New worm exploits critical Windows bug

Attacks PCs vulnerable to flaw Microsoft fixed with emergency patch

A worm that exploits the bug Microsoft Corp. patched in an emergency update 11 days ago is actively attacking systems, several security companies and researchers said today.

The worm, which Symantec Corp. called Wecorl but was dubbed MS08-067.g by Kaspersky Lab and Microsoft itself, likely originated in China, said Kevin Haley, director of Symantec's security response team. It appears to target Chinese-language versions of Windows 2000, he noted.

Haley confirmed that the worm, which is different from the information-stealing Trojan horse that prompted Microsoft to issue the out-of-cycle patch on Oct. 23, is 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 Corp., 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 [firewall], 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 that's likely the case.

Symantec assessed the worm as a "very low" risk, although the company kept its ThreatCon global threat rating at Level 2, because Microsoft issued an emergency patch. "It doesn't appear to be very widespread, although that could change, of course," said Haley.

As counterintuitive 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 [the 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.

Copyright © 2008 IDG Communications, Inc.

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