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Title: Flaw in Windows WM_TIMER Message Handling Could Enable
Privilege Elevation (328310)
Date: 11 December 2002
Software: Microsoft Windows NT 4.0, Windows 2000, and Windows XP
Impact: Privilege elevation
Max Risk: Important
Bulletin: MS02-071

Microsoft encourages customers to review the Security Bulletins 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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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 Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-071.asp
  for information on obtaining this patch.

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