Flaw in Windows WM_TIMER Message Handling

August 1, 2007 – 6:09 pm | by Beni | 58 views

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A security vulnerability exists in the WM_TIMER message handling in Windows NT, 2000 and XP. The flaw could give an attacker, who had the ability to log onto a system interactively, complete control over the system.

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.