The company is breaking with its monthly patch cycle to fix a flaw that cybercrooks have been using to attack Windows PCs via Internet Explorer. Malicious software can be loaded, unbeknownst to the user, onto a vulnerable Windows PC when the user clicks on a malicious link on a Web site or in an e-mail message.  

The vulnerability, first reported last week, lies in a Windows component called "vgx.dll." This component is meant to support Vector Markup Language documents in the operating system. VML is used for high-quality vector graphics on the Web and is used for viewing pages in the IE browser that is part of Windows. Microsoft deems the flaw "critical," its highest severity rating.

"An attacker could exploit the vulnerability by constructing a specially crafted Web page or HTML e-mail that could potentially allow remote code execution if a user visited the Web page or viewed the message," Microsoft said in security bulletin MS06-055. E-mail messages that use HTML, or HyperText Markup Language, look like a Web page.

The vulnerability does not apply to IE 7, the upcoming version of IE that is available right now in a pre-release form, Microsoft said.

Microsoft typically releases fixes each second Tuesday of the month, which has become known as Patch Tuesday. The last time the software maker rushed out a fix was in January, when another image-related flaw in IE was being used to compromise Windows PCs through malicious Web sites.

Security experts had pushed Microsoft to rush out a fix for the VML flaw. A group of security professionals even crafted an unofficial fix for the problem, which was released on Friday.

"Exploitation has already eclipsed that of the last out-of-cycle patch," said Ken Dunham, director of the rapid response team at VeriSign's iDefense. "It appears that there were several million domains that were redirecting to malicious VML sites."

Microsoft's security update is being pushed out to Windows users via Automatic Updates and will also be available on Windows Update.