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Title:      Flaw in Windows WM_TIMER Message Handling Could Enable
            Privilege Elevation (328310)
Released:   11 December 2002
Revised:    07 February 2003 (version 2.0)
Software:   Microsoft Windows NT 4.0 
            Microsoft Windows NT 4.0, Terminal Server Edition 
            Microsoft Windows 2000 
            Microsoft Windows XP
Impact:     Privilege elevation
Max Risk:   Important  
Bulletin:   MS02-071

Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS02-071.asp
http://www.microsoft.com/security/security_bulletins/ms02-071.asp
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Reason for Revision:
====================
Subsequent to the release of this bulletin it was determined
that the patch for Microsoft Windows NT 4.0 machines introduced
an error that could, under certain configurations, cause NT 4.0
to fail. Microsoft has investigated this issue and is releasing
an updated patch for Windows NT 4.0. The bulletin has been updated
to include the new download links for the NT 4.0 patch. Customers
who have installed the patch on Microsoft Windows 2000 and Windows
XP are unaffected by this error.

Issue:
======
Windows messages provide a way for interactive processes to react
to user events (e.g., keystrokes or mouse movements) and communicate
with other interactive processes. One such message, WM_TIMER, is sent
at the expiration of a timer, and can be used to cause a process to
execute a timer callback function. A security vulnerability results
because it's possible for one process in the interactive desktop to
use a WM_TIMER message to cause another process to execute a callback
function at the address of its choice, even if the second process did
not set a timer. If that second process had higher privileges than
the first, this would provide the first process with a way of
exercising them.
 
By default, several of the processes running in the interactive
desktop do so with LocalSystem privileges. As a result, an attacker
who had the ability to log onto a system interactively could
potentially run a program that would levy a WM_TIMER request upon
such a process, causing it to take any action the attacker specified.
This would give the attacker complete control over the system. 

In addition to addressing this vulnerability, the patch also
makes changes to several processes that run on the interactive
desktop with high privileges. Although none of these would, in
the absence of the TM_TIMER vulnerability, enable an attacker to
gain privileges on the system, we have included them in the patch
to make the services more robust.

Mitigating Factors:
====================
 - An attacker would need valid logon credentials to exploit the
   vulnerability. It could not be exploited remotely. 
 - Properly secured servers would be at little risk from this
   vulnerability. Standard best practices recommend only allowing
   trusted administrators to log onto such systems interactively;
   without such privileges, an attacker could not exploit the
   vulnerability.

Risk Rating:
============
 - Important

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletins at
   http://www.microsoft.com/technet/security/bulletin/ms02-071.asp
   http://www.microsoft.com/security/security_bulletins/ms02-071.asp
   for information on obtaining this patch.


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