A worm that exploits the bug just patched by Microsoft is now actively attacking systems, according to security researchers.

The worm, which Symantec called "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 23 October, and circulating in the wild.

Other researchers echoed Symantec's take that the worm installs multiple components on victimized PCs, including a Trojan downloader and rootkit code to mask it from security software. Helsinki-based 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 Inc. "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."

Within days of the emergency patch, hackers had published working attack code on the Internet.

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.