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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.

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

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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.
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