Microsoft issued a formal security advisory late Tuesday on a reported zero-day flaw in Windows Vista and Windows Server 2008. However, the software maker also said that the flaw does not affect the final version of Windows 7, contrary to earlier reports.

"Microsoft is investigating new public reports of a possible vulnerability in Microsoft Server Message Block (SMB) implementation," Microsoft said in the advisory. "We are not aware of attacks that try to use the reported vulnerabilities or of customer impact at this time."

The flaw could allow an attacker to gain control of a system, although Microsoft said that "most attempts to exploit this vulnerability will cause an affected system to stop responding and restart."

The software maker said it is working with security software partners to provide information that can be used to create protections. Once its investigation is wrapped up, Microsoft said it will take action, which could include releasing a patch during its next monthly cycle or doing an "out-of-band" release, if necessary. Tuesday was Microsoft's monthly release for patches, which included five critical Windows updates addressing eight vulnerabilities.

The software maker said the latest issue affects the "release candidate" version of Windows 7, but not the final version that was completed in July. Also, the recently completed Windows Server 2008 R2 is not vulnerable, Microsoft said, nor are the earlier Windows XP and Windows 2000 operating systems.

Microsoft is already dealing with a separate, still unpatched flaw reported last week. Attacks have already been seen based on that vulnerability. Microsoft has taken issue with the fact that that flaw, like the latest one, was reported publicly as opposed to being privately disclosed to Microsoft, giving the company time to patch it.