Hackers cannot use the exploit to remotely compromise a PC, however, as it requires local access, a fact that Microsoft stressed. "Because this is a local elevation-of-privilege issue, it requires attackers to be already able to execute code on a targeted machine," said Bryant.

"On its own, this bug does not allow remote code execution, but does enable non-administrator accounts to execute code as if they were an administrator," added Wisniewski.

Although many Windows XP users, especially consumers and those in very small businesses, run the OS via administrator accounts, Microsoft added UAC to Vista and later operating systems as one way to limit user privileges, and thus malware's access to the PC.

Attackers would have to combine the exploit with other malicious code that takes advantage of another vulnerability on the machine -- not necessarily one in Windows, but in any commonly-installed application, such as Adobe Reader, for example -- to hijack a PC and bypass UAC.

"This exploit allows malware that has already been dropped on the system to bypass [UAC] and get the full control of the system," said Prevx researcher in an entry on that security company's blog Thursday.