A security vendor has found vulnerability with a 'medium' security rating in Microsoft's new operating system Windows Vista.

The flaw, which is similar to a buffer overflow problem, is a privilege escalation bug, according to Marc Maiffret, co-founder and chief hacking officer of eEye Digital Security, an endpoint security company based in Aliso Viejo, Calif.

The flaw, which eEye first reported as an Upcoming Advisory, is one of the first to be found in the brand new operating system. Earlier this month, Microsoft patched a flaw in Windows Defender, which is a built-in spyware and security component in different applications, including Windows XP and Vista. Maiffret points out that this new flaw is in the Vista operating system itself, not in a component that has been used in various programs.

eEye researches found the vulnerability on Jan. 9 and reported it to Microsoft on Jan. 19. Vista wasn't released for retail until the end of January.

The vulnerability enables regular users to grab more power on the system.

"A main security feature added to Vista is that regular users have a lower level of privileges," says Maiffret. "They have fewer privileges in Vista than they did in Windows XP. When regular users are running the operating system, they have regular user-level access, but with this vulnerability, you can elevate yourself to system-level access. Any normal user can do anything they want to the system."

Maiffret says they gave it a "medium" security rating because it doesn't enable a remote user to control the system. But he also says it wouldn't take much to elevate it.

"If it was coupled with a virus or a different remote vulnerability, it would be a lot more serious," he adds. "Viruses are very prevalent and there are plenty of other vulnerabilities you can couple it with. In a real world context, it's high because there are a lot of other things you can couple it with to make it pretty nasty. On its own, though, it's only medium."

A spokesman with Microsoft said researchers still are investigating the vulnerability.

"Microsoft is aware of a responsibly disclosed report indicating a possible vulnerability in Windows Vista," said a Microsoft spokesman in a statement e-mailed to InformationWeek. "The company is not aware of any public discussion of the report itself. It is also unaware of any attacks attempting to use the reported vulnerability or of customer impact at this time, and will continue to investigate the public report to help provide additional guidance for customers as necessary."