OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-18
Web Authorization Protocol T. Lodderstedt
Internet-Draft yes.com
Intended status: Best Current Practice J. Bradley
Expires: 14 October 2021 Yubico
A. Labunets
D. Fett
yes.com
12 April 2021
OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-18
Abstract
This document describes best current security practice for OAuth 2.0.
It updates and extends the OAuth 2.0 Security Threat Model 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Structure . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Protecting Redirect-Based Flows . . . . . . . . . . . . . 5
2.1.1. Authorization Code Grant . . . . . . . . . . . . . . 6
2.1.2. Implicit Grant . . . . . . . . . . . . . . . . . . . 7
2.2. Token Replay Prevention . . . . . . . . . . . . . . . . . 7
2.2.1. Access Tokens . . . . . . . . . . . . . . . . . . . . 7
2.2.2. Refresh Tokens . . . . . . . . . . . . . . . . . . . 7
2.3. Access Token Privilege Restriction . . . . . . . . . . . 8
2.4. Resource Owner Password Credentials Grant . . . . . . . . 8
2.5. Client Authentication . . . . . . . . . . . . . . . . . . 9
2.6. Other Recommendations . . . . . . . . . . . . . . . . . . 9
3. The Updated OAuth 2.0 Attacker Model . . . . . . . . . . . . 10
4. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 12
4.1. Insufficient Redirect URI Validation . . . . . . . . . . 12
4.1.1. Redirect URI Validation Attacks on Authorization Code
Grant . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.2. Redirect URI Validation Attacks on Implicit Grant . . 14
4.1.3. Countermeasures . . . . . . . . . . . . . . . . . . . 15
4.2. Credential Leakage via Referer Headers . . . . . . . . . 16
4.2.1. Leakage from the OAuth Client . . . . . . . . . . . . 16
4.2.2. Leakage from the Authorization Server . . . . . . . . 17
4.2.3. Consequences . . . . . . . . . . . . . . . . . . . . 17
4.2.4. Countermeasures . . . . . . . . . . . . . . . . . . . 17
4.3. Credential Leakage via Browser History . . . . . . . . . 18
4.3.1. Authorization Code in Browser History . . . . . . . . 18
4.3.2. Access Token in Browser History . . . . . . . . . . . 18
4.4. Mix-Up Attacks . . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Attack Description . . . . . . . . . . . . . . . . . 19
4.4.2. Countermeasures . . . . . . . . . . . . . . . . . . . 21
4.5. Authorization Code Injection . . . . . . . . . . . . . . 23
4.5.1. Attack Description . . . . . . . . . . . . . . . . . 23
4.5.2. Discussion . . . . . . . . . . . . . . . . . . . . . 24
4.5.3. Countermeasures . . . . . . . . . . . . . . . . . . . 25
4.5.4. Limitations . . . . . . . . . . . . . . . . . . . . . 26
4.6. Access Token Injection . . . . . . . . . . . . . . . . . 27
4.6.1. Countermeasures . . . . . . . . . . . . . . . . . . . 27
4.7. Cross Site Request Forgery . . . . . . . . . . . . . . . 27
4.7.1. Countermeasures . . . . . . . . . . . . . . . . . . . 28
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4.8. PKCE Downgrade Attack . . . . . . . . . . . . . . . . . . 28
4.8.1. Attack Description . . . . . . . . . . . . . . . . . 29
4.8.2. Countermeasures . . . . . . . . . . . . . . . . . . . 29
4.9. Access Token Leakage at the Resource Server . . . . . . . 30
4.9.1. Access Token Phishing by Counterfeit Resource
Server . . . . . . . . . . . . . . . . . . . . . . . 30
4.9.2. Compromised Resource Server . . . . . . . . . . . . . 35
4.10. Open Redirection . . . . . . . . . . . . . . . . . . . . 36
4.10.1. Client as Open Redirector . . . . . . . . . . . . . 36
4.10.2. Authorization Server as Open Redirector . . . . . . 37
4.11. 307 Redirect . . . . . . . . . . . . . . . . . . . . . . 37
4.12. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 38
4.13. Refresh Token Protection . . . . . . . . . . . . . . . . 39
4.13.1. Discussion . . . . . . . . . . . . . . . . . . . . . 39
4.13.2. Recommendations . . . . . . . . . . . . . . . . . . 40
4.14. Client Impersonating Resource Owner . . . . . . . . . . . 41
4.14.1. Countermeasures . . . . . . . . . . . . . . . . . . 41
4.15. Clickjacking . . . . . . . . . . . . . . . . . . . . . . 41
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
7. Security Considerations . . . . . . . . . . . . . . . . . . . 43
8. Normative References . . . . . . . . . . . . . . . . . . . . 43
9. Informative References . . . . . . . . . . . . . . . . . . . 44
Appendix A. Document History . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 ("OAuth"
in the following) has gotten massive traction in the market and
became the standard for API protection and the basis for federated
login using OpenID Connect [OpenID]. 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. 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.
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* 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 a static relationship between client,
authorization server and resource servers. The URLs of AS and RS
were known to the client at deployment time and built an anchor
for the trust relationship among those parties. The validation
whether the client talks to a legitimate server was based on TLS
server authentication (see [RFC6819], Section 4.5.4). 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 e-mail or
OpenID Connect) or serve as a frontend to a particular tenant in a
multi-tenancy 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 in order to
support the usage 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 does not supplant the security advice given in
[RFC6749], [RFC6750], and [RFC6819], but complements those documents.
1.1. Structure
The remainder of this document is organized as follows: The next
section summarizes the most important recommendations of the OAuth
working group for every OAuth implementor. Afterwards, the updated
the OAuth attacker model is presented. Subsequently, a detailed
analysis of the threats and implementation issues that can be found
in the wild today is given 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.
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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].
2. Recommendations
This section describes the set of security mechanisms the OAuth
working group recommends to OAuth implementers.
2.1. Protecting Redirect-Based Flows
When comparing client redirect 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 AS MUST NOT expose URLs that forward the user's browser
to arbitrary URIs obtained from a query parameter ("open
redirector"). Open redirectors can enable exfiltration of
authorization codes and access tokens, see Section 4.10.1.
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 a malicious third
party (see Section 4.4.1.8. of [RFC6819] for details). Clients that
have ensured that the authorization server supports PKCE [RFC7636]
MAY rely 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
[I-D.ietf-oauth-iss-auth-resp], 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] or in [JARM] responses), processing it as described in
[I-D.ietf-oauth-iss-auth-resp].
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In the absence of these options, clients MAY instead use distinct
redirect URIs to identify authorization endpoints and token
endpoints, as described in Section 4.4.2.
An AS that redirects a request potentially containing user
credentials MUST avoid forwarding these user credentials accidentally
(see Section 4.11 for details).
2.1.1. Authorization Code Grant
Clients MUST prevent injection (replay) of authorization codes into
the authorization response by attackers. Public clients MUST use
PKCE [RFC7636] to this end. For confidential clients, the use of
PKCE [RFC7636] is RECOMMENDED. With additional precautions,
described in Section 4.5.3.2, confidential clients MAY use the OpenID
Connect "nonce" parameter and the respective Claim in the ID Token
[OpenID] 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.
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 A4 in Section 3) can break the security provided by PKCE.
Currently, "S256" is the only such method.
Authorization servers MUST support PKCE [RFC7636].
Authorization servers MUST provide a way to detect their support for
PKCE. It is RECOMMENDED for AS to publish the element
"code_challenge_methods_supported" in their AS metadata ([RFC8414])
containing the supported PKCE challenge methods (which can be used by
the client to detect PKCE support). AS MAY instead provide a
deployment-specific way to ensure or determine PKCE support by the
AS.
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.
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2.1.2. Implicit Grant
The implicit grant (response type "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 Section 4.1, Section 4.2,
Section 4.3, and Section 4.6.
Moreover, no viable mechanism exists to cryptographically bind access
tokens issued in the authorization response to a certain client as it
is recommended in Section 2.2. This makes replay detection for such
access tokens at resource servers impossible.
In order to avoid these issues, clients SHOULD NOT use the implicit
grant (response type "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" (aka
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 next
section).
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 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 to prevent token replay, such as Mutual
TLS for OAuth 2.0 [RFC8705] (see Section 4.9.1.1.2).
2.2.2. Refresh Tokens
Refresh tokens MUST be sender-constrained or use refresh token
rotation as described in Section 4.13.
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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 restricted to certain resource
servers (audience restriction), preferably to a single resource
server. 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 not, the resource server MUST refuse to serve
the respective request. Clients and authorization servers MAY
utilize the parameters "scope" or "resource" as specified in
[RFC6749] and [I-D.ietf-oauth-resource-indicators], 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 [I-D.ietf-oauth-rar] to
determine those resources and/or actions.
2.4. Resource Owner Password Credentials Grant
The resource owner password credentials grant MUST NOT be used. This
grant type insecurely exposes the credentials of the resource owner
to the client. Even if the client is benign, this results in an
increased attack surface (credentials can leak in more places than
just the AS) and users are trained to enter their credentials in
places other than the AS.
Furthermore, adapting the resource owner password credentials grant
to two-factor authentication, authentication with cryptographic
credentials (cf. WebCrypto [webcrypto], WebAuthn [webauthn]), and
authentication processes that require multiple steps can be hard or
impossible.
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2.5. Client Authentication
Authorization servers SHOULD use client authentication if possible.
It is RECOMMENDED to use asymmetric (public-key based) methods for
client authentication such as mTLS [RFC8705] or "private_key_jwt"
[OpenID]. When asymmetric methods for client authentication are
used, authorization servers do not need to store sensitive symmetric
keys, making these methods more robust against a number of attacks.
2.6. Other Recommendations
The use of OAuth 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 AS publish OAuth metadata according
to [RFC8414] and that clients make use of this metadata to configure
themselves when available.
Authorization servers SHOULD NOT allow clients to influence their
"client_id" or "sub" value or any other Claim if that can cause
confusion with a genuine resource owner (see Section 4.14).
It is RECOMMENDED to use end-to-end TLS. If TLS traffic needs to be
terminated at an intermediary, refer to Section 4.12 for further
security advice.
Authorization responses MUST NOT be transmitted over unencrypted
network connections. To this end, AS MUST NOT allow redirect URIs
that use the "http" scheme except for native clients that use
Loopback Interface Redirection as described in [RFC8252],
Section 7.3.
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3. The Updated OAuth 2.0 Attacker Model
In [RFC6819], an attacker model is laid out that describes the
capabilities of attackers against which OAuth deployments must be
protected. In the following, this attacker model is updated 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.
OAuth MUST ensure that the authorization of the resource owner (RO)
(with a user agent) at the authorization server (AS) and the
subsequent usage of the access token at the resource server (RS) is
protected at least against the following attackers:
* (A1) Web Attackers that can set up and operate an arbitrary number
of network endpoints including browsers and servers (except for
the concrete RO, AS, and RS). Web attackers may set up web sites
that are visited by the RO, operate their own user agents, and
participate in the protocol.
Web attackers may, in particular, operate OAuth clients that are
registered at AS, and operate their own authorization and resource
servers that can be used (in parallel) by the RO and other
resource owners.
It must also be assumed that web attackers can lure the user to
open 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 legit-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 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 AS).
* (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.
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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 (wifi) network using
ARP spoofing, or a state-sponsored attacker with access to internet
exchange points, for instance.
These attackers 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 in which their OAuth
implementations are expected to run. Previous attacks on OAuth have
shown that OAuth deployments SHOULD in particular consider the
following, stronger attackers in addition to those listed above:
* (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 for such attacks include open redirector attacks,
problems existing on mobile operating 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 a attacker-controlled AS, 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 AS. For
example, a resource server can be compromised by an attacker, an
access token may be sent to an attacker-controlled resource server
due to a misconfiguration, or an RO is social-engineered into
using a attacker-controlled RS. See also 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 in this attacker model, an attacker (see A1) can be a RO or
act as one. For example, an attacker can use his own browser to
replay tokens or authorization codes obtained by any of the attacks
described above at the client or RS.
This document focusses on threats resulting from these attackers.
Attacks in an even stronger attacker model are discussed, for
example, in [arXiv.1901.11520].
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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.
4.1. Insufficient Redirect URI Validation
Some authorization servers allow clients to register redirect URI
patterns instead of complete redirect URIs. The authorization
servers then match the redirect 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 URIs.
This approach turned out to be more complex to implement and more
error prone to manage than exact redirect URI matching. Several
successful attacks exploiting flaws in the pattern matching
implementation or concrete configurations have been observed in the
wild . Insufficient validation of the redirect 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 attackers
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 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 URI for the client, for example
"https://app1.somesite.example/redirect". A naive implementation on
the authorization server, however, might interpret the wildcard "*"
as "any character" and not "any character valid for a domain name".
The authorization server, therefore, might permit
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"https://attacker.example/.somesite.example" as a redirect URI,
although "attacker.example" is a different domain potentially
controlled by a malicious party.
The attack can then be conducted as follows:
First, the attacker needs to trick the user into opening a tampered
URL in his browser that launches a page under the attacker's control,
say "https://www.evil.example" (see Attacker A1.)
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 URI and compares it
to the registered redirect URL patterns for the client "s6BhdRkqt3".
The authorization request is processed and presented to the user.
If the user does not see the redirect 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 impersonated 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. The attacker can, however, use the legitimate
confidential client to redeem the code by performing an authorization
code injection attack, see Section 4.5.
Note: Vulnerabilities of this kind can also exist if the
authorization server handles wildcards properly. For example, assume
that the client registers the redirect URL pattern
"https://*.somesite.example/*" and the authorization server
interprets this as "allow redirect URIs pointing to any host residing
in the domain "somesite.example"". If an attacker manages to
establish a host or subdomain in "somesite.example", he can
impersonate the legitimate client. This could be caused, for
example, by a subdomain takeover attack [subdomaintakeover], where an
outdated CNAME record (say, "external-service.somesite.example")
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points to an external DNS name that does no longer exist (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 a URI
under his control, he will directly get access to the fragment
carrying the access token.
Additionally, implicit clients can be subject to a further kind of
attack. It utilizes the fact that user agents re-attach fragments to
the destination URL of a redirect if the location header does not
contain a fragment (see [RFC7231], Section 9.5). The attack
described here combines this behavior with the client as an open
redirector (see Section 4.10.1) in order to get access to access
tokens. This allows circumvention even of very narrow redirect URI
patterns, but not 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:
First, and as above, the attacker needs to trick the user into
opening a tampered URL in his 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 URI and it uses the response type "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
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Now, since the redirect 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 example.com, 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 re-attach 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 now 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 to simplify the required logic and configuration by using
exact redirect URI matching. This means the authorization server
MUST compare the two URIs using simple string comparison as defined
in [RFC3986], Section 6.2.1. The only exception are native apps
using a "localhost" URI: In this case, the AS MUST allow variable
port numbers as described in [RFC8252], Section 7.3.
Additional recommendations:
* Servers on which callbacks are hosted MUST NOT expose open
redirectors (see Section 4.10).
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* 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 causing access token issuance at the authorization
endpoint. This offers countermeasures against reuse of leaked
credentials through the exchange process with the authorization
server and token replay through sender-constraining of the access
tokens.
If the origin and integrity of the authorization request containing
the redirect URI can be verified, for example when using
[I-D.ietf-oauth-jwsreq] or [I-D.ietf-oauth-par] with client
authentication, the authorization server MAY trust the redirect 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 [RFC7231], Section 5.5.2), by leaking
either from the AS's or the client's web site, respectively. Most
importantly, authorization codes or "state" values can be disclosed
in this way. Although specified otherwise in [RFC7231],
Section 5.5.2, the same may happen to access tokens conveyed in URI
fragments due to browser implementation issues as illustrated by
Chromium Issue 168213 [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").
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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 Section 4.1.1,
Section 4.5, and Section 4.6. If the attacker learns "state", the
CSRF protection achieved by using "state" is lost, resulting in CSRF
attacks as described in [RFC6819], Section 4.4.1.8.
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 [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 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 [RFC6749], Section 4.1.2, authorization codes MUST
be invalidated by the AS after their first use at the token
endpoint. For example, if an AS invalidated the code after the
legitimate client redeemed it, the attacker would fail exchanging
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 AS SHOULD revoke all tokens
issued previously based on that code.
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* 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 web site, 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 AS's web site, 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-v2-form-post-response-mode]).
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 [RFC6819],
Section 4.4.1.1, and Section 4.5.
* Use form post response mode instead of redirect for the
authorization response (see [oauth-v2-form-post-response-mode]).
4.3.2. Access Token in Browser History
An access token may end up in the browser history if a client or a
web site 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 to transfer tokens via a
header, but in practice web sites often pass access tokens in query
parameters.
In case of the 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.
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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-v2-form-post-response-mode] can be used to
this end.
4.4. Mix-Up Attacks
Mix-up is an attack on 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 his 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, we assume that
* the implicit or authorization code grant are used with multiple AS
of which one is considered "honest" (H-AS) and one is operated by
the attacker (A-AS), and
* the client stores the AS chosen by the user in a session bound to
the user's browser and uses the same redirection endpoint URI for
each AS.
In the following, we assume 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 shorted for presentation to only
include parameters relevant for the attack.
Attack on the authorization code grant:
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1. The user selects to start the grant using A-AS (e.g., by clicking
on a button at 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 her resources at H-AS.
(Note that a vigilant user might at this point detect that she
intended to use A-AS instead of H-AS. The first attack variant
listed below avoids this.) 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 AS and is then redirected by the client to that
AS), as in Attacker A2. This capability can, for example, be the
result of a man-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 Steps 2
and following above.
* *Implicit Grant*: In the implicit grant, the attacker receives an
access token instead of the code; the rest of the attack works as
above.
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* *Per-AS Redirect URIs*: If clients use different redirect URIs for
different ASs, do not store the selected AS in the user's session,
and ASs do not check the redirect URIs properly, attackers can
mount an attack called "Cross-Social Network Request Forgery".
These attacks have been observed in practice. Refer to
[oauth_security_jcs_14] for details.
* *OpenID Connect*: There are variants that can be used to attack
OpenID Connect. In these attacks, the attacker misuses features
of the OpenID Connect Discovery [OpenIDDisc] mechanism or replays
access tokens or ID Tokens to conduct a mix-up attack. The
attacks are described in detail in [arXiv.1704.08539], Appendix A,
and [arXiv.1508.04324v2], Section 6 ("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 in
the following.
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 metadata [RFC8414] nor OpenID Connect Discovery
[OpenIDDisc] are 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.
Note: Just storing the authorization server URL is not sufficient to
identify mix-up attacks. An attacker might declare an uncompromised
AS's authorization endpoint URL as "his" AS URL, but declare a token
endpoint under his own control.
4.4.2.1. Mix-Up Defense via Issuer Identification
This defense requires that the authorization server sends his 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.
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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
[I-D.ietf-oauth-iss-auth-resp].
* 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
[I-D.ietf-oauth-iss-auth-resp].
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 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 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"
AS using the redirect URI that the client assigned to the attacker's
AS. The attacker could then run the attack as described above,
replacing the client ID with the client ID of his newly created
client.
This defense SHOULD therefore only be used if other options are not
available.
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4.5. Authorization Code Injection
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.
This attack is useful if the attacker cannot exchange the
authorization code for an access token himself. Examples include:
* The code is bound to a particular confidential client and the
attacker is unable to obtain the required client credentials to
redeem the code himself.
* The attacker wants to access certain functions in this particular
client. As an example, the attacker wants to impersonate his
victim in a certain app or on a certain web site.
* The authorization or resource servers are limited to certain
networks that the attacker is unable to access directly.
In the following attack description and discussion, we assume the
presence of a web (A1) or network attacker (A2).
4.5.1. Attack Description
The attack works as follows:
1. The attacker obtains an authorization code by performing any of
the attacks described above.
2. He starts a regular OAuth authorization process with the
legitimate client from his device.
3. The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client.
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 client's client ID,
client secret and actual "redirect_uri".
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5. The authorization server checks the client secret, whether the
code was issued to the particular client, and whether the actual
redirect 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 his
session with the legitimate client with the victim's resources
and/or identity.
4.5.2. Discussion
Obviously, the check in 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 URI
should also be detected if the authorization server stored the
complete redirect URI used in the authorization request and compares
it with the "redirect_uri" parameter.
[RFC6749], Section 4.1.3, requires the AS 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 above, the legitimate
client would use the correct redirect URI it always uses for
authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not
land at the attackers page). So the authorization server would
detect the attack and refuse to exchange the code.
Note: This check could also detect attempts to inject an
authorization code which had been obtained from another instance of
the same client on another device, if certain conditions are
fulfilled:
* the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
* the client has bound this data to this particular instance of the
client.
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But this approach conflicts with the idea to enforce 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 URI pattern. This saves the
authorization server from storing the link between the actual
redirect URI and the respective authorization code for every
transaction. But this kind of check obviously does not fulfill the
intent of the specification, since the tampered redirect 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 [RFC6749],
Section 4.1.3, increase client implementation complexity as clients
need to store or re-construct the correct redirect URI for the call
to the token endpoint.
This document therefore recommends to instead bind 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 achieve this goal, outlined
in the following.
4.5.3.1. PKCE
The PKCE parameter "code_challenge" along with the corresponding
"code_verifier" as specified in [RFC7636] can be used as a
countermeasure. When the attacker attempts to inject an
authorization code, the verifier check fails: the client uses its
correct verifier, but the code is associated with a challenge that
does not match this verifier. PKCE is a deployed OAuth feature,
although its originally intended use was solely focused on securing
native apps, not the broader use recommended by this document.
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4.5.3.2. Nonce
OpenID Connect's existing "nonce" parameter can be used for the same
purpose. The "nonce" value is one-time use and created by the
client. The client is supposed to bind it to the user agent session
and sends it with the initial request to the OpenID Provider (OP).
The OP binds "nonce" to the authorization code and attests this
binding in the ID Token, which is issued as part of the code exchange
at the token endpoint. If an attacker injected an authorization code
in the authorization response, the nonce value in the client session
and the nonce value in the ID token 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, where he 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 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.
4.5.3.3. Other Solutions
Other solutions, like binding "state" to the code, using token
binding for the code, 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 today (originally intended for OAuth native apps) whereas
"nonce" is appropriate for OpenID Connect clients.
4.5.4. Limitations
An attacker can circumvent the countermeasures described above if he
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 his own
session in Step 2 of the attack above. (This requires that the
victim's session with the client begins after the attacker started
his session with the client.) If the attacker is then able to
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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 Section 4.1, Section 4.2,
Section 4.3, Section 4.4, and Section 4.10.
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 makes 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 on the OAuth
protocol level, since the token is issued without any binding to the
transaction or the particular user agent.
The recommendation is therefore to use the authorization code grant
type instead of relying on response types issuing acess tokens at the
authorization endpoint. Authorization code injection can be detected
using one of the countermeasures discussed in Section 4.5.
4.7. Cross Site Request Forgery
An attacker might attempt to inject a request to the redirect 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).
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4.7.1. Countermeasures
The traditional countermeasure are CSRF tokens that are bound to the
user agent and passed in the "state" parameter to the authorization
server as described in [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 AS 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 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
state values [I-D.bradley-oauth-jwt-encoded-state].
AS therefore MUST provide a way to detect their support for PKCE.
Using AS metadata according to [RFC8414] is RECOMMENDED, but AS MAY
instead provide a deployment-specific way to ensure or determine PKCE
support.
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 AS enables and enforces PKCE if this parameter is present in the
authorization request, but 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.
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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: He injects an authorization code (and with that, an
access token) that is bound to his resources into a session between
his victim and the client.
4.8.1. Attack Description
1. The user has started an OAuth session using some client at an AS.
In the authorization request, the client has set the parameter
"code_challenge=sha256(abc)" as the PKCE code challenge. The
client is now waiting to receive the authorization response from
the user's browse.
2. To conduct the attack, the attacker uses his own device to start
an authorization flow with the targeted client. The client now
uses another PKCE code challenge, say
"code_challenge=sha256(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 which contains the code for the attacker's session
with the AS.
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
AS. 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 that 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, AS MUST take precautions against this threat.
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Note that from the view of the AS, 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 AS 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 AS 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.
Note: AS 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 setup his 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). 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 e-Mail, calendar, health,
or banking) and where the client is configured by a user or
administrator for a service which this user or company uses.
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4.9.1.1. Countermeasures
There are several potential mitigation strategies, which will be
discussed in the following sections.
4.9.1.1.1. Metadata
An authorization server could provide the client with additional
information about the location where it is safe to use its access
tokens.
In the simplest form, this would require the AS to publish a list of
its known resource servers, illustrated in the following example
using a non-standard 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 AS 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",
...
}
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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 [oauth_security_ubc] and [oauth_security_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. Clearly, the
client has to contribute to the overall security. But there are
alternative countermeasures, as described in the next sections, which
provide a better balance between the involved parties.
4.9.1.1.2. Sender-Constrained Access Tokens
As the name suggests, sender-constrained access token scope the
applicability of an access token to a certain sender. This sender is
obliged to demonstrate knowledge of a certain secret as 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 identity, but in most cases the AS
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 AS creates the binding or the
AS creates ephemeral keys. The way pre-existing 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 RS must implement the actual proof of possession check. This
is typically done on the application level, often tied to
specific material provided by transport layer (e.g., TLS). The
RS must also ensure that replay of the proof of possession is not
possible.
There exist several proposals to demonstrate the proof of possession
in the scope of the OAuth working group:
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* *OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound
Access Tokens* ([RFC8705]): The approach as 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 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.
* *DPoP* ([I-D.ietf-oauth-dpop]): DPoP (Demonstration of Proof-of-
Possession at the Application Layer) outlines an application-level
sender-constraining for access and refresh tokens that can be used
in cases where neither mTLS nor OAuth Token Binding (see below)
are available. It uses proof-of-possession based on a public/
private key pair and application-level signing. DPoP can be used
with public clients and, in case of confidential clients, can be
combined with any client authentication method.
* *OAuth Token Binding* ([I-D.ietf-oauth-token-binding]): In this
approach, an access token is, via the token binding ID, bound to
key material representing a long term association between a client
and a certain TLS host. Negotiation of the key material and proof
of possession in the context of a TLS handshake is taken care of
by the TLS stack. The client needs to determine the token binding
ID of the target resource server and pass this data to the access
token request. The authorization server then associates the
access token with this ID. The resource server checks on every
invocation that the token binding ID of the active TLS connection
and the token binding ID of associated with the access token
match. Since all crypto-related functions are covered by the TLS
stack, this approach is very client developer friendly. As a
prerequisite, token binding as described in [RFC8473] (including
federated token bindings) must be supported on all ends (client,
authorization server, resource server).
* *Signed HTTP Requests* ([I-D.ietf-oauth-signed-http-request]):
This approach utilizes [I-D.ietf-oauth-pop-key-distribution] and
represents the elements of the signature in a JSON object. The
signature is built using JWS. The mechanism has built-in support
for signing of HTTP method, query parameters and headers. It also
incorporates a timestamp as basis for replay prevention.
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* *JWT Pop Tokens* ([I-D.sakimura-oauth-jpop]): This draft describes
different ways to constrain access token usage, namely TLS or
request signing. Note: Since the authors of this draft
contributed the TLS-related proposal to [RFC8705], this document
only considers the request signing part. For request signing, the
draft utilizes [I-D.ietf-oauth-pop-key-distribution] and
[RFC7800]. The signature data is represented in a JWT and JWS is
used for signing. Replay prevention is provided by building the
signature over a server-provided nonce, client-provided nonce and
a nonce counter.
At the time of writing, OAuth Mutual TLS is the most widely
implemented and the only standardized sender-constraining method.
The use of OAuth Mutual TLS therefore is RECOMMENDED.
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 a 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 4.13).
4.9.1.1.3. 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 SHOULD verify the intended audience. If the access token
fails the intended audience validation, the resource server must
refuse to serve the respective request.
In general, audience restrictions limit 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). In order 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.
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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.
The client SHOULD tell the authorization server the intended resource
server. The proposed mechanism [I-D.ietf-oauth-resource-indicators]
could be used or by encoding the information in the scope value.
Instead of the URL, it is also possible to utilize the fingerprint of
the resource server's X.509 certificate as audience value. This
variant would also allow to detect 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 crypto on the client-side. Still, since every access token is
bound to a specific resource server, the client also needs to obtain
a single RS-specific access token when accessing several resource
servers. (Resource indicators, as specified in
[I-D.ietf-oauth-resource-indicators], can help to achieve this.)
[I-D.ietf-oauth-token-binding] has the same property since different
token binding ids must be associated with the access token. Using
[RFC8705], on the other hand, allows a client to use the access token
at multiple resource servers.
It shall 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, has additional benefits beyond the
scope of token leakage prevention. It allows the authorization
server to create different access token whose format and content is
specifically minted for the respective server. This has huge
functional and privacy advantages in deployments using structured
access tokens.
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 of the respective server.
If the attacker were able to gain full control, including shell
access, all controls can be circumvented and all resources 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.
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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 focuses on the impact of
OAuth-related breaches and the replaying of captured access tokens.
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 4.9.1.1.2
SHOULD be used to prevent the attacker from replaying the access
tokens on other resource servers. Depending on the severity of
the penetration, sender-constrained access tokens will also
prevent replay on the compromised system.
* Audience restriction as described in Section 4.9.1.1.3 SHOULD be
used to prevent replay of captured access tokens on other resource
servers.
* The resource server MUST treat access tokens like any other
credentials. It is considered good practice to not log them and
not store them in plain text.
The first and second recommendation also apply to other scenarios
where access tokens leak (see Attacker A5).
4.10. Open Redirection
The following attacks can occur when an AS or client has an open
redirector. An open redirector is an endpoint that forwards a user's
browser to an arbitrary URI obtained from a query parameter.
4.10.1. 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 follow 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 whitelisted or if the origin and integrity of a
request can be authenticated. Countermeasures against open
redirection are described by OWASP [owasp_redir].
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4.10.2. 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 web sites (the clients), but must do so in a
safe way.
[RFC6749], Section 4.1.2.1, already prevents open redirects by
stating that the AS 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 URI to perform phishing attacks. The attacker could, for
example, register a client via dynamic client registration [RFC7591]
and intentionally send an erroneous authorization request, e.g., by
using an invalid scope value, thus instructing the AS to redirect the
user agent to its phishing site.
The AS MUST take precautions to prevent this threat. Based on its
risk assessment, the AS needs to decide whether it can trust the
redirect URI and SHOULD only automatically redirect the user agent if
it trusts the redirect URI. If the URI is not trusted, the AS MAY
inform the user and rely on the user to make the correct decision.
4.11. 307 Redirect
At the authorization endpoint, a typical protocol flow is that the AS
prompts the user to enter her credentials in a form that is then
submitted (using the HTTP POST method) back to the authorization
server. The AS checks the credentials and, if successful, redirects
the user agent to the client's redirection endpoint.
In [RFC6749], the HTTP status code 302 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 credentials
via HTTP POST to the client.
This discloses the sensitive credentials to the client. If the
relying party is malicious, it can use the credentials to impersonate
the user at the AS.
The behavior might be unexpected for developers, but is defined in
[RFC7231], Section 6.4.7. This status code 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.
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In the HTTP standard [RFC7231], only the status code 303
unambigiously 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 the user credentials to the client. (In practice, however,
most user agents will only show this behaviour for 307 redirects.)
AS which redirect a request that potentially contains user
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, AS SHOULD use HTTP status
code 303 "See Other".
4.12. 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 custom HTTP headers added to the upstream request.
If the reverse proxy would pass 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 a whitelisted IP address to the header and
render a IP whitelist 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 was able to get access to the internal network between
proxy and application server, the attacker could also try to
circumvent security controls in place. It is, therefore, essential
to ensure the authenticity of the communicating entities.
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Furthermore, the communication link between reverse proxy and
application server must be protected against eavesdropping,
injection, and replay of messages.
4.13. Refresh Token Protection
Refresh tokens are a convenient and user-friendly way to obtain new
access tokens after the expiration of access tokens. Refresh tokens
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.
4.13.1. Discussion
Refresh tokens are an attractive target for attackers since they
represent the overall grant a resource owner delegated to a certain
client. 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 a 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 (i.e., "client_id"),
* 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 behavior.
This specification gives recommendations beyond the scope of
[RFC6749] and clarifications.
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4.13.2. Recommendations
Authorization servers SHOULD 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 refresh access tokens 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 server 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 by utilizing [RFC8705] or [I-D.ietf-oauth-token-binding].
* *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:
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* password change
* 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).
4.14. Client Impersonating Resource Owner
Resource servers may make access control decisions based on the
identity of the resource owner as communicated in the "sub" Claim
returned by the authorization server in a token introspection
response [RFC7662] or other mechanisms. If a client is able to
choose its own "client_id" during registration with the authorization
server, then there is a risk that it can register with the same "sub"
value as a privileged user. A subsequent access token obtained under
the client credentials grant may be mistaken for an access token
authorized by the privileged user if the resource server does not
perform additional checks.
4.14.1. Countermeasures
Authorization servers SHOULD NOT allow clients to influence their
"client_id" or "sub" value or any other Claim if that can cause
confusion with a genuine resource owner. Where this cannot be
avoided, authorization servers MUST provide other means for the
resource server to distinguish between access tokens authorized by a
resource owner from access tokens authorized by the client itself.
4.15. Clickjacking
As described in Section 4.4.1.9 of [RFC6819], the authorization
request is susceptible to clickjacking. 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 [CSP-2] or greater.
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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 [CSP-2] for details). Level 2 of this standard
provides a robust mechanism for protecting against clickjacking by
using policies that restrict the origin of frames (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 [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.
5. Acknowledgements
We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen,
Vittorio Bertocci, David Waite, Nov Matake, Tomek Stojecki, Dominick
Baier, Neil Madden, William Dennis, Dick Hardt, Petteri Stenius,
Annabelle Richard Backman, Aaron Parecki, George Fletscher, Brian
Campbell, Konstantin Lapine, Tim Würtele, Guido Schmitz, Hans
Zandbelt, Jared Jennings, Michael Peck, Pedram Hosseyni, Michael B.
Jones, Travis Spencer, and Karsten Meyer zu Selhausen for their
valuable feedback.
6. IANA Considerations
This draft includes no request to IANA.
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7. Security Considerations
All relevant security considerations have been given in the
functional specification.
8. Normative References
[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>.
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", February 2020,
<https://www.rfc-editor.org/info/rfc8705>.
[oauth-v2-form-post-response-mode]
Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
Mode", 27 April 2015, <http://openid.net/specs/oauth-v2-
form-post-response-mode-1_0.html>.
[OpenID] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", 8 November 2014,
<https://openid.net/specs/openid-connect-core-1_0.html>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[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>.
[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>.
[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>.
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[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[OpenIDDisc]
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0 incorporating errata set 1", 8
November 2014, <https://openid.net/specs/openid-connect-
discovery-1_0.html>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[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>.
9. Informative References
[webcrypto]
Watson, M., "Web Cryptography API", 26 January 2017,
<https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126/>.
[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", 7 January 2016,
<http://arxiv.org/abs/1508.04324v2/>.
[I-D.ietf-oauth-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://tools.ietf.org/html/draft-ietf-oauth-token-
binding-08>.
[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>.
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[I-D.ietf-oauth-iss-auth-resp]
Selhausen, K. and D. Fett, "OAuth 2.0 Authorization Server
Issuer Identifier in Authorization Response", Work in
Progress, Internet-Draft, draft-ietf-oauth-iss-auth-resp-
00, 6 January 2021, <https://tools.ietf.org/html/draft-
ietf-oauth-iss-auth-resp-00>.
[oauth_security_jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
Maffeis, "Discovering concrete attacks on website
authorization by formal analysis", 23 April 2014,
<https://www.doc.ic.ac.uk/~maffeis/papers/jcs14.pdf>.
[I-D.sakimura-oauth-jpop]
Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
Authorization Framework: JWT Pop Token Usage", Work in
Progress, Internet-Draft, draft-sakimura-oauth-jpop-05, 22
July 2019, <https://tools.ietf.org/html/draft-sakimura-
oauth-jpop-05>.
[oauth_security_ubc]
Sun, S.-T. and K. Beznosov, "The Devil is in the
(Implementation) Details: An Empirical Analysis of OAuth
SSO Systems", October 2012,
<http://passwordresearch.com/papers/paper267.html>.
[CSP-2] West, M., Barth, A., and D. Veditz, "Content Security
Policy Level 2", July 2015, <https://www.w3.org/TR/CSP2>.
[arXiv.1901.11520]
Fett, D., Hosseyni, P., and R. Küsters, "An Extensive
Formal Security Analysis of the OpenID Financial-grade
API", 31 January 2019, <http://arxiv.org/abs/1901.11520/>.
[I-D.ietf-oauth-par]
Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
Work in Progress, Internet-Draft, draft-ietf-oauth-par-05,
14 December 2020,
<https://tools.ietf.org/html/draft-ietf-oauth-par-05>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
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[webappsec-referrer-policy]
Eisinger, J. and E. Stark, "Referrer Policy", 20 April
2017, <https://w3c.github.io/webappsec-referrer-policy>.
[JARM] Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
Secured Authorization Response Mode for OAuth 2.0 (JARM)",
17 October 2018,
<https://openid.net/specs/openid-financial-api-jarm.html>.
[bug.chromium]
"Referer header includes URL fragment when opening link
using New Tab",
<https://bugs.chromium.org/p/chromium/issues/
detail?id=168213/>.
[I-D.ietf-oauth-signed-http-request]
Richer, J., Bradley, J., and H. Tschofenig, "A Method for
Signing HTTP Requests for OAuth", Work in Progress,
Internet-Draft, draft-ietf-oauth-signed-http-request-03, 8
August 2016, <https://tools.ietf.org/html/draft-ietf-
oauth-signed-http-request-03>.
[owasp_redir]
"OWASP Cheat Sheet Series - Unvalidated Redirects and
Forwards",
<https://cheatsheetseries.owasp.org/cheatsheets/
Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.
[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>.
[arXiv.1601.01229]
Fett, D., Küsters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0", 6 January 2016,
<http://arxiv.org/abs/1601.01229/>.
[webauthn] Balfanz, D., Czeskis, A., Hodges, J., Jones, J.C., Jones,
M.B., Kumar, A., Liao, A., Lindemann, R., and E. Lundberg,
"Web Authentication: An API for accessing Public Key
Credentials Level 1", 4 March 2019,
<https://www.w3.org/TR/2019/REC-webauthn-1-20190304/>.
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[I-D.ietf-oauth-jwsreq]
Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
Authorization Framework: JWT Secured Authorization Request
(JAR)", Work in Progress, Internet-Draft, draft-ietf-
oauth-jwsreq-30, 10 September 2020,
<https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-30>.
[I-D.ietf-oauth-dpop]
Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof-
of-Possession at the Application Layer (DPoP)", Work in
Progress, Internet-Draft, draft-ietf-oauth-dpop-02, 18
November 2020,
<https://tools.ietf.org/html/draft-ietf-oauth-dpop-02>.
[RFC8473] Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
J. Hodges, "Token Binding over HTTP", RFC 8473,
DOI 10.17487/RFC8473, October 2018,
<https://www.rfc-editor.org/info/rfc8473>.
[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>.
[I-D.ietf-oauth-resource-indicators]
Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", Work in Progress, Internet-
Draft, draft-ietf-oauth-resource-indicators-08, 11
September 2019, <https://tools.ietf.org/html/draft-ietf-
oauth-resource-indicators-08>.
[arXiv.1704.08539]
Fett, D., Küsters, R., and G. Schmitz, "The Web SSO
Standard OpenID Connect: In-Depth Formal Security Analysis
and Security Guidelines", 27 April 2017,
<http://arxiv.org/abs/1704.08539/>.
[I-D.bradley-oauth-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://tools.ietf.org/html/draft-bradley-oauth-jwt-
encoded-state-09>.
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[oauth_security_cmu]
Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
Tague, "OAuth Demystified for Mobile Application
Developers", November 2014,
<http://css.csail.mit.edu/6.858/2012/readings/oauth-
sso.pdf>.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M.
Meszaros, "OAuth 2.0 Proof-of-Possession: Authorization
Server to Client Key Distribution", Work in Progress,
Internet-Draft, draft-ietf-oauth-pop-key-distribution-07,
27 March 2019, <https://tools.ietf.org/html/draft-ietf-
oauth-pop-key-distribution-07>.
[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>.
[I-D.ietf-oauth-rar]
Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
Rich Authorization Requests", Work in Progress, Internet-
Draft, draft-ietf-oauth-rar-03, 18 October 2020,
<https://tools.ietf.org/html/draft-ietf-oauth-rar-03>.
[subdomaintakeover]
Liu, D., Hao, S., and H. Wang, "All Your DNS Records Point
to Us: Understanding the Security Threats of Dangling DNS
Records", 24 October 2016,
<https://www.eecis.udel.edu/~hnw/paper/ccs16a.pdf>.
Appendix A. Document History
[[ To be removed from the final specification ]]
-18
* Fix editorial and spelling issues.
* Change wording for disallowing HTTP redirect URIs.
-17
* Make the use of metadata RECOMMENDED for both servers and clients
* Make announcing PKCE support in metadata the RECOMMENDED way
(before: either metadata or deployment-specific way)
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* AS also MUST NOT expose open redirectors.
* Mention that attackers can collaborate.
* Update recommendations regarding mix-up defense, building upon
[I-D.ietf-oauth-iss-auth-resp].
* Improve description of mix-up attack.
* Make HTTPS mandatory for most redirect URIs.
-16
* Make MTLS a suggestion, not RECOMMENDED.
* Add important requirements when using nonce for code injection
protection.
* Highlight requirements for refresh token sender-constraining.
* Make PKCE a MUST for public clients.
* Describe PKCE Downgrade Attacks and countermeasures.
* Allow variable port numbers in localhost redirect URIs as in
RFC8252, Section 7.3.
-15
* Update reference to DPoP
* Fix reference to RFC8414
* Move to xml2rfcv3
-14
* Added info about using CSP to prevent clickjacking
* Changes from WGLC feedback
* Editorial changes
* AS MUST announce PKCE support either in metadata or using
deployment-specific ways (before: SHOULD)
-13
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* Discourage use of Resource Owner Password Credentials Grant
* Added text on client impersonating resource owner
* Recommend asymmetric methods for client authentication
* Encourage use of PKCE mode "S256"
* PKCE may replace state for CSRF protection
* AS SHOULD publish PKCE support
* Cleaned up discussion on auth code injection
* AS MUST support PKCE
-12
* Added updated attacker model
-11
* Adapted section 2.1.2 to outcome of consensus call
* more text on refresh token inactivity and implementation note on
refresh token replay detection via refresh token rotation
-10
* incorporated feedback by Joseph Heenan
* changed occurrences of SHALL to MUST
* added text on lack of token/cert binding support tokens issued in
the authorization response as justification to not recommend
issuing tokens there at all
* added requirement to authenticate clients during code exchange
(PKCE or client credential) to 2.1.1.
* added section on refresh tokens
* editorial enhancements to 2.1.2 based on feedback
-09
* changed text to recommend not to use implicit but code
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* added section on access token injection
* reworked sections 3.1 through 3.3 to be more specific on implicit
grant issues
-08
* added recommendations re implicit and token injection
* uppercased key words in Section 2 according to RFC 2119
-07
* incorporated findings of Doug McDorman
* added section on HTTP status codes for redirects
* added new section on access token privilege restriction based on
comments from Johan Peeters
-06
* reworked section 3.8.1
* incorporated Phil Hunt's feedback
* reworked section on mix-up
* extended section on code leakage via referrer header to also cover
state leakage
* added Daniel Fett as author
* replaced text intended to inform WG discussion by recommendations
to implementors
* modified example URLs to conform to RFC 2606
-05
* Completed sections on code leakage via referrer header, attacks in
browser, mix-up, and CSRF
* Reworked Code Injection Section
* Added reference to OpenID Connect spec
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* removed refresh token leakage as respective considerations have
been given in section 10.4 of RFC 6749
* first version on open redirection
* incorporated Christian Mainka's review feedback
-04
* Restructured document for better readability
* Added best practices on Token Leakage prevention
-03
* Added section on Access Token Leakage at Resource Server
* incorporated Brian Campbell's findings
-02
* Folded Mix up and Access Token leakage through a bad AS into new
section for dynamic OAuth threats
* reworked dynamic OAuth section
-01
* Added references to mitigation methods for token leakage
* Added reference to Token Binding for Authorization Code
* incorporated feedback of Phil Hunt
* fixed numbering issue in attack descriptions in section 2
-00 (WG document)
* turned the ID into a WG document and a BCP
* Added federated app login as topic in Other Topics
Authors' Addresses
Torsten Lodderstedt
yes.com
Email: torsten@lodderstedt.net
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John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Email: isciurus@gmail.com
Daniel Fett
yes.com
Email: mail@danielfett.de
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