the authorization server, a malicious client may set it to a
value identifying a resource owner (e.g., a sub value if OpenID
Connect is used). If the resource server cannot properly distinguish
between access tokens obtained with involvement of the resource owner
and those without, the client may accidentally be able to access
resources belonging to the resource owner.
This attack potentially affects not only implementations using
[RFC9068], but also similar, bespoke solutions.
4.15.1. Countermeasures
Authorization servers SHOULD NOT allow clients to influence their
client_id or any other claim that could cause confusion with a
genuine resource owner if a common namespace for client IDs and user
identifiers exists, such as in the sub claim example from [RFC9068]
shown in Section 4.15 above. Where this cannot be avoided,
authorization servers MUST provide other means for the resource
server to distinguish between the two types of access tokens.
4.16. Clickjacking
As described in Section 4.4.1.9 of [RFC6819], the authorization
request is susceptible to clickjacking attacks, also called user
interface redressing. In such an attack, an attacker embeds the
authorization endpoint user interface in an innocuous context. A
user believing to interact with that context, for example, by
clicking on buttons, inadvertently interacts with the authorization
endpoint user interface instead. The opposite can be achieved as
well: A user believing to interact with the authorization endpoint
might inadvertently type a password into an attacker-provided input
field overlaid over the original user interface. Clickjacking
attacks can be designed such that users can hardly notice the attack,
for example, using almost invisible iframes overlaid on top of other
elements.
An attacker can use this vector to obtain the user's authentication
credentials, change the scope of access granted to the client, and
potentially access the user's resources.
Authorization servers MUST prevent clickjacking attacks. Multiple
countermeasures are described in [RFC6819], including the use of the
X-Frame-Options HTTP response header field and frame-busting
JavaScript. In addition to those, authorization servers SHOULD also
use Content Security Policy (CSP) level 2 [W3C.CSP-2] or greater.
To be effective, CSP must be used on the authorization endpoint and,
if applicable, other endpoints used to authenticate the user and
authorize the client (e.g., the device authorization endpoint, login
pages, error pages, etc.). This prevents framing by unauthorized
origins in user agents that support CSP. The client MAY permit being
framed by some other origin than the one used in its redirection
endpoint. For this reason, authorization servers SHOULD allow
administrators to configure allowed origins for particular clients
and/or for clients to register these dynamically.
Using CSP allows authorization servers to specify multiple origins in
a single response header field and to constrain these using flexible
patterns (see [W3C.CSP-2] for details). Level 2 of CSP provides a
robust mechanism for protecting against clickjacking by using
policies that restrict the origin of frames (by using frame-
ancestors) together with those that restrict the sources of scripts
allowed to execute on an HTML page (by using script-src). A non-
normative example of such a policy is shown in the following listing:
HTTP/1.1 200 OK
Content-Security-Policy: frame-ancestors https://ext.example.org:8000
Content-Security-Policy: script-src 'self'
X-Frame-Options: ALLOW-FROM https://ext.example.org:8000
...
Because some user agents do not support [W3C.CSP-2], this technique
SHOULD be combined with others, including those described in
[RFC6819], unless such legacy user agents are explicitly unsupported
by the authorization server. Even in such cases, additional
countermeasures SHOULD still be employed.
4.17. Attacks on In-Browser Communication Flows
If the authorization response is sent with in-browser communication
techniques like postMessage [WHATWG.postmessage_api] instead of HTTP
redirects, messages may inadvertently be sent to malicious origins or
injected from malicious origins.
4.17.1. Examples
The following non-normative pseudocode examples of attacks using in-
browser communication are described in [research.rub].
4.17.1.1. Insufficient Limitation of Receiver Origins
When sending the authorization response or token response via
postMessage, the authorization server sends the response to the
wildcard origin "*" instead of the client's origin. When the window
to which the response is sent is controlled by an attacker, the
attacker can read the response.
window.opener.postMessage(
{
code: "ABC",
state: "123"
},
"*" // any website in the opener window can receive the message
)
4.17.1.2. Insufficient URI Validation
When sending the authorization response or token response via
postMessage, the authorization server may not check the receiver
origin against the redirect redirection URI and instead, for example, may send
the response to an origin provided by an attacker. This is analogous
to the attack described in Section 4.1.
window.opener.postMessage(
{
code: "ABC",
state: "123"
},
"https://attacker.example" // attacker-provided value
)
4.17.1.3. Injection after Insufficient Validation of Sender Origin
A client that expects the authorization response or token response
via postMessage may not validate the sender origin of the message.
This may allow an attacker to inject an authorization response or
token response into the client.
In the case of a maliciously injected authorization response, the
attack is a variant of the CSRF attacks described in Section 4.7.
The countermeasures described in Section 4.7 apply to this attack as
well.
In the case of a maliciously injected token response, sender-
constrained access tokens as described in Section 4.10.1 may prevent
the attack under some circumstances, but additional countermeasures
as described in Section 4.17.2 are generally required.
4.17.2. Recommendations
When comparing client receiver origins against pre-registered
origins, authorization servers MUST utilize exact string matching as
described in Section 4.1.3. Authorization servers MUST send
postMessages to trusted client receiver origins, as shown in the
following, non-normative example:
window.opener.postMessage(
{
code: "ABC",
state: "123"
},
"https://client.example" // use explicit client origin
)
Wildcard origins like "*" in postMessage MUST NOT be used, as
attackers can use them to leak a victim's in-browser message to
malicious origins. Both measures contribute to the prevention of
leakage of authorization codes and access tokens (see Section 4.1).
Clients MUST prevent injection of in-browser messages on the client
receiver endpoint. Clients MUST utilize exact string matching to
compare the initiator origin of an in-browser message with the
authorization server origin, as shown in the following, non-normative
example:
window.addEventListener("message", (e) => {
// validate exact authorization server origin
if (e.origin === "https://honest.as.example") {
// process e.data.code and e.data.state
}
})
Since in-browser communication flows only apply a different
communication technique (i.e., postMessage instead of HTTP redirect),
all measures protecting the authorization response listed in
Section 2.1 MUST be applied equally.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
Security considerations are described in Sections 2, 3, and 4.
7. References
7.1. Normative References
[BCP195] Best Current Practice 195,
<https://www.rfc-editor.org/info/bcp195>.
At the time of writing, this BCP comprises the following:
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[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>.
[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>.
[RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
May 2015, <https://www.rfc-editor.org/info/rfc7521>.
[RFC7523] Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
2015, <https://www.rfc-editor.org/info/rfc7523>.
[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>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>.
[RFC9068] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
2021, <https://www.rfc-editor.org/info/rfc9068>.
7.2. Informative References
[arXiv.1508.04324v2]
Mladenov, V., Mainka, C., and J. Schwenk, "On the security
of modern Single Sign-On Protocols: Second-Order
Vulnerabilities in OpenID Connect", arXiv:1508.04324v2,
DOI 10.48550/arXiv.1508.04324, 7 January 2016,
<https://arxiv.org/abs/1508.04324v2/>.
[arXiv.1601.01229]
Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0", arXiv:1601.01229,
DOI 10.48550/arXiv.1601.01229, 6 January 2016,
<https://arxiv.org/abs/1601.01229/>.
[arXiv.1704.08539]
Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
Standard OpenID Connect: In-Depth Formal Security Analysis
and Security Guidelines", arXiv:1704.08539,
DOI 10.48550/arXiv.1704.08539, 27 April 2017,
<https://arxiv.org/abs/1704.08539/>.
[arXiv.1901.11520]
Fett, D., Hosseyni, P., and R. Küsters, "An Extensive
Formal Security Analysis of the OpenID Financial-grade
API", arXiv:1901.11520, DOI 10.48550/arXiv.1901.11520, 31
January 2019, <https://arxiv.org/abs/1901.11520/>.
[bug.chromium]
"Referer header includes URL fragment when opening link
using New Tab", Chromium Issue Tracker, Issue ID:
40076763, <https://issues.chromium.org/issues/40076763>.
[JWT-ENCODED-STATE]
Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
claims in the OAuth 2 state parameter using a JWT", Work
in Progress, Internet-Draft, draft-bradley-oauth-jwt-
encoded-state-09, 4 November 2018,
<https://datatracker.ietf.org/doc/html/draft-bradley-
oauth-jwt-encoded-state-09>.
[TOKEN-BINDING]
Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", Work in Progress, Internet-
Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
token-binding-08>.
[OAUTH-V2.1]
Hardt, D., Parecki, A., and T. Lodderstedt, "The OAuth 2.1
Authorization Framework", Work in Progress, Internet-
Draft, draft-ietf-oauth-v2-1-11, 14 May draft-ietf-oauth-v2-1-12, 15 November 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
v2-1-11>.
v2-1-12>.
[OAuth.Post]
Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
Mode", The OpenID Foundation, 27 April 2015,
<https://openid.net/specs/oauth-v2-form-post-response-
mode-1_0.html>.
[OAuth.Responses]
de Medeiros, B., Ed., Scurtescu, M., Tarjan, P., and M.
Jones, "OAuth 2.0 Multiple Response Type Encoding
Practices", The OpenID Foundation, 25 February 2014,
<https://openid.net/specs/oauth-v2-multiple-response-
types-1_0.html>.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 2", The OpenID Foundation, 15 December 2023,
<https://openid.net/specs/openid-connect-core-1_0.html>.
[OpenID.Discovery]
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0 incorporating errata set 2", The
OpenID Foundation, 15 December 2023,
<https://openid.net/specs/openid-connect-discovery-
1_0.html>.
[OpenID.JARM]
Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
Secured Authorization Response Mode for OAuth 2.0 (JARM)",
The OpenID Foundation, 17 October 2018,
<https://openid.net/specs/openid-financial-api-jarm.html>.
[owasp.redir]
OWASP Foundation, "Unvalidated Redirects and Forwards
Cheat Sheet", OWASP Cheat Sheet Series,
<https://cheatsheetseries.owasp.org/cheatsheets/
Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.
[research.cmu]
Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
Tague, "OAuth Demystified for Mobile Application
Developers", CCS '14: Proceedings of the 2014 ACM SIGSAC
Conference on Computer and Communications Security, pp.
892-903, DOI 10.1145/2660267.2660323, November 2014,
<https://www.microsoft.com/en-us/research/wp-
content/uploads/2016/02/OAuthDemystified.pdf>.
[research.jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
Maffeis, "Discovering concrete attacks on website
authorization by formal analysis", Journal of Computer
Security, vol. 22, no. 4, pp. 601-657, DOI 10.3233/JCS-
140503, 23 April 2014,
<https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.
[research.rub]
Jannett, L., Mladenov, V., Mainka, C., and J. Schwenk,
"DISTINCT: Identity Theft using In-Browser Communications
in Dual-Window Single Sign-On", CCS '22: Proceedings of
the 2022 ACM SIGSAC Conference on Computer and
Communications Security, DOI 10.1145/3548606.3560692, 7
November 2022,
<https://dl.acm.org/doi/pdf/10.1145/3548606.3560692>.
[research.rub2]
Fries, C., "Security Analysis of Real-Life OpenID Connect
Implementations", Master's thesis, Ruhr-Universität Bochum
(RUB), 20 December 2020,
<https://www.nds.rub.de/media/ei/arbeiten/2021/05/03/
masterthesis.pdf>.
[research.ubc]
Sun, S.-T. and K. Beznosov, "The Devil is in the
(Implementation) Details: An Empirical Analysis of OAuth
SSO Systems", Proceedings of the 2012 ACM conference on
Computer and communications security (CCS '12), pp.
378-390, DOI 10.1145/2382196.2382238, October 2012,
<https://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>.
[research.udel]
Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
to Us: Understanding the Security Threats of Dangling DNS
Records", CCS '16: Proceedings of the 2016 ACM SIGSAC
Conference on Computer and Communications Security, pp.
1414-1425, DOI 10.1145/2976749.2978387, 24 October 2016,
<https://dl.acm.org/doi/pdf/10.1145/2976749.2978387>.
[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>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
February 2020, <https://www.rfc-editor.org/info/rfc8707>.
[RFC9101] Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
Authorization Framework: JWT-Secured Authorization Request
(JAR)", RFC 9101, DOI 10.17487/RFC9101, August 2021,
<https://www.rfc-editor.org/info/rfc9101>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9126] Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
RFC 9126, DOI 10.17487/RFC9126, September 2021,
<https://www.rfc-editor.org/info/rfc9126>.
[RFC9207] Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0
Authorization Server Issuer Identification", RFC 9207,
DOI 10.17487/RFC9207, March 2022,
<https://www.rfc-editor.org/info/rfc9207>.
[RFC9396] Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
Rich Authorization Requests", RFC 9396,
DOI 10.17487/RFC9396, May 2023,
<https://www.rfc-editor.org/info/rfc9396>.
[RFC9440] Campbell, B. and M. Bishop, "Client-Cert HTTP Header
Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
<https://www.rfc-editor.org/info/rfc9440>.
[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/info/rfc9449>.
[TOKEN-BINDING]
Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", Work in Progress, Internet-
Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
token-binding-08>.
[W3C.CSP-2]
West, M., Barth, A., and D. Veditz, "Content Security
Policy Level 2", W3C Recommendation, December 2016,
<https://www.w3.org/TR/2016/REC-CSP2-20161215/>. Latest
version available at <https://www.w3.org/TR/CSP2/>.
[W3C.webappsec-referrer-policy]
Eisinger, J. and E. Stark, "Referrer Policy", 26 January
2017,
<https://www.w3.org/TR/2017/CR-referrer-policy-20170126/>.
Latest version available at
<https://www.w3.org/TR/referrer-policy/>.
[W3C.WebAuthn]
Hodges, J., Jones, J.C., Jones, M.B., Kumar, A., and E.
Lundberg, "Web Authentication: An API for accessing Public
Key Credentials Level 2", W3C Recommendation, 8 April
2021,
<https://www.w3.org/TR/2021/REC-webauthn-2-20210408/>.
Latest version available at
<https://www.w3.org/TR/webauthn-2/>.
[W3C.WebCrypto]
Watson, M., Ed., "Web Cryptography API", W3C
Recommendation, 26 January 2017,
<https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>.
Latest version available at
<https://www.w3.org/TR/WebCryptoAPI/>.
[W3C.webappsec-referrer-policy]
Eisinger, J. and E. Stark, "Referrer Policy", 26 January
2017,
<https://www.w3.org/TR/2017/CR-referrer-policy-20170126/>.
Latest version available at
<https://www.w3.org/TR/referrer-policy/>.
[WHATWG.CORS]
WHATWG, "CORS protocol", Fetch: Living Standard,
Section 3.2, 17 June 2024,
<https://fetch.spec.whatwg.org/#http-cors-protocol>.
[WHATWG.postmessage_api]
WHATWG, "Cross-document messaging", HTML: Living Standard,
Section 9.3, 19 August 2024,
<https://html.spec.whatwg.org/multipage/web-
messaging.html#web-messaging>.
[arXiv.1508.04324v2]
Mladenov, V., Mainka, C., and J. Schwenk, "On the security
of modern Single Sign-On Protocols: Second-Order
Vulnerabilities in OpenID Connect", arXiv:1508.04324v2,
DOI 10.48550/arXiv.1508.04324, 7 January 2016,
<https://arxiv.org/abs/1508.04324v2/>.
[arXiv.1601.01229]
Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0", arXiv:1601.01229,
DOI 10.48550/arXiv.1601.01229, 6 January 2016,
<https://arxiv.org/abs/1601.01229/>.
[arXiv.1704.08539]
Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
Standard OpenID Connect: In-Depth Formal Security Analysis
and Security Guidelines", arXiv:1704.08539,
DOI 10.48550/arXiv.1704.08539, 27 April 2017,
<https://arxiv.org/abs/1704.08539/>.
[arXiv.1901.11520]
Fett, D., Hosseyni, P., and R. Küsters, "An Extensive
Formal Security Analysis of the OpenID Financial-grade
API", arXiv:1901.11520, DOI 10.48550/arXiv.1901.11520, 31
January 2019, <https://arxiv.org/abs/1901.11520/>.
[bug.chromium]
"Referer header includes URL fragment when opening link
using New Tab", Chromium Issue Tracker, Issue ID:
40076763, <https://issues.chromium.org/issues/40076763>.
[owasp.redir]
OWASP Foundation, "Unvalidated Redirects and Forwards
Cheat Sheet", OWASP Cheat Sheet Series,
<https://cheatsheetseries.owasp.org/cheatsheets/
Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.
[research.cmu]
Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
Tague, "OAuth Demystified for Mobile Application
Developers", November 2014,
<https://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>.
[research.jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
Maffeis, "Discovering concrete attacks on website
authorization by formal analysis", Journal of Computer
Security, vol. 22, no. 4, pp. 601-657, DOI 10.3233/JCS-
140503, 23 April 2014,
<https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.
[research.rub]
Jannett, L., Mladenov, V., Mainka, C., and J. Schwenk,
"DISTINCT: Identity Theft using In-Browser Communications
in Dual-Window Single Sign-On", CCS '22: Proceedings of
the 2022 ACM SIGSAC Conference on Computer and
Communications Security, DOI 10.1145/3548606.3560692, 7
November 2022,
<https://dl.acm.org/doi/pdf/10.1145/3548606.3560692>.
[research.rub2]
Fries, C., "Security Analysis of Real-Life OpenID Connect
Implementations", Master's thesis, Ruhr-Universität Bochum
(RUB), 20 December 2020,
<https://www.nds.rub.de/media/ei/arbeiten/2021/05/03/
masterthesis.pdf>.
[research.ubc]
Sun, S.-T. and K. Beznosov, "The Devil is in the
(Implementation) Details: An Empirical Analysis of OAuth
SSO Systems", Proceedings of the 2012 ACM conference on
Computer and communications security (CCS '12), pp.
378-390, DOI 10.1145/2382196.2382238, October 2012,
<https://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>.
[research.udel]
Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
to Us: Understanding the Security Threats of Dangling DNS
Records", CCS '16: Proceedings of the 2016 ACM SIGSAC
Conference on Computer and Communications Security, pp.
1414-1425, DOI 10.1145/2976749.2978387, 24 October 2016,
<https://dl.acm.org/doi/pdf/10.1145/2976749.2978387>.
Appendix A.
Acknowledgements
We would like to thank Brock Allen, Annabelle Richard Backman,
Dominick Baier, Vittorio Bertocci, Brian Campbell, Bruno Crispo,
William Dennis, George Fletcher, Matteo Golinelli, Dick Hardt, Joseph
Heenan, Pedram Hosseyni, Phil Hunt, Tommaso Innocenti, Louis Jannett,
Jared Jennings, Michael B. Jones, Engin Kirda, Konstantin Lapine,
Neil Madden, Christian Mainka, Jim Manico, Nov Matake, Doug McDorman,
Karsten Meyer zu Selhausen, Ali Mirheidari, Vladislav Mladenov, Kaan
Onarioglu, Aaron Parecki, Michael Peck, Johan Peeters, Nat Sakimura,
Guido Schmitz, Jörg Schwenk, Rifaat Shekh-Yusef, Travis Spencer,
Petteri Stenius, Tomek Stojecki, David Waite, Tim Würtele, and Hans
Zandbelt for their valuable feedback.
Authors' Addresses
Torsten Lodderstedt
SPRIND
Email: torsten@lodderstedt.net
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Independent Researcher
Email: isciurus@gmail.com
Daniel Fett
Authlete
Email: mail@danielfett.de