Security assessment vendor, Immunity, has discovered a way to exploit a recently disclosed bug in Microsoft's Windows operating system, and researchers are concerned that a new worm attack, similar to last August's Zotob outbreak, may be imminent.

The bug, which Microsoft patched on Tuesday, primarily affects users of Windows 2000, the same operating system that was targeted in the Zotob attacks. It concerns the Microsoft Distributed Transaction Coordinator (MSDTC), a component of the operating system that is commonly used by database software to help manage transactions.

Microsoft rated the vulnerability as critical for Windows 2000 users and said that, if exploited, it could be used to seize control of an unpatched system.

Late Tuesday, just hours after Microsoft disclosed the vulnerability, Immunity began distributing the exploit to members of the company's Immunity Partners program, which is comprised primarily of security researchers, the company's CEO, Justine Aitel, said.

The code would be included in the next release of the company's Canvas security assessment software, scheduled for release on November 1, she said.

"Right now we have a proof of concept for that particular vulnerability," Aitel said. "The vulnerability can be exploited."

Immunity's proof of concept release makes it more likely that hackers would begin targeting the same bug, team leader of the X-Force research team with Internet Security Systems (ISS), Neel Mehta, said.

"The fact that they created an exploit for it will spur other hackers to go ahead and do the same, knowing that it's possible," he said.

Based on its own research, ISS claimed that the bug might enable a fairly widespread Windows 2000 worm, Mehta said. "It can be very reliably exploited, and it has the potential to be another worm," he said.

Microsoft executives were not available to comment on this matter, but the company released a statement in which it said that it was unaware of any active attacks that used this exploit code.

Though Microsoft is encouraging all Windows 2000 users to install its software patch, security experts claim the problem may be more acute for Windows Server 2000 customers than it is for users of the Windows 2000 Pro client software. This is because the MSDTC service is turned on by default on the server software, but turned off by default on the client, according to Microsoft.

Though consumer users have largely moved on to Windows XP, which is not considered to be nearly as vulnerable to this attack, Windows 2000 is still widely used in corporate environments. And as the Zotob attacks proved two months ago, attacks that focus on the older system can still be fairly disruptive.

Zotob exploited a vulnerability in the Windows Plug-and-Play system and variations of the worm disrupted hundreds of thousands of Windows 2000 machines at companies such as American Express and Cable News Network (CNN).

More details on the MSDTC bug, including Microsoft's patch, can be found in Microsoft Security Bulletin MS05-051: http://www.microsoft.com/technet/security/bulletin/MS05-051.mspx

Microsoft did not respond to requests for comment on this story.