Online criminals are scanning the internet and attacking Windows 2000 machines that haven't had a recent Windows Media Service patch installed, according to Symantec.
Symantec first spotted the attacks on Monday, saying that they are extremely limited. Symantec's detection network picked up just a handful of attempts, but the code used in the attacks is new and gives hackers a way to take over a system.
The attacks are the latest development in a troublesome patch for Microsoft. The company released its MS010-025 update on April 13, but was forced to reissue the patch two weeks later after discovering that its original patch didn't fix the problem.
The Metasploit open-source hacking toolkit has published code that exploits the flaw, but whoever is behind these attacks isn't using that software, Symantec Security Intelligence Manager Joshua Talbot, said. "Somebody's done some homework on their own and developed their own exploit," he said.
When the attack works - Symantec says that it often doesn't and simply causes the victim's computer to crash - the attacker uploads several password logging tools, and also enables remote desktop access.
Because the attack only works on Windows 2000 users who have not blocked the Windows Media Service's port 1755 at the firewall, the vast majority of Microsoft's customers have nothing to fear.
"It's interesting to know that someone out there is taking the time to write an exploit for this and then scan for uncommon configurations," Talbot said. "People should keep that in mind, even though this isn't necessarily a very high-profile issue."
Microsoft couldn't immediately be reached for comment about the attacks.