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UNIRAS Brief - 405/06 - Cisco Security Response: Document ID: 70469 - WebVPN Cross-Site Scripting Vulnerability



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   UNIRAS (UK Govt CERT) Briefing Notice - 405/06 dated 14.06.06  Time: 14:25  
  UNIRAS is part of NISCC (National Infrastructure Security Co-ordination Centre)
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  UNIRAS material is also available from its website at www.uniras.gov.uk and
         Information about NISCC is available from www.niscc.gov.uk
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Title
=====

Cisco Security Response: Document ID: 70469 - WebVPN Cross-Site Scripting Vulnerability

Detail
====== 

This is the response of the Cisco Product Security Incident Response
Team (PSIRT) to the statements made by Michal Zalewski
<lcamtuf@xxxxxxxxxxxx> in his message entitled "SSL VPNs and security",


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Cisco Security Response: WebVPN Cross-Site Scripting Vulnerability

Document ID: 70469

http://www.cisco.com/warp/public/707/cisco-sr-20060613-webvpn-xss.shtml

Revision 1.0

For Public Release 2006 June 13 2200 UTC (GMT)

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Contents
========

    Cisco Response
    Additional Information
    Revision History
    Cisco Security Procedures

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Cisco Response
==============

This is the response of the Cisco Product Security Incident Response
Team (PSIRT) to the statements made by Michal Zalewski
<lcamtuf@xxxxxxxxxxxx> in his message entitled "SSL VPNs and security",
which he posted to the Bugtraq and full-disclosure mailing lists on
June 8, 2006.

The original emails are available at 
http://www.securityfocus.com/archive/1/436479/30/0/threaded (Bugtraq 
archive) and at 
http://archives.neohapsis.com/archives/fulldisclosure/2006-06/0094.html
(full-disclosure archive).

In his posting to Bugtraq and full-disclosure, Michal Zalewski mentions
a Cross-Site Scripting (XSS) vulnerability that exists in "Cisco SSL
VPN."

Cisco confirms the existence of an XSS vulnerability in the clientless
mode of the WebVPN feature of the Cisco VPN 3000 Series Concentrators
and the Cisco ASA 5500 Series Adaptive Security Appliances (ASA).

Please note that the technology affected by the XSS vulnerability
covered in Michal Zalewski's message is what Cisco calls "WebVPN
clientless mode" and not "WebVPN full-network-access mode", which is a
different encrypted tunnel technology that is more similar to IPSec and
that requires the installation of the Cisco SSL VPN Client.

For a description of the differences between the clientless and
full-network-access modes of Cisco WebVPN please refer to:

http://www.cisco.com/en/US/products/ps6635/products_data_sheet0900aecd80405e25.html

Cisco is tracking this issue using the following Cisco bug IDs:

  * CSCsd81095 - VPN3k vulnerable to cross-site scripting when using WebVPN
  * CSCse48193 - ASA vulnerable to cross-site scripting when using WebVPN

Additional Information
======================

The vulnerability happens when certain error conditions occur and the
device tries to make the user aware of the problem. Under these error
conditions the WebVPN feature presents the user with an HTML page that
indicates the error and the URL the user was trying to access.

Because the pages displayed also output the URL that caused the
problem, it is possible to embed scripting code in the URL that then
will be executed by the user's web browser.

In his posting to Bugtraq and full-disclosure, Michal Zalewski provides
the example "https://<vpnhost>/webvpn/dnserror.html?domain=<u>foo</u>".
In this example, the vulnerability is triggered when the device
displays a DNS resolution problem ("dnserror.html"). The other possible
page where this problem can happen is "connecterror.html", which is
displayed when the device has trouble connecting to the URL specified
by the user.

Cisco bugs CSCsd81095 and CSCse48193 will address the issue for all 
WebVPN error conditions.

To exploit these issues an attacker would have to entice authenticated
users to follow a specially crafted, malicious URL. A successful attack
would result in the execution of arbitrary script code in the user's
web browser.

As Michal Zalewski points out, SSL VPN technologies have their own set
of challenges. The whitepaper on SSL VPN Security that is mentioned in
the original posting to Bugtraq and full-disclosure is a good resource
on this topic that attempts to address the nature of these challenges
and increase awareness. This whitepaper is located at:

http://www.cisco.com/web/about/security/intelligence/05_08_SSL-VPN-Security.html

This issue was independently reported to Cisco by Michal Zalewski and
two other customers. We would like to thank them for bringing this
issue to our attention.

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE
INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT
YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS
DOCUMENT AT ANY TIME.

Revision History
================

+----------------------------------------+
| Revision |              | Initial      |
| 1.0      | 2006-June-13 | public       |
|          |              | release.     |
+----------------------------------------+

Cisco Security Procedures
=========================

Complete information on reporting security vulnerabilities in Cisco
products, obtaining assistance with security incidents, and registering
to receive security information from Cisco, is available on Cisco's
worldwide website at 
http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html.
This includes instructions for press inquiries regarding Cisco security 
notices. All Cisco security advisories are available at 
http://www.cisco.com/go/psirt/.

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All contents are Copyright 1992-2006 Cisco Systems, Inc. All rights
reserved. 

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Updated: Jun 13, 2006                                Document ID: 70469

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UNIRAS wishes to acknowledge the contributions of Cisco for the information 
contained in this Briefing. 
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This Briefing contains the information released by the original author. Some 
of the information may have changed since it was released. If the vulnerability 
affects you, it may be prudent to retrieve the advisory from the site of the
original source to ensure that you receive the most current information concerning 
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<End of UNIRAS Briefing>

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