Open Authentication Protocol T. Lodderstedt, Ed.
Internet-Draft Deutsche Telekom AG
Intended status: Informational J. Bradley
Expires: May 16, 2017 Ping Identity
A. Labunets
Facebook
November 12, 2016
OAuth Security Topics
draft-lodderstedt-oauth-security-topics-00
Abstract
This draft gives a comprehensive overview on open OAuth security
topics. It is intended to serve as a tool for the OAuth working
group to systematically address these open security topics,
recommending mitigations, and potentially also defining OAuth
extensions needed to cope with the respective security threats. This
draft will potentially become a BCP over time.
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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This Internet-Draft will expire on May 16, 2017.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. OAuth Credentials Leakage . . . . . . . . . . . . . . . . . . 3
2.1. Redirect URI validation of authorization requests . . . . 3
2.1.1. Authorization Code Grant . . . . . . . . . . . . . . 3
2.1.2. Implicit Grant . . . . . . . . . . . . . . . . . . . 4
2.1.3. Countermeasure: exact redirect URI matching . . . . . 6
2.2. Authorization code leakage via referrer headers . . . . . 7
2.2.1. Countermeasures . . . . . . . . . . . . . . . . . . . 7
2.3. Code in browser history (TBD) . . . . . . . . . . . . . . 8
2.4. Access token in browser history (TBD) . . . . . . . . . . 8
2.5. Access token on bad resource servers (TBD) . . . . . . . 8
2.6. Mix-Up (TBD) . . . . . . . . . . . . . . . . . . . . . . 9
3. OAuth Credentials Injection . . . . . . . . . . . . . . . . . 9
3.1. Code Injection . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Proposed Counter Measures . . . . . . . . . . . . . . 11
3.1.2. Access Token Injection (TBD) . . . . . . . . . . . . 13
3.1.3. XSRF (TBD) . . . . . . . . . . . . . . . . . . . . . 13
4. Other Attacks . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Normative References . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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
protection and, as foundation of OpenID Connect, identity providing.
o OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (XSRF, referrer
header). Although most of these threats are discussed in RFC 6819
[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.
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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 both RFC 6749
[RFC6749] and RFC 6819 [RFC6819].
This remainder of the document is organized as follows: The next
section describes various scenarios how OAuth credentials (namely
access tokens and authorization codes) may be disclosed to attackers
and proposes countermeasures. Afterwards, the document discusses
attacks possible with captured credential and how they may be
prevented. The last sections discuss additional threats.
2. OAuth Credentials Leakage
2.1. Redirect URI validation of authorization requests
The following implementation issue has been observed: Some
authorization servers allow clients to register redirect URI patterns
instead of complete redirect URIs. In those cases, the authorization
servers, at runtime, match 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 usually via the client as open redirector in conjunction with
fragment handling (implicit grant) carrying the response including
the respective OAuth credentials.
2.1.1. Authorization Code Grant
For a public client using the grant type code, an attack would look
as follows:
Let's assume the pattern "https://*.example.com/*" had been
registered for the client "s6BhdRkqt3". This pattern allows redirect
URI from any host residing in the domain example.com. So if an
attacker manager to establish a host or subdomain in "example.com" he
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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.client.example.com%2Fcb HTTP/1.1
Host: server.example.com
(3) 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.
(4) If the user does not recognize the attack, the code is issued
and directly sent to the attacker's client.
(5) 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 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. This and other kinds of injections are covered in
Section OAuth Credentials Injection.
2.1.2. Implicit Grant
The attack described above for grant type authorization code works
similarly for the implicit grant. 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, it is possible to conduct an attack utilizing the way
user agents treat fragments in case of redirects. 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). In
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this attack this behavior is combined with the client as an open
redirector in order to get access to access tokens. This allows
circumvention even of strict redirect URI patterns.
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
(3) 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&...
(4) 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
(5) Since the redirector at example.com does not include a fragment
in the Location header, the user agent will re-attach the
original fragment
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"#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and will
navigate to the following URL:
https://client.evil.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...
(6) The attacker's page at client.evil.com can access the fragment
and obtain the access token.
2.1.3. Countermeasure: exact redirect URI matching
Since the cause of the implementation and management issues is the
complexity of the pattern matching, this document proposes to
recommend general use of exact redirect URI matching instead, i.e.
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 whether this feature is used in deployed reality
today.
o The working group might also consider this change as a step
towards improved interoperability for OAuth implementations since
RFC 6749 is somehow vague on redirect URI validation. There is
especially no rule for pattern matching. So one may 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?
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.
Note: Does redirect uri validation solve any problem for native
apps? Effective against impersonation when used in conjunction
with claimed HTTPS redirect URIs only.
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Additional recommendations:
o It is also advisable that the domains on which callbacks are
hosted should not expose open redirectors (see respective
section).
o As a further recommendation, clients may drop fragments via
intermediary URL with fix fragment (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: authenticate client
using digital signatures (JAR? https://tools.ietf.org/html/draft-
ietf-oauth-jwsreq-09), ...
2.2. Authorization code leakage via referrer headers
The section above already discussed use of the referrer header for
one kind of attack to obtain OAuth credentials. It is also 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 <img> (f.e. if this is blog web site
etc.): the implication is obviously the same - loading this content
by browser results in leaking referrer with a code.
2.2.1. Countermeasures
There are some means to prevent leakage as described above:
o Use of the HTML link attribute rel="noreferrer" (Chrome
52.0.2743.116, FF 49.0.1, Edge 38.14393.0.0, IE/Win10)
o Use of the "referrer" meta link attribute (possible values e.g.
noreferrer, origin, ...) (cf. https://w3c.github.io/webappsec-
referrer-policy/ - work in progress (seems Google, Chrome and Edge
support it))
o Redirect to intermediate page (sanitize history) before sending
user agent to other pages
Note: double check redirect/referrer header behavior
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o Use form post mode instead of redirect for authorization response
Note: There shouldn't be a referer header when loading HTTP content
from a HTTPS -loaded page (e.g. help/faq pages)
Note: This kind of attack is not applicable to the implicit grant
since fragments are not be included in referrer headers (cf.
https://tools.ietf.org/html/rfc7231#section-5.5.2)
2.3. Code in browser history (TBD)
When browser navigates to "client.com/redirection_endpoint?code=abcd"
as a result of a redirect from a provider's authorization endpoint.
Proposal for counter-measures: code is one time use, has limited
duration, is bound to client id/secret (confidential clients only)
2.4. Access token in browser history (TBD)
When 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
When browser navigates to client.com/
redirection_endpoint#access_token=abcef as a result of a redirect
from a provider's authorization endpoint.
Proposal: replace implicit flow with postmessage communication
2.5. Access token on bad resource servers (TBD)
In the beginning, the basic assumption of OAuth 2.0 was that the
OAuth client is implemented for and tightly bound to a certain
deployment. It therefore knows the URLs of the authorization and
resource servers upfront, at development/deployment time. So well-
known URLs to resource servers serve as trust anchor. The validation
whether the client talks to a legitimate resource server is based on
TLS server authentication (see [RFC6819], Section 4.5.4).
As OAuth clients nowadays more and more bind dynamically at runtime
to authorization and resource servers, there need to be alternative
solutions to ensure, client do not deliver access tokens to bad
resource servers.
...
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Potential mitigations:
o PoP
o Token Binding
o AS-provided Meta Data
o ...
2.6. Mix-Up (TBD)
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 counter-measures
is given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-
mitigation-01.
Potential mitigations:
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 (so requires OpenID Connect) carries client id and issuer
o register AS-specific redirect URIs, bind transaction to AS
o ...
3. OAuth Credentials Injection
Credential injection means an attacker somehow obtained a valid OAuth
credential (code or token) and is able to utilize this to impersonate
the legitimate resource owner or to cause a victim to access
resources under the attacker's control (XSRF).
3.1. 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. Example are:
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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 and/or
o The attacker wants to access certain functions in this particular
client. As an example, the attacker potentially wants to
impersonate his victim in a certain app.
o Another example could be that access to the authorization and
resource servers is some how limited to networks, the attackers is
unable to access directly.
How does an attack look like?
(1) The attacker obtains an authorization code by executing any of
the attacks described above (OAuth Credentials Leakage).
(2) It performs an 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.
(6) If all checks succeed, the authorization server issues access
and other tokens to the client.
(7) The attacker just impersonated the victim.
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.
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
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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:
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.
3.1.1. Proposed Counter Measures
The general proposal is to bind every particular authorization code
to a certain client 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:
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Nonce OpenID Connect's existing "nonce" parameter is 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
hold of the user agent state on the victims device, where he
has stolen the respective authorization code.
pro:
- existing feature, used in the wild
con:
- OAuth does not have an ID Token - would need to push that
down the stack
State It has been discussed in the security workshop in December to
use the OAuth state value much similar in the way as
described above. In the case of the state value, the idea is
to add a further parameter state to the code exchange
request. The authorization server 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. Note: a variant of this
solution would be send a hash of the state (in order to
prevent bulky requests and DoS).
pro:
- use existing concept
con:
- state needs to fulfil certain requirements (one time use,
complexity)
- new parameter means normative spec change
PKCE Basically, the PKCE challenge/verifier 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.
pro:
- existing and deployed OAuth feature
con:
- currently used and recommended for native apps, not web
apps
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Token Binding Code must be bind to UA-AS and UA-Client legs -
requires further data (extension to response) to manifest
binding id for particular code.
pro:
- highly secure
con:
- highly sophisticated, requires browser support, will it
work for native apps?
per instance client id/secret ...
Note on pre-warmed secrets: An attacker can circumvent the counter-
measures described above if he is able to create the respective
secret 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, for native apps with claimed URLs.
What about other native apps? Treat nonce or PKCE challenge as
replay detection tokens (needs to ensure cluster-wide one-time use)?
3.1.2. Access Token Injection (TBD)
Note: An attacker in possession of an access token can access any
resources the access token gives him the permission to. This kind of
attacks simply illustrates the fact that bearer tokens utilized by
OAuth are reusable similar to passwords unless they are protected by
further means.
(where do we treat access token replay/use at the resource server?
https://tools.ietf.org/html/rfc6819#section-4.6.4 has some text about
it but is it sufficient?)
The attack described in this section is about injecting a stolen
access token into a legitimate client on a device under the
adversaries control. The attacker wants to impersonate a victim and
cannot use his own client, since he wants to access certain functions
in this particular client.
Proposal: token binding, hybrid flow+nonce(OIDC), other
cryptographical binding between access token and user agent instance
3.1.3. XSRF (TBD)
injection of code or access token on a victim's device (e.g. to cause
client to access resources under the attacker's control)
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mitigation: XSRF tokens (one time use) w/ user agent binding (cf.
https://www.owasp.org/index.php/
CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)
4. Other Attacks
Using the AS as Open Redirector - error handling AS (redirects)
(draft-ietf-oauth-closing-redirectors-00)
Using the Client as Open Redirector
redirect via status code 307 - use 302
5. Acknowledgements
We would like to thank ... 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. 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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc6819>.
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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,
<http://www.rfc-editor.org/info/rfc7231>.
Authors' Addresses
Torsten Lodderstedt (editor)
Deutsche Telekom AG
Email: torsten@lodderstedt.net
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Facebook
Email: isciurus@fb.com
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