Security researchers Tuesday said they'd discredited Microsoft's claim that the year's first critical Windows vulnerability would be "difficult and unlikely" to be exploited by attackers.

On Tuesday, Immunity updated a working exploit for the TCP/IP flaw spelled out Jan. 8 in Microsoft's MS08-001 security bulletin, and posted a Flash demonstration of the attack on its Web site. The exploit, which was released to customers of its CANVAS penetration testing software -- but is not available to the public -- was a revised version of code first issued two weeks ago.

"This demonstrates conclusively that the MS08-001 IGMPv3 vulnerability is highly exploitable," said Dave Aitel, Immunity's chief technology officer, in a message to his Dailydave security mailing list.

Aitel's assertion challenged Microsoft's earlier assessment that "there are a number of factors that make exploitation of this issue difficult and unlikely in real-world conditions."

Immunity did acknowledge that its newest exploit was not 100% reliable, however.

Other security companies reacted to the revamped attack code and Flash proof by issuing new alerts. Symantec Corp., for instance, sent a new warning to customers of its DeepSight threat network. "The exploit demonstrates remote code execution," noted Symantec. "The exploit works against Windows XP SP2 English Default [and shows] two Windows XP SP2 computers on a local subnet with firewall enabled being compromised."

It urged users who have not already deployed the patches Microsoft issued Jan. 8 to do immediately.

Previously, Aitel had called out the IGMP (Internet Group Management Protocol) vulnerability as a potential blockbuster for 2008. In a detailed analysis of the flaw and its exploitation, Symantec agreed that the reward to hackers would be large even if replicating Immunity's work might be tough.

"Although exploitation of a remote kernel flaw is considered quite difficult, over the past few years numerous papers have been released on the subject," the Symantec analysis noted. "Examples of exploits successfully leveraging such flaws have also been released to the public. Therefore, we assume that this issue will be exploited in the wild to execute arbitrary code. Deploy patches immediately."

Successful attacks by the Immunity exploit -- and any similar to it developed by others -- allows arbitrary code to execute within the context of the Windows kernel, said Symantec, an especially egregious scenario for Windows Vista.

"This is especially critical on Vista, due to its enhanced kernel security mechanisms," said Symantec. "A local user, even an admin, may have difficultly introducing unsigned code into the kernel, but in this case, it can be done remotely without any authentication whatsoever.

"This vulnerability presents an opportunity to not only execute arbitrary code on the system, but also to install backdoors and other malicious tools as well as a rootkit, which may normally be more difficult with a typical remote userland vulnerability."

In its Jan. 8 MS08-001 bulletin, Microsoft ranked the IGMP flaw as "Critical" for Windows XP SP2, Windows Vista, Windows Small Business Server and Windows Home Server. On other versions, including Windows Server 2003, the bug rated an "Important" label.