Microsoft has released an out-of-band security update to address a critical flaw that could allow a remote attacker to take over Windows computers without any user interaction.

"This security update resolves a vulnerability in the Server service that affects all currently supported versions of Windows," said Christopher Budd, a MSRC security program manager, in a blog post. "Windows XP and older versions are rated as 'Critical' while Windows Vista and newer versions are rated as 'Important.' Because the vulnerability is potentially wormable on those older versions of Windows, we're encouraging customers to test and deploy the update as soon as possible."

In Microsoft Security Bulletin MS08-067, the company warns that the vulnerability resides in its Server service. It could allow an attacker to execute arbitrary code using a specially crafted remote procedure call request.

"This was completely unannounced to anyone in the industry," said Jason Miller, security and data team manager at Shavlik Technologies, a patch management company. "It's is not unheard of for Microsoft, but it's pretty uncommon."

The last time Microsoft released a patch outside of its regularly scheduled cycle, the second Tuesday of every month, was in April 2007.

Miller said that the reason Microsoft issued this emergency patch is because it has seen attacks based on this vulnerability being used online.

As Budd explained in his blog post, "We discovered this vulnerability as part of our research into a limited series of targeted malware attacks against Windows XP systems that we discovered about two weeks ago through our ongoing monitoring. As we investigated these attacks we found they were utilizing a new vulnerability and initiated our Software Security Incident Response Process (SSIRP)."

Budd identified the malware used in these attacks as "TrojanSpy:Win32/Gimmiv.A" and "TrojanSpy:Win32/Gimmiv.A.dll." He said that Microsoft has released corresponding signatures through the Microsoft Malware Protection Center.

"This vulnerability is pretty nasty," said Miller. "It affects every version of the Windows operating system. Attackers don't need to have any credentials or access to the machine. They don't have to trick the user into doing anything."

His biggest fear, he said, is that a worm will be developed to take over vulnerable machines en masse. And he fully expects that to happen. "You're talking about a vulnerability that does not need user interaction," he said. "That's a gold mine if you're trying to build a botnet."

Echoing Budd's call to action, Miller advises everyone to apply the patch immediately. "Normally I'll tell people you should test the patch," he said. "On something like this, I would definitely just deploy it as soon as possible."