Microsoft has warned that hackers are attacking Internet Explorer by exploiting an unpatched bug in DirectX, the second such attack in six weeks. Microsoft's security team has issued an advisory acknowledging reports of in-the-wild attacks and providing more information about who is vulnerable. The bug was first identified by Chinese security forums and antivirus firms, including Kingsoft.

"A browse-and-get-owned attack vector exists," said Chengyun Chu, of the Microsoft Security Response Center's engineering team, in a blog post. "A user needs to be lured to navigate to a malicious website or a compromised legitimate webssite 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-by 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 14 July, 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.

Users running a non-Microsoft browser, such as Mozilla's Firefox or Google's Chrome, are safe from attack.