The bug, which concerns the way IE processes Web pages using the createTextRange() method, is now being exploited by attackers on hundreds of malicious Web sites (see "Update: Microsoft tests fix for IE bug as exploits appear"). Users who might be tricked into visiting these Web sites could have unauthorized software installed on their computers, security experts warn.

Though Microsoft has described these attacks as "limited" in scope, the problem is being taken seriously by the software giant because the exploits can be used to seize control of a user's machine.

"We're working day and night on development of a cumulative security update for Internet Explorer that addresses the vulnerability," wrote Stephen Toulouse, head of Microsoft's Security Response Center, in a Saturday blog posting.

That patch is expected to be released as part of an April 11 security update, although the software could come earlier if the threat grows, Toulouse said.

The possibility that it could be more than two weeks before this bug is fixed prompted eEye to release the patch, said Marc Maiffret, the security company's chief hacking officer. "That's a long time to leave several million Windows users without any sort of protection."

Microsoft says that users can avoid the attack by disabling Active Scripting in their browsers, but this is not a viable option for the many users who use sites that employ scripting, Maiffret said.

EEye's patch, which is available free of charge, will automatically remove itself when Microsoft's official patch is delivered.

This not the first time security researchers have rushed to patch IE ahead of Microsoft. In late December, Ilfak Guilfanov, a developer with Liege Belgium's DataRescue SA, wrote a widely-distributed patch that fixed a similarly critical bug in Internet Explorer.