The company's security team issued an advisory Monday around 1 p.m. ET acknowledging reports of in-the-wild attacks and providing more information about who is vulnerable.

Earlier today, security researchers at a pair of Danish firms had announced that thousands of legitimate Web sites hacked over the weekend were conducting drive-by attacks on IE users with an exploit of a critical unpatched vulnerability in Windows' DirectShow, part of DirectX.

"A browse-and-get-owned attack vector exists," Chengyun Chu, of the Microsoft Security Response Center's engineering team, said in a blog post this afternoon. "A user needs to be lured to navigate to a malicious Web site or a compromised legitimate Web site to be affected ... [but] no further user interaction is needed."

Users running IE6 or IE7 on Windows XP and Windows Server 2003 are vulnerable to the drive-bys attacks, Microsoft said. Vista and Server 2008 are not at risk, however, nor are people running IE8, Microsoft's newest browser.

Although Microsoft promised it would patch the bug, a company spokesman declined to say whether that patch would be ready by July 14, the next regularly-scheduled security update release day.

To protect at-risk PCs in the meantime, the company urged users to set 45 "kill bits" in the flawed ActiveX control that contains the vulnerability. That ActiveX control, Microsoft admitted, wasn't intended to be used by IE. "We identified that none of the ActiveX Control Objects hosted by msvidctl.dll are meant to be used in IE," said Chu. "Therefore, we recommend to kill-bit all of these controls as a defense-in-depth practice. The side effect is minimal."

Setting ActiveX kill bits can be dangerous, as it involves editing the Windows registry. "If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system," Microsoft warned in its advisory. "Use Registry Editor at your own risk."

An easier way to set the kill bits is to run a custom downloadable automated tool that Microsoft's crafted. The company offered a similar tool as a workaround for the other DirectShow bug it acknowledged in late May.

The new tool can be downloaded from Microsoft's support site.

An earlier report in Computerworld credited the Danish company CSIS Security Group with first publicizing the DirectShow vulnerability. Actually, Chinese security forums and antivirus firms, including Kingsoft ( Google Translate translation), were the first to document the bug.