In a , Microsoft acknowledged that a bug in SMB (Server Message Block), a Microsoft-made network file- and print-sharing protocol, could be used by attackers to cripple Windows 7 and Windows Server 2008 R2 machines.

The zero-day vulnerability was last Wednesday, when he revealed the bug and posted proof-of-concept attack code to the Full Disclosure security mailing list and his blog. According to Gaffie, exploiting the flaw crashes Windows 7 and Server 2008 R2 systems so thoroughly that the only recourse is to manually power off the computers.

At the time, Microsoft only said it was investigating Gaffie's reports.

Then on Friday, it took the next step and issued the advisory. "Microsoft is aware of public, detailed exploit code that would cause a system to stop functioning or become unreliable," Dave Forstrom, a spokesman for Microsoft security group, said in an e-mail. "The company is not aware of attacks to exploit the reported vulnerability at this time."

Forstrom echoed Gaffie's comments earlier in the week that while an exploit could incapacitate a PC, the vulnerability could not be used by hackers to install malicious code on a Windows 7 system.