A security firm has discovered one of the first security flaws to directly affect Windows Vista, a bug that it claims allows local users to escalate their privileges.

The flaw involves Windows' system for managing user security levels, User Account Control (UAC), which was introduced with Vista. UAC is designed to limit the damage that can be caused by mass attacks such as worms by giving standard users limited privileges, a practice common with other operating systems.

Combined with a remote vulnerability, the newly discovered bug could essentially render UAC useless, escalating standard user privileges to system-level access, according to eEye.

"A flaw exists within Windows Vista that allows local privilege escalation to System," eEye said in a note on its website. The company said it reported the bug to Microsoft on Jan. 19, and plans to disclose further details once a fix is available.

According to eEye co-founder Marc Maiffret, the flaw is similar to a buffer overflow.

Microsoft said in a statement it is aware of the report and is investigating. "The company is not aware of any public discussion of the report itself," Microsoft stated.

UAC is by far the most visible change in Vista's security system, to the point where some have criticized it as too intrusive. At the same time, researchers have already begun picking holes in the system.

What's more, Microsoft recently made it clear that it doesn't consider UAC a security feature, since it has deliberately left particular holes in the system for ease of use. That means bugs in UAC aren't security flaws, Microsoft says.

"Neither UAC elevations nor Protected Mode IE define new Windows security boundaries," wrote Mark Russinovich, a Technical Fellow in Microsoft's Platform and Services Division, in a blog post earlier this month. "Because elevations and ILs (Integrity Levels) don't define a security boundary, potential avenues of attack, regardless of ease or scope, are not security bugs."

Instead of being a security barrier, UAC is intended "to get us to a world where everyone runs as standard user by default and all software is written with that assumption," Russinovich wrote.