The U.S.-CERT is warning users of a vulnerability in Microsoft's Windows libraries that could let a hacker run malicious code on compromised computers.

The security organization posted an online advisory about the buffer overflow vulnerability in the MFC42 and MFC71 libraries offered natively in Windows.

"Specifically, the vulnerability exists due to the "FindFile" function failing to properly validate the length of user supplied input," U.S.-CERT researchers warned. "By passing an overly long argument to the "FindFile" function, an attacker may be able to cause a buffer overflow and execute arbitrary code on an affected system."

Jonathan Sarba, a researcher with the GoodFellas Security Research Team, discovered the flaw. He noted in an online posting that any application that uses the libraries and allows users to manipulate the arguments being passed to the API may be affected.

"The FindFile method allocates memory for the buffer," wrote Sarba. "The buffer then is used to store the contents of the first argument of the function without checking if the argument actually fits in the allocated buffer. This data is in turn used to start a search. Both the unicode and ascii versions of the library use a very similar function and have the same bug, the only real difference is the size of the allocated buffer. The unicode version allocates 592 bytes, and the ascii version, 320 bytes."

The Goodfellas Security site said there is no known workaround for the vulnerability.

In an e-mail response, Christopher Budd, security program manager for Microsoft, told InformationWeek that the company is investigating reports of a possible vulnerability in Microsoft Windows.

"We're currently unaware of any attacks trying to use the claimed vulnerability or of customer impact," he added. "We will take steps to determine how customers can protect themselves should we confirm the vulnerability. Once we're done investigating, we will take appropriate action to help protect customers. This may include providing a security update through the monthly release process, an out-of-cycle update or additional guidance to help customers protect themselves."

Secunia, a Danish research firm, noted in an advisory that the vulnerability is confirmed on a fully-patched Windows XP SP2 including mfc42.dll version 6.2.4131.0 and mfc42u.dll version 6.2.8071.0.

For a workaround, according to Secunia, users would need to restrict access to applications allowing user-controlled input to be passed to the vulnerable function. And applications using the vulnerable library should check the length of the user input before passing it to the affected function.