For the third time in the last 90 days, Microsoft Corp. has warned that hackers are exploiting an unpatched critical vulnerability in its software.

Late Thursday, Microsoft issued a security advisory that said malicious hackers were already using attack code that leveraged a bug in DirectX, a Windows subsystem crucial to games and used when streaming video from Web sites.

Hackers are using malicious QuickTime files -- QuickTime is rival Apple Inc.'s default video format -- to hijack PCs, Microsoft said. "The vulnerability could allow remote code execution if [the] user opened a specially crafted QuickTime media file," the company said in the advisory. "Microsoft is aware of limited, active attacks that use this exploit code."

According to Christopher Budd, a spokesman for the Microsoft Security Response Center, QuickTime itself is not flawed. Instead, the QuickTime parser in DirectShow, a component of DirectX, contains the bug. "An attacker would try and exploit the vulnerability by crafting a specially formed video file and then posting it on a website or sending it as an attachment in e-mail,," Budd said in an entry on the MSRC blog.

Because the bug is in DirectShow, any browser using a plug-in that relies on DirectShow is also vulnerable.

DirectX 7, 8 and 9 in Windows 2000, XP and Server 2003 are at risk, Budd said, but Vista, Server 2008 and Windows 7 are not. "Our investigation has shown that the vulnerable code was removed as part of our work building Windows Vista," Budd said.

Until a patch is available, users can protect their PCs by disabling QuickTime parsing. To do that requires editing the Windows registry, normally a task most users shy from, but Microsoft has automated the workaround. "We've gone ahead and built a 'Fix it' that implements the 'Disable the parsing of QuickTime content in quartz.dll' registry change," Budd said. "We have also built a 'Fix it' that will undo the workaround automatically."

"Fix it" is the name Microsoft's given to downloadable tools it has added to some of the support documents on its Knowledge Base (KB), the online collection of troubleshooting instructions. Microsoft debuted "Fix it" buttons late last year.

The DirectX bug is the third since late February that Microsoft has acknowledged in a security advisory. The other two involved Excel and PowerPoint.

Budd said that Microsoft is working on a patch for the problem, but he didn't give a time table. It's unlikely the company will deliver a fix on June 9, the next scheduled date for its security updates. Microsoft patched the Excel bug, which it disclosed in February, two months later; it patched the PowerPoint vulnerability about five weeks after it warned users.