Web Authorization Protocol T. Lodderstedt
Internet-Draft yes.com
Intended status: Best Current Practice J. Bradley
Expires: January 9, 2020 Yubico
A. Labunets
Facebook
D. Fett
yes.com
July 8, 2019
OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-13
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Structure . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. The Updated OAuth 2.0 Attacker Model . . . . . . . . . . . . 4
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Protecting Redirect-Based Flows . . . . . . . . . . . . . 6
3.1.1. Authorization Code Grant . . . . . . . . . . . . . . 7
3.1.2. Implicit Grant . . . . . . . . . . . . . . . . . . . 8
3.2. Token Replay Prevention . . . . . . . . . . . . . . . . . 8
3.3. Access Token Privilege Restriction . . . . . . . . . . . 9
3.4. Resource Owner Password Credentials Grant . . . . . . . . 9
3.5. Client Authentication . . . . . . . . . . . . . . . . . . 10
3.6. Other Recommendations . . . . . . . . . . . . . . . . . . 10
4. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 10
4.1. Insufficient Redirect URI Validation . . . . . . . . . . 10
4.1.1. Redirect URI Validation Attacks on Authorization Code
Grant . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.2. Redirect URI Validation Attacks on Implicit Grant . . 12
4.1.3. Proposed Countermeasures . . . . . . . . . . . . . . 13
4.2. Credential Leakage via Referrer Headers . . . . . . . . . 14
4.2.1. Leakage from the OAuth Client . . . . . . . . . . . . 14
4.2.2. Leakage from the Authorization Server . . . . . . . . 14
4.2.3. Consequences . . . . . . . . . . . . . . . . . . . . 14
4.2.4. Proposed Countermeasures . . . . . . . . . . . . . . 14
4.3. Attacks through the Browser History . . . . . . . . . . . 15
4.3.1. Code in Browser History . . . . . . . . . . . . . . . 16
4.3.2. Access Token in Browser History . . . . . . . . . . . 16
4.4. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Attack Description . . . . . . . . . . . . . . . . . 17
4.4.2. Countermeasures . . . . . . . . . . . . . . . . . . . 18
4.5. Authorization Code Injection . . . . . . . . . . . . . . 19
4.5.1. Attack Description . . . . . . . . . . . . . . . . . 19
4.5.2. Discussion . . . . . . . . . . . . . . . . . . . . . 20
4.5.3. Proposed Countermeasures . . . . . . . . . . . . . . 21
4.6. Access Token Injection . . . . . . . . . . . . . . . . . 23
4.6.1. Proposed Countermeasures . . . . . . . . . . . . . . 23
4.7. Cross Site Request Forgery . . . . . . . . . . . . . . . 23
4.7.1. Proposed Countermeasures . . . . . . . . . . . . . . 23
4.8. Access Token Leakage at the Resource Server . . . . . . . 24
4.8.1. Access Token Phishing by Counterfeit Resource Server 24
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4.8.2. Compromised Resource Server . . . . . . . . . . . . . 29
4.9. Open Redirection . . . . . . . . . . . . . . . . . . . . 30
4.9.1. Authorization Server as Open Redirector . . . . . . . 30
4.9.2. Clients as Open Redirector . . . . . . . . . . . . . 31
4.10. 307 Redirect . . . . . . . . . . . . . . . . . . . . . . 31
4.11. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 32
4.12. Refresh Token Protection . . . . . . . . . . . . . . . . 32
4.13. Client Impersonating Resource Owner . . . . . . . . . . . 34
4.13.1. Proposed Countermeasures . . . . . . . . . . . . . . 35
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Normative References . . . . . . . . . . . . . . . . . . 35
8.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Document History . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 has
gotten massive traction in the market and became the standard for API
protection and, as the foundation of OpenID Connect [OpenID],
identity providing. While OAuth was used in a variety of scenarios
and different kinds of deployments, the following challenges could be
observed:
o OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (CSRF, referrer
header). Although most of these threats are discussed in the
OAuth 2.0 Threat Model and Security Considerations [RFC6819],
continued exploitation demonstrates there may be a need for more
specific recommendations, that the existing mitigations may be too
difficult to deploy, and that more defense in depth is needed.
o Technology has changed, e.g., the way browsers treat fragments in
some situations, which changes the implicit grant's underlying
security model.
o 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.
o 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].
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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 serves as a frontend to a particular tenant in a multi-
tenancy. Extensions of OAuth, such as [RFC7591] and [RFC8414] were
developed in order to support the usage of OAuth in dynamic
scenarios. As a challenge to the community, such usage scenarios
open up new attack angles, which are discussed in this document.
1.1. Structure
The remainder of the document is organized as follows: The next
section updates the OAuth attacker model. Afterwards, the most
important recommendations of the OAuth working group for every OAuth
implementor are summarized. Subsequently, a detailed analysis of the
threats and implementation issues which 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.
2. The Updated OAuth 2.0 Attacker Model
In [RFC6819], an attacker model was laid out that described the
capabilities of attackers against which OAuth deployments must
defend. In the following, this attacker model is updated to account
for the potentially dynamic relationships involving multiple parties
(as described above), to include new types of attackers, and to
define the attacker model more clearly.
OAuth 2.0 MUST ensure that the authorization of the resource owner
(RO) (with a user agent) at an authorization server (AS) and the
subsequent usage of the access token at the resource server (RS) is
protected at least against the following attackers:
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o (A1) Web Attackers that control an arbitrary number of network
endpoints (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, participate in the protocol using their own user
credentials, etc.
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 ROs.
It must also be assumed that web attackers can lure the user to
open arbitrary attacker-chosen URIs at any time. This can be
achieved through many ways, for example, by injecting malicious
advertisements into advertisement networks, or by sending legit-
looking emails.
o (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.
These attackers conform to the attacker model that was used in formal
analysis efforts for OAuth [arXiv.1601.01229]. Previous attacks on
OAuth have shown that OAuth deployments SHOULD protect against an
even stronger attacker model that is described as follows:
o (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), so-called mix-up
attacks, 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.
o (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).
o (A5) Attackers that control a resource server used by RO with 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.
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
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replay tokens or authorization codes obtained by any of the attacks
described above at the client or RS.
This document discusses the additional threats resulting from these
attackers in detail and recommends suitable mitigations. Attacks in
an even stronger attacker model are discussed, for example, in
[arXiv.1901.11520].
This is a minimal attacker model. Implementers MUST take into
account all possible attackers in the environment in which their
OAuth implementations are expected to run.
3. Recommendations
This section describes the set of security mechanisms the OAuth
working group recommends to OAuth implementers.
3.1. Protecting Redirect-Based Flows
Authorization servers MUST utilize exact matching of client redirect
URIs against pre-registered URIs. This measure contributes to the
prevention of leakage of authorization codes and access tokens
(depending on the grant type). It also helps to detect mix-up
attacks.
Clients SHOULD avoid forwarding the user's browser to a URI obtained
from a query parameter since such a function could be utilized to
exfiltrate authorization codes and access tokens. If there is a
strong need for this kind of redirects, clients are advised to
implement appropriate countermeasures against open redirection, e.g.,
as described by OWASP [owasp].
Clients MUST prevent CSRF. One-time use CSRF tokens carried in the
"state" parameter, which are securely bound to the user agent, SHOULD
be used for that purpose. If PKCE [RFC7636] is used by the client
and the authorization server supports PKCE, clients MAY opt to not
use "state" for CSRF protection, as such protection is provided by
PKCE. In this case, "state" MAY be used again for its original
purpose, namely transporting data about the application state of the
client (see Section 4.7.1).
In order to prevent mix-up attacks, clients MUST only process
redirect responses of the OAuth authorization server they sent the
respective request to and from the same user agent this authorization
request was initiated with. Clients MUST memorize which
authorization server they sent an authorization request to and bind
this information to the user agent and ensure any sub-sequent
messages are sent to the same authorization server. Clients SHOULD
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use AS-specific redirect URIs as a means to identify the AS a
particular response came from.
Note: [I-D.bradley-oauth-jwt-encoded-state] gives advice on how to
implement CSRF prevention and AS matching using signed JWTs in the
"state" parameter.
AS which redirect a request that potentially contains user
credentials MUST avoid forwarding these user credentials accidentally
(see Section 4.10).
3.1.1. Authorization Code Grant
Clients utilizing the authorization grant type MUST use PKCE
[RFC7636] in order to (with the help of the authorization server)
detect and prevent attempts to inject (replay) authorization codes
into the authorization response. The PKCE challenges must be
transaction-specific and securely bound to the user agent in which
the transaction was started and the respective client. OpenID
Connect clients MAY use the "nonce" parameter of the OpenID Connect
authentication request as specified in [OpenID] in conjunction with
the corresponding ID Token claim for the same purpose.
Note: although PKCE so far was recommended as a mechanism to protect
native apps, this advice applies to all kinds of OAuth clients,
including web applications.
Clients SHOULD use PKCE code challenge methods that do not expose the
PKCE verifier in the authorization request. (Otherwise, the attacker
A4 can trivially break the security provided by PKCE.) Currently,
"S256" is the only such method.
AS MUST support PKCE [!@RFC7636].
AS SHOULD provide a way to detect their support for PKCE. To this
end, they SHOULD either (a) publish, in their AS metadata
([!@RFC8418]), the element "code_challenge_methods_supported"
containing the supported PKCE challenge methods (which can be used by
the client to detect PKCE support) or (b) provide a deployment-
specific way to ensure or determine PKCE support by the AS.
Authorization servers SHOULD furthermore consider the recommendations
given in [RFC6819], Section 4.4.1.1, on authorization code replay
prevention.
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3.1.2. 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 3.2. 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 any other response type issuing
access tokens in the authorization response, such as "token id_token"
and "code token id_token", unless the issued access tokens are
sender-constrained and access token injection in the authorization
response is prevented.
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).
Clients SHOULD instead use the response type "code" (aka
authorization code grant type) as specified in Section 3.1.1 or any
other response type that causes the authorization server to issue
access tokens in the token response. This allows the authorization
server to detect replay attempts 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.
3.2. Token Replay Prevention
Authorization servers SHOULD use TLS-based methods for sender-
constrained access tokens as described in Section 4.8.1.2, such as
token binding [I-D.ietf-oauth-token-binding] or Mutual TLS for OAuth
2.0 [I-D.ietf-oauth-mtls] in order to prevent token replay. Refresh
tokens MUST be sender-constrained or use refresh token rotation as
described in Section 4.12.
It is recommended to use end-to-end TLS whenever possible. If TLS
traffic needs to be terminated at an intermediary, refer to
Section 4.11 for further security advice.
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3.3. 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 limit the impact of token leakage although more
effective counter-measures are described in Section 3.2.
In particular, access tokens SHOULD be restricted to certain resource
servers, 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] to determine
those resources and/or actions.
3.4. 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, and authentication processes that require multiple steps
can be hard or impossible (WebCrypto, WebAuthn).
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3.5. 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 [I-D.draft-ietf-oauth-mtls] or
"private_key_jwt" [OIDC]. 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. Additionally, these methods enable non-repudation
and work well with sender-constrained access tokens (without
requiring additional keys to be distributed).
3.6. Other Recommendations
Authorization servers SHOULD NOT allow clients to influence their
"client_id" or "sub" value or any other claim that might cause
confusion with a genuine resource owner (see Section 4.13).
4. Attacks and Mitigations
This section gives a detailed description of attacks on OAuth
implementations, along with potential countermeasures. This section
complements and enhances the description given in [RFC6819].
4.1. Insufficient Redirect URI Validation
Some authorization servers allow clients to register redirect URI
patterns instead of complete redirect URIs. In those cases, the
authorization server, at runtime, matches the actual redirect URI
parameter value at the authorization endpoint against this pattern.
This approach allows clients to encode transaction state into
additional redirect URI parameters or to register just a single
pattern for multiple redirect URIs. As a downside, it turned out to
be more complex to implement and error prone to manage than exact
redirect URI matching. Several successful attacks, utilizing 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
o by directly sending the user agent to a URI under the attackers
control, or
o 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.
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4.1.1. Redirect URI Validation Attacks on Authorization Code Grant
For a public client using the grant type code, an attack would look
as follows:
Let's assume the redirect URL pattern "https://*.somesite.example/*"
had been registered for the client "s6BhdRkqt3". This pattern allows
redirect URIs pointing to any host residing in the domain
somesite.example. So if an attacker manages to establish a host or
subdomain in somesite.example he can impersonate the legitimate
client. Assume the attacker sets up the host
"evil.somesite.example".
The attack can then be conducted as follows:
First, the attacker needs to trick the user into opening a tampered
URL in his browser, which launches a page under the attacker's
control, say "https://www.evil.example". (See Attacker A1.)
This URL initiates an authorization request with the client id of a
legitimate client to the authorization endpoint. This is the example
authorization request (line breaks are for display purposes only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
&redirect_uri=https%3A%2F%2Fevil.somesite.example%2Fcb HTTP/1.1
Host: server.somesite.example
Afterwards, the authorization server validates the redirect URI in
order to identify the client. Since the pattern allows arbitrary
host names in "somesite.example", the authorization request is
processed under the legitimate client's identity. This includes the
way the request for user consent is presented to the user. If auto-
approval is allowed (which is not recommended for public clients
according to [RFC6749]), the attack can be performed even easier.
If the user does not recognize the attack, the code is issued and
immediately sent to the attacker's client.
Since the attacker impersonated a public client, it can exchange the
code for tokens at the respective token endpoint.
Note: This attack will not work as easily for confidential clients,
since the code exchange requires authentication with the legitimate
client's secret. The attacker will need to impersonate or utilize
the legitimate client to redeem the code (e.g., by performing a code
injection attack). This kind of injections is covered in
Section 4.5.
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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 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 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, which launches a page under
the attacker's control, say "https://www.evil.example".
Afterwards, the website initiates an authorization request, which 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://client.evil.example" into the redirect URI and
it uses the response type "token" (line breaks are for display
purposes 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
%252Fclient.evil.example%252Fcb HTTP/1.1
Host: server.somesite.example
Now, since the redirect URI matches the registered pattern, the
authorization server allows the request and sends the resulting
access token with a 303 redirect (some response parameters are
omitted for better readability)
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HTTP/1.1 303 See Other
Location: https://client.somesite.example/cb?
redirect_to%3Dhttps%3A%2F%2Fclient.evil.example%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&...
At example.com, the request arrives at the open redirector. It will
read the redirect parameter and will issue an HTTP 303 Location
header redirect to the URL "https://client.evil.example/cb".
HTTP/1.1 303 See Other
Location: https://client.evil.example/cb
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://client.evil.example/cb#access_token=2YotnFZFEjr1z...
The attacker's page at "client.evil.example" can now access the
fragment and obtain the access token.
4.1.3. Proposed Countermeasures
The complexity of implementing and managing pattern matching
correctly obviously causes security issues. This document therefore
proposes to simplify the required logic and configuration by using
exact redirect URI matching only. This means the authorization
server must compare the two URIs using simple string comparison as
defined in [RFC3986], Section 6.2.1.
Additional recommendations:
o Servers on which callbacks are hosted must not expose open
redirectors (see Section 4.9).
o Clients MAY drop fragments via intermediary URLs with "fix
fragments" (see [fb_fragments]) to prevent the user agent from
appending any unintended fragments.
o 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 certificate binding of the access
tokens.
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As an alternative to exact redirect URI matching, the AS could also
authenticate clients, e.g., using [I-D.ietf-oauth-jwsreq].
4.2. Credential Leakage via Referrer Headers
Authorization codes or values of "state" can unintentionally be
disclosed to attackers through the referrer header, by leaking either
from a client's web site or from an AS's web site. Note: even if
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
o contains links to other pages under the attacker's control (ads,
faq, ...) and a user clicks on such a link, or
o includes third-party content (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", "access token", or "state".
4.2.2. Leakage from the Authorization Server
In a similar way, an attacker can learn "state" 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
referrer 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. Proposed 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.
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The following measures further reduce the chances of a successful
attack:
o Bind authorization code to a confidential client or PKCE
challenge. In this case, the attacker lacks the secret to request
the code exchange.
o 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.
o 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 referrer 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.)
o Suppress the referrer header by adding the attribute
"rel="noreferrer"" to HTML links or 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).
o Use authorization code instead of response types causing access
token issuance from the authorization endpoint. This provides
countermeasures against leakage on the OAuth protocol level
through the code exchange process with the authorization server.
o Additionally, one might use the form post response mode instead of
redirect for authorization response (see
[oauth-v2-form-post-response-mode]).
4.3. Attacks through the 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.
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4.3.1. 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.
Proposed countermeasures:
o Authorization code replay prevention as described in [RFC6819],
Section 4.4.1.1, and Section 4.5
o Use form post response mode instead of redirect for 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 just
a web site, which already has a token, deliberately navigates to a
page like "provider.com/get_user_profile?access_token=abcdef.".
Actually [RFC6750] discourages this practice and asks to transfer
tokens via a header, but in practice web sites often just pass access
token in query parameters.
In case of implicit grant, a URL like "client.example/
redirection_endpoint#access_token=abcdef" may also end up in the
browser history as a result of a redirect from a provider's
authorization endpoint.
Proposed countermeasures:
o Replace implicit flow with postmessage communication or the
authorization code grant
o Never pass access tokens in URL query parameters
4.4. Mix-Up
Mix-up is an attack on scenarios where an OAuth client interacts with
multiple authorization servers, as is usually the case when dynamic
registration is used. The goal of the attack is to obtain an
authorization code or an access token by tricking the client into
sending those credentials to the attacker instead of using them at
the respective endpoint at the authorization/resource server.
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4.4.1. Attack Description
For a detailed attack description, refer to [arXiv.1601.01229] and
[I-D.ietf-oauth-mix-up-mitigation]. The description here closely
follows [arXiv.1601.01229], with variants of the attack outlined
below.
Preconditions: For the attack to work, we assume that
o 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),
o 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, and
o the attacker can 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.
Some of the attack variants described below require different
preconditions.
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").
Attack on the authorization code grant:
1. The user selects to start the grant using H-AS (e.g., by clicking
on a button at the client's website).
2. The attacker intercepts this request and changes the user's
selection to "A-AS".
3. The client stores in the user's session that the user selected
"A-AS" and redirects the user to A-AS's authorization endpoint by
sending the response code "303 See Other" with a Location header
containing the URL "https://attacker.example/
authorize?response_type=code&client_id=666RVZJTA".
4. Now the attacker intercepts this response and changes the
redirection such that the user is being redirected to H-AS. The
attacker also 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
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"https://honest.as.example/
authorize?response_type=code&client_id=7ZGZldHQ"
5. Now, the user authorizes the client to access her resources at
H-AS. H-AS issues a code and sends it (via the browser) back to
the client.
6. 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.
7. The attacker therefore obtains code and can either exchange the
code for an access token (for public clients) or perform a code
injection attack as described in Section 4.5.
Variants:
o *Implicit Grant*: In the implicit grant, the attacker receives an
access token instead of the code; the rest of the attack works as
above.
o *Mix-Up Without Interception*: A variant of the above attack works
even if the first request/response pair cannot be intercepted (for
example, because TLS is used to protect these messages): Here, we
assume that the user wants to start the grant using A-AS (and not
H-AS, see Attacker A1). After the client redirected the user to
the authorization endpoint at A-AS, the attacker immediately
redirects the user to H-AS (changing the client id to "7ZGZldHQ").
(A vigilant user might at this point detect that she intended to
use A-AS instead of H-AS.) The attack now proceeds exactly as in
Steps 3ff. of the attack description above.
o *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".
Refer to [oauth_security_jcs_14] for details.
o *OpenID Connect*: There are several variants that can be used to
attack OpenID Connect. They are described in detail in
[arXiv.1704.08539], Appendix A, and [arXiv.1508.04324v2],
Section 6 ("Malicious Endpoints Attacks").
4.4.2. Countermeasures
In scenarios where an OAuth client interacts with multiple
authorization servers, clients MUST prevent mix-up attacks.
Potential countermeasures:
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o Configure authorization servers to return an AS identitifier
("iss") and the "client_id" for which a code or token was issued
in the authorization response. This enables clients to compare
this data to their own client id and the "iss" identifier of the
AS it believed it sent the user agent to. This mitigation is
discussed in detail in [I-D.ietf-oauth-mix-up-mitigation]. In
OpenID Connect, if an ID token is returned in the authorization
response, it carries client id and issuer. It can be used for
this mitigation.
o As it can be seen in the preconditions of the attacks above,
clients can prevent mix-up attack by (1) using AS-specific
redirect URIs with exact redirect URI matching, (2) storing, for
each authorization request, the intended AS, and (3) comparing the
intended AS with the actual redirect URI where the authorization
response was received.
4.5. Authorization Code Injection
In such an attack, the adversary attempts to inject a stolen
authorization code into a legitimate client on a device under his
control. In the simplest case, the attacker would want to use the
code in his own client. But there are situations where this might
not be possible or intended. Examples are:
o 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.
o 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.
o 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 or network attacker, but not of an attacker with
advanced capabilities (A3-A5).
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.
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2. He performs a regular OAuth authorization process with the
legitimate client on his device.
3. The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client.
4. The client sends the code to the authorization server's token
endpoint, along with client id, client secret and actual
"redirect_uri".
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. If all checks succeed, the authorization server issues access and
other tokens to the client, so now the attacker is able to
impersonate the legitimate user.
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 a code which
had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled:
o the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
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o the client has bound this data to this particular instance.
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 spec, since the tampered redirect URI is not
considered. So any attempt to inject a code obtained using the
"client_id" of a legitimate client or by utilizing the legitimate
client on another device won't 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 memorize or re-construct the correct redirect URI for the
call to the tokens 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.
4.5.3. Proposed Countermeasures
There are multiple technical solutions to achieve this goal:
o *Nonce*: OpenID Connect's existing "nonce" parameter can be used
for the purpose of detecting authorization code injection attacks.
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. The main advantage of
this option is that "nonce" is an existing feature used in the
wild. On the other hand, leveraging "nonce" by the broader OAuth
community would require AS and clients to adopt ID Tokens.
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o *Code-bound State*: The "state" parameter as specified in
[RFC6749] could be used similarly to what is described above.
This would require to add a further parameter "state" to the code
exchange token endpoint request. The authorization server would
then compare the "state" value it associated with the code and the
"state" value in the parameter. If those values do not match, it
is considered an attack and the request fails. The advantage of
this approach would be to utilize an existing OAuth parameter.
But it would also mean to re-interpret the purpose of "state" and
to extend the token endpoint request.
o *PKCE*: The PKCE parameter "code_challenge" along with the
corresponding "code_verifier" as specified in [RFC7636] could be
used in the same way as "nonce" or "state". In contrast to its
original intention, the verifier check would fail although the
client uses its correct verifier but the code is associated with a
challenge, which does not match. PKCE is a deployed OAuth
feature, even though it is used today to secure native apps only.
o *Token Binding*: Token binding [I-D.ietf-oauth-token-binding]
could also be used. In this case, the code would need to be bound
to two legs, between user agent and AS and the user agent and the
client. This requires further data (extension to response) to
manifest binding id for particular code. Token binding is
promising as a secure and convenient mechanism (due to its browser
integration). As a challenge, it requires broad browser support
and use with native apps is still under discussion.
o *Per-instance client id/secret*: One could use per instance
"client_id" and secrets and bind the code to the respective
"client_id". Unfortunately, this does not fit into the web
application programming model (would need to use per-user client
IDs).
PKCE seems to be the most obvious solution for OAuth clients as it is
available and effectively used today for similar purposes for OAuth
native apps whereas "nonce" is appropriate for OpenId Connect
clients.
Note on pre-warmed secrets: An attacker can circumvent the
countermeasures described above if he is able to create or capture
the respective secret or code_challenge on a device under his
control, which is then used in the victim's authorization request.
Exact redirect URI matching of authorization requests can prevent the
attacker from using the pre-warmed secret in the faked authorization
transaction on the victim's device.
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Unfortunately, it does not work for all kinds of OAuth clients. It
is effective for web and JS apps and for native apps with claimed
URLs. Attacks on native apps using custom schemes or redirect URIs
on localhost cannot be prevented this way, except if the AS enforces
one-time use for PKCE verifier or "nonce" values.
4.6. Access Token Injection
In such an attack, the adversary attempts to inject a stolen access
token into a legitimate client on a device under his 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 adversary starts an OAuth flow with the
client 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 and will use the access token injected by the
attacker.
4.6.1. Proposed 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. 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.
4.7.1. Proposed Countermeasures
Use of CSRF tokens which are bound to the user agent and passed in
the "state" parameter to the authorization server as described in
[!@RFC6819]. Alternatively, PKCE provides CSRF protection.
It is important to note that:
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o Clients MUST ensure that the AS supports PKCE before using PKCE
for CSRF protection. If an authorization server does not support
PKCE, "state" MUST be used for CSRF protection.
o 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].
The recommendation therefore is that AS publish their PKCE support
either in AS metadata according to [RFC8418] or provide a deployment-
specific way to ensure or determine PKCE support.
Additionally, standard CSRF defenses MAY be used to protect the
redirection endpoint, for example the Origin header.
For more details see [owasp_csrf].
4.8. Access Token Leakage at the Resource Server
Access tokens can leak from a resource server under certain
circumstances.
4.8.1. 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.
There are several potential mitigation strategies, which will be
discussed in the following sections.
4.8.1.1. Metadata
An authorization server could provide the client with additional
information about the location where it is safe to use its access
tokens.
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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 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 return parameter
"access_token_resource_server":
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"access_token_resource_server":
"https://hostedresource.somesite.example/path1",
...
}
This mitigation strategy would rely on the client to enforce the
security policy and to only send access tokens to legitimate
destinations. Results of OAuth related security research (see for
example [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.
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4.8.1.2. 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
which 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, it may utilize
capabilities of the transport layer (e.g., TLS). Note: replay
prevention is required as well!
There exist several proposals to demonstrate the proof of possession
in the scope of the OAuth working group:
o *OAuth Token Binding* ([I-D.ietf-oauth-token-binding]): In this
approach, an access token is, via the so-called 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]
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(including federated token bindings) must be supported on all ends
(client, authorization server, resource server).
o *OAuth Mutual TLS* ([I-D.ietf-oauth-mtls]): 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.
o *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.
o *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 [I-D.ietf-oauth-mtls],
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.
Mutual TLS and OAuth Token Binding are built on top of TLS and this
way continue the successful OAuth 2.0 philosophy to leverage TLS to
secure OAuth wherever possible. Both mechanisms allow prevention of
access token leakage in a fairly client developer friendly way.
There are some differences between both approaches: To start with,
for OAuth Token Binding, all key material is automatically managed by
the TLS stack whereas mTLS requires the developer to create and
maintain the key pairs and respective certificates. Use of self-
signed certificates, which is supported by the draft, significantly
reduces the complexity of this task. Furthermore, OAuth Token
Binding allows to use different key pairs for different resource
servers, which is a privacy benefit. On the other hand,
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[I-D.ietf-oauth-mtls] only requires widely deployed TLS features,
which means it might be easier to adopt in the short term.
Application level signing approaches, like
[I-D.ietf-oauth-signed-http-request] and [I-D.sakimura-oauth-jpop]
have been debated for a long time in the OAuth working group without
a clear outcome.
As one advantage, application-level signing allows for end-to-end
protection including non-repudiation even if the TLS connection is
terminated between client and resource server. But deployment
experiences have revealed challenges regarding robustness (e.g.,
reproduction of the signature base string including correct URL) as
well as state management (e.g., replay prevention).
This document therefore recommends implementors to consider one of
TLS-based approaches wherever possible.
4.8.1.3. Audience Restricted Access Tokens
An audience restriction essentially restricts the resource server a
particular access token can be used at. The authorization server
associates the access token with a certain resource server and every
resource server is obliged to verify for every request, whether the
access token sent with that request was meant to be used at the
particular resource server. If not, the resource server must refuse
to serve the respective request. In the general case, audience
restrictions limit the impact of a 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 basically 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.
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 needs to tell the authorization server, at which URL it
will use the access token it is requesting. It could use the
mechanism proposed [I-D.ietf-oauth-resource-indicators] or encode the
information in the scope value.
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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 legit
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 seems easy to use since it does not require any
crypto on the client side. But since every access token is bound to
a certain resource server, the client also needs to obtain different
RS-specific access tokens, if it wants to access several resource
services. [I-D.ietf-oauth-token-binding] has the same property,
since different token binding ids must be associated with the access
token. [I-D.ietf-oauth-mtls] 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.
4.8.2. Compromised Resource Server
An attacker may compromise a resource server in order to get access
to its resources and other resources of the respective deployment.
Such a compromise may range from partial access to the system, e.g.,
its logfiles, to full control of the respective server.
If the attacker was able to take over full control including shell
access it will be able to circumvent all controls in place and access
resources without access control. It will also get access to access
tokens, which are sent to the compromised system and which
potentially are valid for access to other resource servers as well.
Even if the attacker "only" is able to access logfiles or databases
of the server system, it may get access to valid access tokens.
Preventing server breaches by way of hardening and monitoring server
systems is considered a standard operational procedure and therefore
out of scope of this document. This section will focus on the impact
of such breaches on OAuth-related parts of the ecosystem, which is
the replay of captured access tokens on the compromised resource
server and other resource servers of the respective deployment.
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The following measures should be taken into account by implementors
in order to cope with access token replay:
o The resource server must treat access tokens like any other
credentials. It is considered good practice to not log them and
not to store them in plain text.
o Sender-constrained access tokens as described in Section 4.8.1.2
will prevent the attacker from replaying the access tokens on
other resource servers. Depending on the severity of the
penetration, it will also prevent replay on the compromised
system.
o Audience restriction as described in Section 4.8.1.3 may be used
to prevent replay of captured access tokens on other resource
servers.
4.9. Open Redirection
The following attacks can occur when an AS or client has an open
redirector, i.e., a URL which causes an HTTP redirect to an attacker-
controlled web site.
4.9.1. Authorization Server as Open Redirector
Attackers could try to utilize a user's trust in the authorization
server (and its URL in particular) for performing phishing attacks.
[RFC6749], Section 4.1.2.1, already prevents open redirects by
stating the AS MUST NOT automatically redirect the user agent in case
of an invalid combination of client_id and redirect_uri.
However, as described in [I-D.ietf-oauth-closing-redirectors], an
attacker could also utilize a correctly registered redirect URI to
perform phishing attacks. It 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, to cause the AS to automatically 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 or not and SHOULD only automatically redirect the user
agent, if it trusts the redirect URI. If not, it MAY inform the user
that it is about to redirect her to the another site and rely on the
user to decide or MAY just inform the user about the error.
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4.9.2. Clients as Open Redirector
Client MUST NOT expose URLs which could be utilized as open
redirector. Attackers may use an open redirector to produce URLs
which appear to point to the client, which might trick users to trust
the URL and follow it in her browser. Another abuse case is to
produce URLs pointing to the client and utilize them to impersonate a
client with an authorization server.
In order to prevent open redirection, clients should only expose such
a function, if the target URLs are whitelisted or if the origin of a
request can be authenticated.
4.10. 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". However, when the status code 307 is used
for redirection, the user agent will send the form data (user
credentials) via HTTP POST to the client since this status code does
not require the user agent to rewrite the POST request to a GET
request (and thereby dropping the form data in the POST request
body). If the relying party is malicious, it can use the credentials
to impersonate the user at the AS.
In the HTTP standard [RFC6749], 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".
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4.11. TLS Terminating Reverse Proxies
A common deployment architecture for HTTP applications is to have the
application server sitting behind a reverse proxy which 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 which are relevant 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.
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 "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, he could also try to circumvent
security controls in place. It is therefore important to ensure the
authenticity of the communicating entities. Furthermore, the
communication link between reverse proxy and application server must
therefore be protected against eavesdropping, injection, and replay
of messages.
4.12. Refresh Token Protection
Refresh tokens are a convenient and UX-friendly way to obtain new
access tokens after the expiration of older 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.
Refresh tokens are an attractive target for attackers since they
represent the overall grant a resource owner delegated to a certain
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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
o confidentiality of the refresh tokens in transit and storage,
o the transmission of refresh tokens over TLS-protected connections
between authorization server and client,
o the authorization server to maintain and check the binding of a
refresh token to a certain client (i.e., "client_id"),
o authentication of this client during token refresh, if possible,
and
o 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 draft gives recommendations beyond the scope of [RFC6749] and
clarifications.
Authorization servers MUST determine based on their 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 legit client and
reduce the impact of refresh token leakage.
Authorization server MUST utilize one of these methods to detect
refresh token replay for public clients:
o *Sender-constrained refresh tokens:* the authorization server
cryptographically binds the refresh token to a certain client
instance by utilizing [I-D.ietf-oauth-token-binding] or
[I-D.ietf-oauth-mtls].
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o *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 can revoke
the active refresh token. This stops the attack at the cost of
forcing the legit client to obtain a fresh authorization grant.
Implementation note: refresh tokens belonging to the same grant
may share a common id. If any of those refresh tokens is used at
the authorization server, the authorization server uses this
common id to look up the currently active refresh token and can
revoke it.
Authorization servers may revoke refresh tokens automatically in case
of a security event, such as:
o password change
o 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).
4.13. 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 mechanism. 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 as an access token
authorized by the privileged user if the resource server does not
perform additional checks.
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4.13.1. Proposed Countermeasures
Authorization servers SHOULD NOT allow clients to influence their
"client_id" or "sub" value or any other claim that might cause
confusion with a genuine resource owner. Where this cannot be
avoided, authorization servers MUST provide another means for the
resource server to distinguish between access tokens authorized by a
resource owner from access tokens authorized by the client itself.
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, and Tim Wuertele for their valuable
feedback.
6. IANA Considerations
This draft includes no request to IANA.
7. Security Considerations
All relevant security considerations have been given in the
functional specification.
8. References
8.1. Normative References
[oauth-v2-form-post-response-mode]
Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
Mode", 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", Nov 2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
[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>.
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[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[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>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC8418] Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
Agreement Algorithm with X25519 and X448 in the
Cryptographic Message Syntax (CMS)", RFC 8418,
DOI 10.17487/RFC8418, August 2018,
<https://www.rfc-editor.org/info/rfc8418>.
8.2. Informative References
[arXiv.1508.04324v2]
Mladenov, V., Mainka, C., and J. Schwenk, "On the security
of modern Single Sign-On Protocols: Second-Order
Vulnerabilities in OpenID Connect", January 2016,
<http://arxiv.org/abs/1508.04324v2/>.
[arXiv.1601.01229]
Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0", January 2016,
<http://arxiv.org/abs/1601.01229/>.
[arXiv.1704.08539]
Fett, D., Kuesters, R., and G. Schmitz, "The Web SSO
Standard OpenID Connect: In-Depth Formal Security Analysis
and Security Guidelines", April 2017,
<http://arxiv.org/abs/1704.08539/>.
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[arXiv.1901.11520]
Fett, D., Hosseyni, P., and R. Kuesters, "An Extensive
Formal Security Analysis of the OpenID Financial-grade
API", January 2019, <http://arxiv.org/abs/1901.11520/>.
[bug.chromium]
"Referer header includes URL fragment when opening link
using New Tab",
<https://bugs.chromium.org/p/chromium/issues/
detail?id=168213/>.
[fb_fragments]
"Facebook Developer Blog",
<https://developers.facebook.com/blog/post/552/>.
[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", draft-
bradley-oauth-jwt-encoded-state-09 (work in progress),
November 2018.
[I-D.ietf-oauth-closing-redirectors]
Bradley, J., Sanso, A., and H. Tschofenig, "OAuth 2.0
Security: Closing Open Redirectors in OAuth", draft-ietf-
oauth-closing-redirectors-00 (work in progress), February
2016.
[I-D.ietf-oauth-jwsreq]
Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
Framework: JWT Secured Authorization Request (JAR)",
draft-ietf-oauth-jwsreq-19 (work in progress), June 2019.
[I-D.ietf-oauth-mix-up-mitigation]
Jones, M., Bradley, J., and N. Sakimura, "OAuth 2.0 Mix-Up
Mitigation", draft-ietf-oauth-mix-up-mitigation-01 (work
in progress), July 2016.
[I-D.ietf-oauth-mtls]
Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual TLS Client Authentication
and Certificate-Bound Access Tokens", draft-ietf-oauth-
mtls-15 (work in progress), July 2019.
[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", draft-ietf-oauth-pop-
key-distribution-07 (work in progress), March 2019.
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[I-D.ietf-oauth-resource-indicators]
Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", draft-ietf-oauth-resource-
indicators-02 (work in progress), January 2019.
[I-D.ietf-oauth-signed-http-request]
Richer, J., Bradley, J., and H. Tschofenig, "A Method for
Signing HTTP Requests for OAuth", draft-ietf-oauth-signed-
http-request-03 (work in progress), August 2016.
[I-D.ietf-oauth-token-binding]
Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", draft-ietf-oauth-token-
binding-08 (work in progress), October 2018.
[I-D.sakimura-oauth-jpop]
Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
Authorization Framework: JWT Pop Token Usage", draft-
sakimura-oauth-jpop-04 (work in progress), March 2017.
[oauth_security_cmu]
Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
Tague, "OAuth Demystified for Mobile Application
Developers", November 2014.
[oauth_security_jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
Maffeis, "Discovering concrete attacks on website
authorization by formal analysis", April 2014.
[oauth_security_ubc]
Sun, S. 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>.
[owasp] "Open Web Application Security Project Home Page",
<https://www.owasp.org/>.
[owasp_csrf]
"Cross-Site Request Forgery (CSRF) Prevention Cheat
Sheet", <https://www.owasp.org/index.php/
Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[webappsec-referrer-policy]
Eisinger, J. and E. Stark, "Referrer Policy", April 2017,
<https://w3c.github.io/webappsec-referrer-policy>.
Appendix A. Document History
[[ To be removed from the final specification ]]
-13
o Discourage use of Resource Owner Password Credentials Grant
o Added text on client impersonating resource owner
o Recommend asymmetric methods for client authentication
o Encourage use of PKCE mode "S256"
o PKCE may replace state for CSRF protection
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o AS SHOULD publish PKCE support
o Cleaned up discussion on auth code injection
o AS MUST support PKCE
-12
o Added updated attacker model
-11
o Adapted section 2.1.2 to outcome of consensus call
o more text on refresh token inactivity and implementation note on
refresh token replay detection via refresh token rotation
-10
o incorporated feedback by Joseph Heenan
o changed occurrences of SHALL to MUST
o 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
o added requirement to authenticate clients during code exchange
(PKCE or client credential) to 2.1.1.
o added section on refresh tokens
o editorial enhancements to 2.1.2 based on feedback
-09
o changed text to recommend not to use implicit but code
o added section on access token injection
o reworked sections 3.1 through 3.3 to be more specific on implicit
grant issues
-08
o added recommendations re implicit and token injection
o uppercased key words in Section 2 according to RFC 2119
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-07
o incorporated findings of Doug McDorman
o added section on HTTP status codes for redirects
o added new section on access token privilege restriction based on
comments from Johan Peeters
-06
o reworked section 3.8.1
o incorporated Phil Hunt's feedback
o reworked section on mix-up
o extended section on code leakage via referrer header to also cover
state leakage
o added Daniel Fett as author
o replaced text intended to inform WG discussion by recommendations
to implementors
o modified example URLs to conform to RFC 2606
-05
o Completed sections on code leakage via referrer header, attacks in
browser, mix-up, and CSRF
o Reworked Code Injection Section
o Added reference to OpenID Connect spec
o removed refresh token leakage as respective considerations have
been given in section 10.4 of RFC 6749
o first version on open redirection
o incorporated Christian Mainka's review feedback
-04
o Restructured document for better readability
o Added best practices on Token Leakage prevention
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Internet-Draft oauth-security-topics July 2019
-03
o Added section on Access Token Leakage at Resource Server
o incorporated Brian Campbell's findings
-02
o Folded Mix up and Access Token leakage through a bad AS into new
section for dynamic OAuth threats
o reworked dynamic OAuth section
-01
o Added references to mitigation methods for token leakage
o Added reference to Token Binding for Authorization Code
o incorporated feedback of Phil Hunt
o fixed numbering issue in attack descriptions in section 2
-00 (WG document)
o turned the ID into a WG document and a BCP
o Added federated app login as topic in Other Topics
Authors' Addresses
Torsten Lodderstedt
yes.com
Email: torsten@lodderstedt.net
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Facebook
Email: isciurus@fb.com
Lodderstedt, et al. Expires January 9, 2020 [Page 42]
Internet-Draft oauth-security-topics July 2019
Daniel Fett
yes.com
Email: mail@danielfett.de
Lodderstedt, et al. Expires January 9, 2020 [Page 43]
