Those websites are either hosting exploit code or are redirecting Internet users to sites with bad code, Websense's blog reported Monday.

The number of websites engineered to exploit the problem has jumped considerably since the vulnerability was publicly disclosed by Microsoft on March 29. It will likely continue to rise until patches are applied across corporate and consumer PCs, said Ross Paul, senior product manager for Websense.

Hackers are hoping to catch some of the millions of unpatched machines.

"What we've seen is that exploits tend to be used as long as they are effective," Paul said.

Last week, Microsoft broke from its regular patching routine and issued an off-schedule fix due to the danger of the vulnerability, which occurs in the way Windows processes .ani or Animated Cursor files, which allow websites to replace the regular cursor with cartoonish alternatives.

The flaw affects nearly all versions of Microsoft's Windows OS and is the third zero-day flaw that Microsoft has patched out of schedule since January 2006.

Companies tend to patch their machines on fixed schedules and may not immediately apply a patch when it's released, Paul said. Home users may automatically receive the patch if they are using Windows XP Service Pack 2, but users of older Windows OSes will not.

That's especially dangerous since the .ani problem doesn't require user interaction for a machine to be infected, said Graham Cluley, senior technology consultant at Sophos. Merely viewing a website engineered to exploit the vulnerability with an unpatched machine can result in an infection.

As a result, security analysts are generally recommending to apply the patch, even though Microsoft said Friday it was fixing compatibility problems with some applications.

"We are recommending this is a patch you really need to install now," Cluley said.

Websense said that attackers from Eastern Europe and China appear to be at the heart of the efforts. Groups in the Asia-Pacific region and China are exploiting the vulnerability, mainly on machines located in Asia, in order to gain credentials for popular online games such as Lineage, Websense said.

A second group in Eastern Europe, which has been known to use other vulnernabilities in Microsoft's software to install malicious software on machines, "have also added the .ani attacks to their arsenal," Websense said. Those attacks are directed at servers and users in the United States.