Microsoft today said that hackers are exploiting a critical, but unpatched, vulnerability in Internet Explorer 8 (IE8) and Internet Explorer 9 (IE9), and that its engineers are working on an update to plug the hole.

As it often does, the company downplayed the threat.

"There are only reports of a limited number of targeted attacks specifically directed at Internet Explorer 8 and 9, although the issue could potentially affect all supported versions," Dustin Childs, a manager in the Trustworthy Computing group and its usual spokesman, said in a blog post Tuesday morning.

"We are actively working to develop a security update to address this issue," Childs added.

According to Childs and the security advisory Microsoft also published today, the vulnerability affects all supported versions of IE, from the 12-year-old IE6 to the not-yet-officially-released IE11, the browser that will accompany Windows 8.1 when it ships Oct. 18.

"There is no escaping this one," said Andrew Storms, director of DevOps at cloud security vendor CloudPassage, referring to the bug affecting all versions of Microsoft's browser. "IE zero-days are never a good thing, especially when they affect every version," Storms added.

Although Microsoft's advisory did not put it in these terms, the vulnerability can be exploited using classic "drive-by" attack tactics. That means hackers need only lure victims running IE to malicious sites -- or legitimate websites that have previously been compromised and loaded with attack code -- to hijack their browser and plant malware on their Windows PCs.