A bug that Microsoft patched in a security release nearly two weeks ago, is actively being exploited by a worm, said security researchers.

The worm, which Symantec labeled 'Wecorl' but was dubbed 'MS08-067.g by Kaspersky Lab and Microsoft itself, likely originated in China, said Kevin Haley, a director with Symantec's security response team.

"It may have come out of China," said Haley, who added that it appeared to target Chinese language versions of Windows 2000.

Haley confirmed that the worm is both different from the information-stealing Trojan horse that prompted Microsoft to issue the out-of-cycle patch on October 23, and circulating in the wild.

Other researchers echoed Symantec's take that the worm installs multiple components on victimised PCs, including a Trojan downloader and rootkit code to mask it from security software. F-Secure, for example, identified the former as 'Trojan-Dropper.Win32.Agent.yhi' and the rootkit bits as 'Rootkit.Win32.KernelBot.dg'.

According to Haley, if the worm manages to infect a Windows PC, it also tries to attack all the machines on the same subnet. "If it can get behind the [fire]wall, then it can infect other systems," Haley said.

"That circumvents the firewall mitigation that Microsoft noted," said Andrew Storms, director of security operations at nCircle Network Security.

"Enterprises typically have laptops configured to be location aware so when they're on the company network, parts of the firewall are disabled, or port 139 is allowed from known IP addresses."

In the security bulletin it released two weeks ago, Microsoft said that "standard default firewall configurations can help protect network resources from attacks that originate outside the enterprise perimeter".

NEXT PAGE: Why the worm is a good thing

A bug that Microsoft patched in a security release nearly two weeks ago , is actively being exploited by a worm, said security researchers.

F-Secure said that the just-released worm is based on the exploit code that had been posted online last week. nCircle's Storms agreed that's likely.

Symantec rated the worm as a 'Very Low' threat, although it maintained its ThreatCon, an all-around indicator of internet security, at '2' because Microsoft issued an emergency patch.

"It doesn't appear to be very widespread, although that could change, of course," said Haley.

As counter-intuitive as it sounds, Storms said that the appearance of a worm is actually a good thing.

"Evidence that we're finding and detecting it means we're in a better situation than we were earlier," he argued.
"If it had gone undetected and unfound [it would have meant] that enterprises didn't have any defense-in-depth. But because we're finding it, that means we have signatures for it."

Storms urged users who had not installed the MS08-067 update to do so immediately.

"The worm may not have many legs, but you should get ahead of the game and deploy now," he said.