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TUCoPS :: Cisco :: b06-3894.htm

Windows VPN Client Local Privilege Escalation Vulnerability
Cisco Security Advisory: Windows VPN Client Local Privilege Escalation Vulnerability
Cisco Security Advisory: Windows VPN Client Local Privilege Escalation Vulnerability

Hash: SHA1

Cisco Security Advisory: 
Windows VPN Client Local Privilege Escalation Vulnerability

Document ID: 70332

Advisory ID: cisco-sa-20060524-vpnclient 

Revision 2.0

Last Updated 2006 July 27 2000 UTC (GMT)

For Public Release 2006 May 24 1600 UTC (GMT)

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    Affected Products
    Software Versions and Fixes
    Obtaining Fixed Software
    Exploitation and Public Announcements
    Status of This Notice: FINAL
    Revision History
    Cisco Security Procedures

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The Cisco VPN Client for Windows is affected by a local privilege
escalation vulnerability that allows non-privileged users to gain
administrative privileges.

A user needs to authenticate and start an interactive Windows session
to be able to exploit this vulnerability.

Cisco has made free software available to address this vulnerability
for affected customers. There is a workaround available to mitigate the
effects of the vulnerability.

This advisory is posted at 

Affected Products
Vulnerable Products

The following versions of the Cisco VPN Client for Windows (excluding
Windows 9x users) are affected:

  * 2.x
  * 3.x
  * 4.0.x
  * 4.6.x
  * 4.7.x with the exception of version (see below in
    Products Confirmed Not Vulnerable.)
  * 4.8.00.x

Products Confirmed Not Vulnerable

All other versions of Cisco VPN Client that are based on an operating
system other than Microsoft Windows are not affected by this
vulnerability. These include:

  * Any version of the Cisco VPN Client for Solaris
  * Any version of the Cisco VPN Client for Linux
  * Any version of the Cisco VPN Client for Macintosh (MacOS X and
    MacOS Classic)

Cisco VPN Client for Windows version 4.8.01.x and later are not
affected by this vulnerability.

In addition, due to a regression, version of the Cisco VPN
Client for Windows is not affected, even though other 4.7.x are
affected. Users running version of the Cisco VPN Client for
Windows do not need to upgrade to version 4.8.01 or later to be
protected from this vulnerability.

No other Cisco products are currently known to be affected by this

Determining the Cisco VPN Client Version

In order to determine which version of the Cisco VPN Client is running
on the Microsoft Windows machine, open the Cisco VPN Client graphical
user interface by selecting "Programs->Cisco Systems VPN Client->VPN
Client" from the Start menu and then select the option "About VPN
Client..." from the "Help" menu. This will display a dialog box
containing text similar to "Cisco Systems VPN Client Version 4.0.5(Rel)
". Please note that the location of the "Cisco Systems VPN Client"
folder mentioned above in Windows' Start menu is where the program is
installed by default; your system administrator may have chosen to use
a different name or location.

The Cisco VPN Client is a software solution for the Sun Solaris, Apple
MacOS Classic and MacOS X, Linux and Microsoft Windows operating
systems that allows users running these operating systems to establish
IPSec VPN tunnels to Cisco VPN-capable devices like Cisco IOS routers,
the PIX Security Appliance, the VPN 3000 Series Concentrators, and the
ASA 5500 Series Adaptive Security Appliances.

A vulnerability in the Cisco VPN Client for Windows graphical user
interface (GUI), also known as the "VPN client dialer", can be
exploited to elevate user privileges and obtain LocalSystem-equivalent

The vulnerability occurs when the Start Before Logon (SBL) feature is
enabled on the Cisco VPN Client. This feature can be enabled by
unprivileged users from the Cisco VPN Client GUI. Once this feature is
enabled, the user will see a Cisco VPN Client GUI in the Windows logon
screen. When the user accesses Cisco VPN Client help in this GUI with
the F1 key, the Cisco VPN Client spawns a web browser process. Since no
user has logged in yet, the Cisco VPN Client is running with
LocalSystem privileges, and therefore so will the spawned web browser
process. After the logon sequence is completed, the user has a web
browser window on the desktop, running with LocalSystem privileges.

Additional information on the SBL feature can be found at: 

This issue is not related to any known issues in Microsoft Windows

This vulnerability is documented by Cisco Bug ID CSCsd79265 

For information about local system level privileges, please refer to:

  * The LocalSystem Account 

Successful exploitation of the vulnerability may result in a normal
user or attacker gaining full control of the system, evading any
controls put in place by the Windows system administrator.

Software Versions and Fixes
When considering software upgrades, also consult and any subsequent advisories to 
determine exposure and a complete upgrade solution.

In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current
hardware and software configurations will continue to be supported
properly by the new release. If the information is not clear, contact
the Cisco Technical Assistance Center ("TAC") or your contracted
maintenance provider for assistance.

This vulnerability is fixed in version of the Cisco VPN
Client for Windows, which can be downloaded from the following

For information on how to upgrade the Cisco VPN Client, including
automatic upgrades, please refer to the following chapter of the VPN
Client Administrator Guide: Updating VPN Client Software.

A workaround for this vulnerability is to prevent the LocalSystem
account (also known as "SYSTEM") from launching the application
associated with files with the ".html" extension. After applying this
workaround, pressing the F1 key on the Cisco VPN Client GUI in the
Windows logon screen has no effect.

This can be accomplished by attaching an Access Control List (ACL)
entry to the Windows registry keys 
HKEY_LOCAL_MACHINE\Software\Classes\.html and HKEY_CLASSES_ROOT\.html 
so that the NT-AUTHORITY\SYSTEM account is denied all access to that key.

The following example uses the tool RegDACL (Copyright(c) 1999-2001
Frank Heyne Software), available from , to apply the Windows 
registry ACL mentioned above:

  C:\>regdacl HKLM\Software\Classes\.html /DGS:F

  RegDACL 5.1 - Permissions Manager for Registry keys for Windows NT 4 and above
Copyright (c) 1999-2001 Frank Heyne Software ( 
  This program is Freeware, use it on your own risk!

  Denying F access for predefined group "System"
   - adding new ACCESS DENY entry
   - removing existing entry

  C:\>regdacl HKCR\.html /DGS:F

  RegDACL 5.1 - Permissions Manager for Registry keys for Windows NT 4 and above
Copyright (c) 1999-2001 Frank Heyne Software ( 
  This program is Freeware, use it on your own risk!

  Denying F access for predefined group "System"
   - adding new ACCESS DENY entry
   - removing existing entry


Another tool that can be used to accomplish the same is the Regini.exe
utility that comes with several versions of the Windows Resource Kit.
For more information on how to use Regini.exe, please visit the
following Microsoft Knowledge Base article:

  * How to Use Regini.exe to Set Permissions on Registry Keys 

Manually editing the Windows registry with RegEdit.exe will also work,
but this method does not scale when the operation needs to be performed
on a large number of machines.

While Cisco has tested the above workaround in Cisco's test
environment, the effectiveness and impact of any workaround is
dependent on each customer's particular environment. Customers electing
to apply any workaround (rather than upgrading to an updated version of
the Cisco VPN Client) are encouraged to test such workaround thoroughly
to ensure, among other things, that it does not negatively impact any
other applications that may rely on applicable functionality (e.g., the
ability of the LocalSystem account to launch HTML files from any

Obtaining Fixed Software
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should
consult their maintenance provider or check the software for feature
set compatibility and known issues specific to their environment.

Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise
using such software upgrades, customers agree to be bound by the terms
of Cisco's software license terms found at, or as otherwise 
set forth at Downloads at 

Do not contact either "" or "" 
for software upgrades.

Customers with Service Contracts

Customers with contracts should obtain upgraded software through their
regular update channels. For most customers, this means that upgrades
should be obtained through the Software Center on Cisco's worldwide
website at 

Customers using Third-party Support Organizations

Customers whose Cisco products are provided or maintained through prior
or existing agreement with third-party support organizations such as
Cisco Partners, authorized resellers, or service providers should
contact that support organization for guidance and assistance with the
appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic
behavior, and organizational mission. Due to the variety of affected
products and releases, customers should consult with their service
provider or support organization to ensure any applied workaround or
fix is the most appropriate for use in the intended network before it
is deployed.

Customers without Service Contracts

Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party vendors
but are unsuccessful at obtaining fixed software through their point of
sale should get their upgrades by contacting the Cisco Technical
Assistance Center (TAC). TAC contacts are as follows.

  * +1 800 553 2447 (toll free from within North America)
  * +1 408 526 7209 (toll call from anywhere in the world)
* e-mail: 

Have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades
for non-contract customers must be requested through the TAC.

Refer to 
for additional TAC contact information, including special localized
telephone numbers and instructions and e-mail addresses for use in
various languages.

Exploitation and Public Announcements
The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

This vulnerability was independently reported to Cisco by Andrew
Christensen from FortConsult and by Johan Ronkainen. Cisco would like
to thank them for working with us towards coordinated disclosure of
this vulnerability.

Applying an ACL to the HKCR\.html registry key in order to prevent the
SYSTEM account from being able to launch a web browser was suggested to
Cisco Systems by Johan Ronkainen.

FortConsult's advisory is available at the following location: 

Status of This Notice: FINAL

A stand-alone copy or Paraphrase of the text of this document that
omits the distribution URL in the following section is an uncontrolled
copy, and may lack important information or contain factual errors.

This advisory is posted on Cisco's worldwide website at 

In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the
following e-mail and Usenet news recipients.


Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on mailing
lists or newsgroups. Users concerned about this problem are encouraged
to check the above URL for any updates.

Revision History
|              |              |   * Added full details about the          |
|              |              |     vulnerability (see the Details        |
|              |              |     section.)                             |
|              |              |   * Provided possible workarounds (see    |
| Revision 2.0 | 2006-July-27 |     the Workarounds section).             |
|              |              |   * Added a link to FortConsult's         |
|              |              |     advisory                              |
|              |              |   * Changed "Status of this Notice" from  |
|              |              |     INTERIM to FINAL.                     |
|              |              |                                           |
| Revision 1.0 | 2006-May-24  | Initial 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 
This includes instructions for press inquiries regarding Cisco security 
notices. All Cisco security advisories are available at 

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

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Updated: Jul 27, 2006                                Document ID: 70332

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Version: GnuPG v1.4.2.2 (GNU/Linux)


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