Open Authentication Protocol T. Lodderstedt, Ed.
Internet-Draft YES.com AG
Intended status: Best Current Practice J. Bradley
Expires: September 19, 2018 Yubico
A. Labunets
Facebook
March 18, 2018
OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-05
Abstract
This document describes best current security practices 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 cover 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
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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
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This Internet-Draft will expire on September 19, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Protecting redirect-based flows . . . . . . . . . . . . . 4
2.2. Token Replay Prevention . . . . . . . . . . . . . . . . . 5
3. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 5
3.1. Insufficient redirect URI validation . . . . . . . . . . 5
3.1.1. Attacks on Authorization Code Grant . . . . . . . . . 5
3.1.2. Attacks on Implicit Grant . . . . . . . . . . . . . . 6
3.1.3. Proposed Countermeasures . . . . . . . . . . . . . . 8
3.2. Authorization code leakage via referrer headers . . . . . 9
3.2.1. Proposed Countermeasures . . . . . . . . . . . . . . 9
3.3. Attacks in the Browser . . . . . . . . . . . . . . . . . 10
3.3.1. Code in browser history . . . . . . . . . . . . . . . 10
3.3.2. Access token in browser history . . . . . . . . . . . 10
3.4. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5. Code Injection . . . . . . . . . . . . . . . . . . . . . 11
3.5.1. Proposed Countermeasures . . . . . . . . . . . . . . 13
3.6. Cross Site Request Forgery . . . . . . . . . . . . . . . 15
3.7. Access Token Leakage at the Resource Server . . . . . . . 15
3.7.1. Access Token Phishing by Counterfeit Resource Server 15
3.7.1.1. Metadata . . . . . . . . . . . . . . . . . . . . 16
3.7.1.2. Sender Constrained Access Tokens . . . . . . . . 17
3.7.1.3. Audience Restricted Access Tokens . . . . . . . . 19
3.7.2. Compromised Resource Server . . . . . . . . . . . . . 20
3.8. Open Redirection . . . . . . . . . . . . . . . . . . . . 21
3.8.1. Authorization Server as Open Redirector . . . . . . . 21
3.8.2. Clients as Open Redirector . . . . . . . . . . . . . 21
3.9. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 22
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Normative References . . . . . . . . . . . . . . . . . . 23
7.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Document History . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
It's been a while since OAuth has been published in RFC 6749
[RFC6749] and RFC 6750 [RFC6750]. Since publication, OAuth 2.0 has
gotten massive traction in the market and became the standard for API
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protection and, as 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 (XSRF, 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 or that the existing mitigations are too
difficult to deploy.
o Technology has changed, e.g. the way browsers treat fragments in
some situations, which may change the implicit grant's underlying
security model.
o OAuth is used in much more dynamic setups than originally
anticipated, creating new challenges with respect to security.
Those challenges go beyond the original scope of RFC 6749
[RFC6749], RFC 6750 [RFC6749], and RFC 6819 [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 relationsship 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
[I-D.ietf-oauth-discovery] 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.
The remainder of the document is organized as follows: The next
section summarizes the most important recommendations of the OAuth
working group for every OAuth implementor. Afterwards, a detailed
analyses of the threats and implementation issues, which can be found
in the wild today, is given along with a discussion of potential
counter measures.
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2. Recommendations
This section describes the set of security mechanisms the authors
believe should be taken into consideration by the OAuth working group
to be recommended to OAuth implementers.
2.1. Protecting redirect-based flows
Authorization servers shall 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 shall avoid any redirects or forwards, which can be
parameterized by URI query parameters, in order to provide a further
layer of defence against token leakage. If there is a need for this
kind of redirects, clients are advised to implement appropriate
counter measures against open redirection, e.g. as described by the
OWASP [owasp].
Clients shall ensure to only process redirect responses of the OAuth
authorization server they send the respective request to and in the
same user agent this request was initiated in. In particular,
clients shall implement appropriate XSRF prevention by utilizing one-
time use XSRF tokens carried in the STATE parameter, which are
securely bound to the user agent. Moreover, the client shall
memorize which authorization server it 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.
Furthermore, clients should use AS-specific redirect URIs as a means
to identify the AS a particular response came from. Matching this
with the before mentioned information regarding the AS the client
sent the request to helps to detect mix-up attacks.
Note: [I-D.bradley-oauth-jwt-encoded-state] gives advice on how to
implement XSRF prevention and AS matching using signed JWTs in the
STATE parameter.
Clients shall 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. 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 of the for the same purpose.
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Note: although PKCE so far was recommended as mechanism to protect
native apps, this advice applies to all kinds of OAuth clients,
including web applications.
Authorization servers shall consider the recommendations given in
[RFC6819], section 4.4.1.1, on authorization code replay prevention.
2.2. Token Replay Prevention
Authorization servers shall use TLS-based methods for sender
constraint access tokens as described in section Section 3.7.1.2,
such as token binding [I-D.ietf-oauth-token-binding] or Mutual TLS
for OAuth 2.0 [I-D.ietf-oauth-mtls]. It is also recommend to use
end-to-end TLS whenever possible.
3. Attacks and Mitigations
3.1. 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 have been observed
in the wild, which utilized flaws in the pattern matching
implementation or concrete configurations. Such a flaw 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.
3.1.1. 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://*.example.com/*" had
been registered for the client "s6BhdRkqt3". This pattern allows
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redirect URIs from any host residing in the domain example.com. So
if an attacker manages to establish a host or subdomain in
"example.com" he can impersonate the legitimate client. Assume the
attacker sets up the host "evil.example.com".
(1) 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.com".
(2) 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=xyz
&redirect_uri=https%3A%2F%2Fevil.example.com%2Fcb HTTP/1.1
Host: server.example.com
(1) The authorization validates the redirect URI in order to
identify the client. Since the pattern allows arbitrary domains
host names in "example.com", 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 RFC 6749), the attack can be performed even
easier.
(2) If the user does not recognize the attack, the code is issued
and directly sent to the attacker's client.
(3) Since the attacker impersonated a public client, it can directly
exchange the code for tokens at the respective token endpoint.
Note: This attack will not directly work for confidential clients,
since the code exchange requires authentication with the legitimate
client's secret. The attacker will need to utilize the legitimate
client to redeem the code (e.g. by performing a code injection
attack). This kind of injections is covered in
Section Code Injection.
3.1.2. 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.
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Additionally, implicit clients can be subject to a further kind of
attacks. 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 strict redirect URI patterns (but not strict
URL matching!).
Assume the pattern for client "s6BhdRkqt3" is
"https://client.example.com/cb?*", i.e. any parameter is allowed for
redirects to "https://client.example.com/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 a HTTP 302.
(1) Same 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.com".
(2) The URL initiates an authorization request, which is very
similar to the attack on the code flow. As differences, it
utilizes the open redirector by encoding
"redirect_to=https://client.evil.com" into the redirect URI and
it uses the response type "token" (line breaks are for display
purposes only):
GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb%26redirect_to
%253Dhttps%253A%252F%252Fclient.evil.com%252Fcb HTTP/1.1
Host: server.example.com
(1) Since the redirect URI matches the registered pattern, the
authorization server allows the request and sends the resulting
access token with a 302 redirect (some response parameters are
omitted for better readability)
HTTP/1.1 302 Found
Location: https://client.example.com/cb?
redirect_to%3Dhttps%3A%2F%2Fclient.evil.com%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&...
(2) At the example.com, the request arrives at the open redirector.
It will read the redirect parameter and will issue a HTTP 302 to
the URL "https://evil.example.com/cb".
HTTP/1.1 302 Found
Location: https://client.evil.com/cb
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(3) Since the redirector at example.com 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.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...
(4) The attacker's page at client.evil.com can access the fragment
and obtain the access token.
3.1.3. 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 shall compare the two URIs using simple string comparison as
defined in [RFC3986], Section 6.2.1..
This would cause the following impacts:
o This change will require all OAuth clients to maintain the
transaction state (and XSRF tokens) in the "state" parameter.
This is a normative change to RFC 6749 since section 3.1.2.2
allows for dynamic URI query parameters in the redirect URI. In
order to assess the practical impact, the working group needs to
collect data on whether this feature is really used in deployments
today.
o The working group may also consider this change as a step towards
improved interoperability for OAuth implementations since RFC 6749
is somewhat vague on redirect URI validation. Notably there are
no rules for pattern matching. One may therefore assume all
clients utilizing pattern matching will do so in a deployment
specific way. On the other hand, RFC 6749 already recommends
exact matching if the full URL had been registered.
o Clients with multiple redirect URIs need to register all of them
explicitly.
Note: clients with just a single redirect URI would not even need
to send a redirect URI with the authorization request. Does it
make sense to emphasize this option? Would that further simplify
use of the protocol and foster security?
o Exact redirect matching does not work for native apps utilizing a
local web server due to dynamic port numbers - at least wild cards
for port numbers are required.
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Question: Does redirect uri validation solve any problem for
native apps? Effective against impersonation when used in
conjunction with claimed HTTPS redirect URIs only.
For Windows token broker exact redirect URI matching is important
as the redirect URI encodes the app identity. For custom scheme
redirects there is a question however it is probably a useful part
of defense in depth.
Additional recommendations:
o Servers on which callbacks are hosted must not expose open
redirectors (see Section 3.8).
o Clients may drop fragments via intermediary URLs with "fix
fragments" (e.g. https://developers.facebook.com/blog/post/552/)
to prevent the user agent from appending any unintended fragments.
Alternatives to exact redirect URI matching:
o Authenticate clients using digital signatures (see
[I-D.ietf-oauth-jwsreq])
3.2. Authorization code leakage via referrer headers
It is possible authorization codes are unintentionally disclosed to
attackers, if a OAuth client renders a page containing links to other
pages (ads, faq, ...) as result of a successful authorization
request.
If the user clicks onto one of those links and the target is under
the control of an attacker, it can get access to the response URL in
the referrer header.
It is also possible that an attacker injects cross-domain content
somehow into the page, such as ![]()
(e.g if this is a blog web
site). The implication is obviously the same: loading this content
by browser results in leaking referrer with a code.
3.2.1. Proposed Countermeasures
There are some means to prevent leakage as described above:
o Make authorization codes one-time use. For example, if the
legitimate client redeemed and invalidated the code in the above
scenario, the attacker would fail exchanging this code later.
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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 Don't include links to external sites into the page rendered as
result of a OAuth authorization response
o Use of the HTML link attribute rel="noreferrer" to suppress
referrer header
o Use of the "referrer" meta link attribute to suppress referrer
header (see [webappsec-referrer-policy])
o Use form post response mode instead of redirect for authorization
response (see [oauth-v2-form-post-response-mode])
3.3. Attacks in the Browser
3.3.1. Code in browser history
When a browser navigates to "client.com/
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 3.5
o Use form post response mode instead of redirect for authorization
response (see [oauth-v2-form-post-response-mode])
3.3.2. Access token in browser history
An access token may end up in the browser history if a 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.com/
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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Proposed countermeasures:
o Replace implicit flow with postmessage communication or the
authorization code grant
o Never pass access tokens in URL query parameters
3.4. Mix-Up
Mix-up is another kind of attack on more dynamic OAuth scenarios (or
at least scenarios where a OAuth client interacts with multiple
authorization servers). 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 (which acts as MITM between
client and authorization server)
A detailed description of the attack and potential countermeasures is
given in [I-D.ietf-oauth-mix-up-mitigation].
Potential mitigations:
o Clients use AS-specific redirect URIs and, for every authorization
request, store intended AS and compare intention with actual
redirect URI where the response was received
o AS returns client_id and its iss in the response. Client compares
this data to AS it believed it sent the user agent to.
o ID token carries client id and issuer (OpenID Connect specific)
3.5. 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, the attackers is unable to access directly.
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How does an attack look like?
(1) The attacker obtains an authorization code by performing any of
the attacks described above.
(2) It 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.
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 malware pretending to be
the legitimate client 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 attempt to inject a code, which
had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled:
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o the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
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 spec.
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.
The authors therefore propose to the working group to drop this
feature in favor of more effective and (hopefully) simpler approaches
to code injection prevention as described in the following section.
3.5.1. Proposed Countermeasures
The general proposal is to 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. There are multiple technical
solutions to achieve this goal:
Nonce OpenID Connect's existing "nonce" parameter could be used for
this 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 associates the 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. assumption: attacker cannot get
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hold of the user agent state on the victims 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 client to adopt
ID Tokens.
Code-bound State The "state" parameter as specified in [RFC6749]
could be used similarly to the way as described above. This
would require to add a further parameter "state" to the code
exchange token endpoint request. The authorization server
would then compares 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.
PKCE The PKCE parameter "challenge" along with the corresponding
"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 so it is today used to secure native apps,
only.
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 still
under discussion.
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 seem to be the most obvious solution for OAuth clients as it
available and effectively used today for similar purposes for OAuth
native apps whereas "nonce" is appropriate for OpenId Connect
clients.
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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.
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.
3.6. 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.
Proposed mitigation: use of XSRF tokens (one-time use), which are
bound to the user agent and passed in the state parameter to the
authorization server. For more details see [owasp_csrf].
3.7. Access Token Leakage at the Resource Server
3.7.1. 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, which are valid for other resource
servers. 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 a certain resource
server (and the respective URL) at development time, but client
instances are configured with an resource server's URL at runtime.
This kind of late binding is typical in situations, where the client
uses a standard API, e.g. for e-Mail, calendar, health, or banking
and is configured by an user or administrator for the standard-based
service, this particular user or company uses.
There are several potential mitigation strategies, which will be
discussed in the following sections.
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3.7.1.1. 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 metadata parameter "resource_servers":
HTTP/1.1 200 OK
Content-Type: application/json
{
"issuer":"https://server.example.com",
"authorization_endpoint":"https://server.example.com/authorize",
"resource_servers":[
"email.example.com",
"storage.example.com",
"video.example.com"]
...
}
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.example.com/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
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client has to contribute to the overall security. But there are
alternative counter measures, as described in the next sections,
which provide a better balance between the involved parties.
3.7.1.2. Sender Constrained Access Tokens
As the name suggests, sender constraint 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 bind 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 happens 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 exists several proposals to demonstrate the proof of possession
in the scope of the OAuth working group:
o [I-D.ietf-oauth-token-binding]: In this approach, an access tokens
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 than 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
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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
[I-D.ietf-tokbind-https] (including federated token bindings) must
be supported on all ends (client, authorization server, resource
server).
o [I-D.ietf-oauth-mtls]: The approach as specified in this document
allow use of mutual TLS for both client authentication and sender
constraint access tokens. For the purpose of sender constraint
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 [I-D.ietf-oauth-signed-http-request] specifies an approach to sign
HTTP requests. It 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 [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 RFC 7800 [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.
[I-D.ietf-oauth-mtls] and [I-D.ietf-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, in
[I-D.ietf-oauth-token-binding] all key material is automatically
managed by the TLS stack whereas [I-D.ietf-oauth-mtls] requires the
developer to create and maintain the key pairs and respective
certificates. Use of self-signed certificates, which is supported by
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the draft, significantly reduce the complexity of this task.
Furthermore, [I-D.ietf-oauth-token-binding] allows to use different
key pairs for different resource servers, which is a privacy benefit.
On the other hand, [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.
3.7.1.3. 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 send 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 see 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.
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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.campbell-oauth-resource-indicators] or encode
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 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.
3.7.2. 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
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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.
The following measures shall 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 constraint access tokens as described in Section 3.7.1.2
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 3.7.1.3 may be used
to prevent replay of captured access tokens on other resource
servers.
3.8. Open Redirection
3.8.1. 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.
The attacker could send an authorization request with an invalid
combination of client_id and redirect_uri. [RFC6749], section
4.1.2.1, already states that the AS MUST NOT automatically redirect
the user agent in this case to prevent open redirection.
But as described in [I-D.ietf-oauth-closing-redirectors], the
attacker could also attempt to register a client and intentionally
send an erroneous authorization request, e.g. by using an invalid
scope value. According to [RFC6749], the AS would send the user
agent to the redirect_uri with an "invalid_request" error response.
This is dangerous because the authorization server would serve as an
open redirector. Therefore this draft recommends that every invalid
authorization request MUST NOT automatically redirect the user agent
to the client's redirect URI.
3.8.2. Clients as Open Redirector
Client MUST not expose URLs which could be utilized as open
redirector. An open redirector is a way to cause the recipient of a
HTTP request to issue a redirect to a target URL that is passed as a
parameter. Attackers may utilize such a mechanism to produce URLs,
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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.
3.9. 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 give 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 would be able to get access to the internal network
between proxy and application server, it 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 tapping and injection (including
replay prevention).
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4. Acknowledgements
We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
Mainka, and Brian Campbell for their valuable feedback.
5. IANA Considerations
This draft includes no request to IANA.
6. Security Considerations
All relevant security considerations have been given in the
functional specification.
7. References
7.1. Normative References
[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,
.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
.
[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,
.
[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,
.
[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,
.
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7.2. Informative References
[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-08 (work in progress),
January 2018.
[I-D.campbell-oauth-resource-indicators]
Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", draft-campbell-oauth-resource-
indicators-02 (work in progress), November 2016.
[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-discovery]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", draft-ietf-oauth-
discovery-10 (work in progress), March 2018.
[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-15 (work in progress), July 2017.
[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-07 (work in progress), January 2018.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-03 (work in progress), February 2017.
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[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-06 (work in progress), March 2018.
[I-D.ietf-tokbind-https]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
tokbind-https-12 (work in progress), January 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-v2-form-post-response-mode]
Microsoft and Ping Identity, "OAuth 2.0 Form Post Response
Mode", April 2015, .
[oauth_security_cmu]
Carnegie Mellon University, Carnegie Mellon University,
Microsoft Research, Carnegie Mellon University, Carnegie
Mellon University, and Carnegie Mellon University, "OAuth
Demystified for Mobile Application Developers", November
2014.
[oauth_security_ubc]
University of British Columbia and University of British
Columbia, "The Devil is in the (Implementation) Details:
An Empirical Analysis of OAuth SSO Systems", October 2012,
.
[OpenID] NRI, Ping Identity, Microsoft, Google, and Salesforce,
"OpenID Connect Core 1.0 incorporating errata set 1", Nov
2014,
.
[owasp] "Open Web Application Security Project Home Page",
.
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[owasp_csrf]
"Cross-Site Request Forgery (CSRF) Prevention Cheat
Sheet", .
[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,
.
[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,
.
[webappsec-referrer-policy]
Google Inc. and Google Inc., "Referrer Policy", April
2017, .
Appendix A. Document History
[[ To be removed from the final specification ]]
-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
-03
o Added section on Access Token Leakage at Resource Server
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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 (editor)
YES.com AG
Email: torsten@lodderstedt.net
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Facebook
Email: isciurus@fb.com
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