---

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.