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Category:   OS (Microsoft)  >   Windows Help System Vendors:   Microsoft
Microsoft Windows Help System Bug in Processing Compiled HTML Help Files Lets Remote Users Execute Arbitrary Commands in the Local Computer Security Zone
SecurityTracker Alert ID:  1005343
SecurityTracker URL:  http://securitytracker.com/id/1005343
CVE Reference:   CVE-2002-0694   (Links to External Site)
Date:  Oct 3 2002
Impact:   Execution of arbitrary code via network, User access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  

Description:   A vulnerability was reported in the Microsoft Windows Help System. A remote user can execute arbitrary commands in the local computer security zone.

A vulnerability was reported in Windows HTML Help System related to the handling of compiled HTML Help files ('.chm' files) that contain shortcuts. A remote user can send a compiled help file via a web page or e-mail message such that the target user's HTML Help facility will execute it from the Temporary Internet Files folder and process it in the Local Computer security zone.

A remote user could cause an arbitrary shortcut to be executed with the privileges of the target user.

Impact:   A remote user can cause arbitrary commands to be executed on the target user's computer with the privileges of the target user.
Solution:   The vendor has released a patch:

For Windows 98 and Windows 98 SE:

http://www.microsoft.com/windows98/downloads/contents/WUCritical/q323255/default.asp

For Windows Me:

Use Windows Update.

For Windows NT 4.0:

http://www.microsoft.com/Downloads/Release.asp?ReleaseID=43308

For Windows NT 4.0, Terminal Server Edition:

http://www.microsoft.com/Downloads/Release.asp?ReleaseID=43308

For Windows 2000:

http://www.microsoft.com/Downloads/Release.asp?ReleaseID=40213

For Windows XP Home Edition and Professional Edition:

http://www.microsoft.com/Downloads/Release.asp?ReleaseID=41834

Microsoft notes that the patch requires that Internet Explorer 5.01, 5.5 or 6 be installed on the system. Also, the patch does not set the Kill Bit on the affected control (see the Vendor URL for more information on this).

According to Microsoft, the Window 98 patch can be installed on Windows 98 Gold. The Windows 98SE patch can be installed on Windows 98SE Gold. The Windows Me patch can be installed on Windows Me Gold. The Windows NT 4.0 patch can be installed on NT SP6a. The Windows NT 4.0 Terminal Server Edition patch can be installed on Windows NT 4.0 Terminal Server Edition SP6. The Windows 2000 patch can be installed on Windows 2000 SP1, SP2, or SP3. The patch for Windows XP can be installed on Windows XP Gold or SP1.

Microsoft plans to include this fix in Windows 2000 SP4 and Windows XP SP2.

This patch supersedes the one referenced in Microsoft Security Bulletin MS00-037.

Microsoft plans to issue Knowledge Base article Q323255 regarding this issue, to be available shortly on the Microsoft Online Support web site:

http://support.microsoft.com/?scid=fh;en-us;kbhowto

Vendor URL:  www.microsoft.com/technet/security/bulletin/MS02-055.asp (Links to External Site)
Cause:   Access control error, State error
Underlying OS:  Windows (Any)

Message History:   None.


 Source Message Contents

Subject:  Microsoft Security Bulletin MS02-055: Unchecked Buffer in Windows Help Facility Could Enable Code Execution (Q323255)


-----BEGIN PGP SIGNED MESSAGE-----

- ----------------------------------------------------------------------
Title:      Unchecked Buffer in Windows Help Facility Could 
            Enable Code Execution (Q323255)
Date:       02 October 2002 
Software:   Microsoft Windows 98 
            Microsoft Windows 98 Second Edition 
            Microsoft Windows Millennium Edition 
            Microsoft Windows NT 4.0 
            Microsoft Windows NT 4.0, Terminal Server Edition 
            Microsoft Windows 2000 
            Microsoft Windows XP
Impact:     Attacker could gain control over user's system
Max Risk:   Critical 
Bulletin:   MS02-055

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-055.asp.
- ----------------------------------------------------------------------

Issue:
======
The HTML Help facility in Windows includes an ActiveX control that
provides much of its functionality. One of the functions exposed via
the control contains an unchecked buffer, which could be exploited by
a web page hosted on an attacker's site or sent to a user as an HTML
mail. An attacker who successfully exploited the vulnerability would
be able to run code in the security context of the user, thereby
gaining the same privileges as the user on the system. 

A second vulnerability exists because of flaws associated with the
handling of compiled HTML Help (.chm) files that contain shortcuts.
Because shortcuts allow HTML Help files to take any desired action on
the system, only trusted HTML Help files should be allowed to use
them. Two flaws allow this restriction to be bypassed. First, the
HTML Help facility incorrectly determines the Security Zone in the
case where a web page or HTML mail delivers a .chm file to the
Temporary Internet Files folder and subsequently opens it. Instead of
handling the .chm file in the correct zone - the one associated with
the web page or HTML mail that delivered it - the HTML Help facility
incorrectly handles it in the Local Computer Zone, thereby
considering it trusted and allowing it to use shortcuts. This error
is compounded by the fact that the HTML Help facility doesn't
consider what folder the content resides in. Were it to do so, it
could recover from the first flaw, as content within the Temporary
Internet Folder is clearly not trusted, regardless of the Security
Zone it renders in. 

The attack scenario for this vulnerability would be complex, and
involves using an HTML mail to deliver a .chm file that contains a
shortcut, then making use of the flaws to open it and allow the
shortcut to execute. The shortcut would be able to perform any action
the user had privileges to perform on the system. 

Before deploying the patch, customers should familiarize themselves
with the caveats discussed in the FAQ and in the Caveats section
below.

Mitigating Factors:
====================
Buffer Overrun in HTML Help ActiveX Control: 
 - The HTML mail-based attack vector could not be exploited on 
   systems where Outlook 98 or Outlook 2000 were used in conjunction
   with the Outlook Email Security Update, or Outlook Express 6 or 
   Outlook 2002 were used in their default configurations. 
 - The vulnerability would convey only the user's privileges on 
   the system. Users whose accounts are configured to have few 
   privileges on the system would be at less risk than ones who 
   operate with administrative privileges. 

Code Execution via Compiled HTML Help File: 
 - The vulnerability could only be exploited if the attacker 
   were able to determine the exact location of the Temporary 
   Internet Files folder. By design, this should not be possible, and
   Microsoft is unaware of any means for doing so which has not 
   already been patched. 
 - The vulnerability would convey only the user's privileges on 
   the system. Users whose accounts are configured to have few 
   privileges on the system would be at less risk than ones who 
   operate with administrative privileges.

Risk Rating:
============
 - Internet systems: Moderate
 - Intranet systems: Moderate
 - Client systems: Critical

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms02-055.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - David Litchfield of Next Generation Security Software Ltd. 
   (http://www.nextgenss.com/)and Thor Larholm, Security Researcher, 
   PivX Solutions, LLC (http://www.pivx.com) for reporting the 
   Buffer Overrun in HTML Help ActiveX Control.

- ---------------------------------------------------------------------

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS 
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE 
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE
FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO
THE FOREGOING LIMITATION MAY NOT APPLY.

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