"It's better than [the vulnerability used by] Conficker," Roger Thompson, chief research officer at AVG, said.
"It exposes the whole world, and can be exploited through the firewall. That's better than Conficker, which mostly did its damage once it got inside a network."
Conficker, the worm that exploded into prominence in January when it infected millions of machines, exploiting an already-patched bug in Windows that Microsoft had thought dire enough to fix outside its usual update schedule.
The worm hijacked a large number of PCs - estimates ranged as high as 12 million at one point - and then assembled them into a massive botnet able to spread malware, plant fake antivirus software or distribute huge amounts of spam.
"I have no doubt that the really bad guys are bustling to get this [new vulnerability] into their exploit toolkits," said Thompson.
"For the Conficker people, this could be the next thing. They waited until they had a really good exploit, then combined that with some smart strategies. So I wouldn't be surprised if they picked up on this."
The vulnerability Thompson's worried about is in the Microsoft Video Controller ActiveX Library, or the 'msvidctl.dll' file, an ActiveX control that can be accessed using Internet Explorer (IE).
Although the bug has been used by hackers since at least June 9, it only made it into the public eye this week, when several security companies, including firms in both China and Denmark, reported that thousands of compromised sites were serving up exploits.
Microsoft acknowledged the vulnerability in a security advisory this week and said it would produce a patch and provided an automated tool to disable the ActiveX control by setting nearly three-dozen "kill bits" in the Windows registry.
"This is a good exploit with a big lump of infectable people," said Thompson.
One reason why the bug is an excellent choice for hackers is that it hasn't been patched. When Conficker first appeared, the flaw it exploited had already been patched by Microsoft. It turned out, however, that there were plenty of PCs that had not been updated with the fix.
Thompson wouldn't hazard a guess as to whether Microsoft would be able to craft a fix in time to add it to the patches slated for delivery next Tuesday, July's regularly-scheduled update day. "But I'm fairly confident that they're trying very hard," he said.
Attack code is readily available, Thompson said, meaning that attackers not yet abusing the bug don't have to figure out an exploit of their own. "If the people who infect banner ads use this before there's a patch, then watch out," he warned.
Today, Microsoft admitted that researchers at IBM's ISS X-Force had reported the vulnerability in 2008, but did not name the date.
"It's not the end of the world or anything," said Thompson. "But I won't be surprised to see Version 2.0 of Conficker with this. It seems custom-made for them."
Systems running Windows XP or Windows Server 2003 are vulnerable to current attacks through IE6 and IE7. Windows Vista and Windows 7 are not at risk; nor are users running IE8 or other browsers, such as Mozilla's Firefox and Google's Chrome.
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See also: Massive botnet targets US govt websites