Microsoft has released an out-of-band security update, MS08-078, to fix a vulnerability in its Internet Explorer Web browser that's being actively exploited.

"At this time, we are aware only of attacks that attempt to use this vulnerability against Windows Internet Explorer 7," said Christopher Budd, Microsoft security response communications lead, in an e-mailed statement. "Our investigation of these attacks so far has verified that they are not successful against customers who have applied the security update. MS08-078 has a maximum severity rating of Critical for all versions of Internet Explorer."

Nonetheless, Microsoft lists Internet Explorer 5.01, 6, and 7 as affected software in its Security Bulletin. It also says separately, in the FAQ section, that Internet Explorer 8 Beta 2 is affected.

The vulnerability can be exploited through JavaScript code posted on malicious Web sites. Internet Explorer users may be redirected to these sites through hacked legitimate sites. If the malicious code is successful, it silently downloads malware onto the victim's computer.

Microsoft typically releases software patches, referred to as Security Bulletins, on the second Tuesday of every month. When critical vulnerabilities emerge and are actively exploited, Microsoft often issues a patch as soon as it's ready.

The last such out-of-band patch, Microsoft Security Bulletin MS08-067, was released on Oct. 23. It addressed a vulnerability in Windows Server service that affected all currently supported versions of Windows. That vulnerability allowed an attacker to take over affected computers remotely.

When Microsoft issued its out-of-band patch in October, it cited the vulnerability's potential "wormability" -- meaning the hole could be exploited on a massive scale using self-copying malware -- as a reason for its action.

But MS08-078 isn't wormable. "[That] shows that the wormability of a vulnerability is no longer a good indicator of the seriousness of a threat and that these Web-based threats are now much more dangerous than network worms," said Roel Schouwenberg, senior antivirus researcher for Kaspersky Lab, Americas, in an e-mailed statement.

Indeed, Microsoft security researchers estimated that as many as 1 in 500 users of Internet Explorer could have been exposed to malware attempting to exploit the flaw.

"The browser flaw had been disclosed roughly one week ago as a zero-day vulnerability, and active exploits have been around the Internet for that timeframe as well," Qualys CTO Wolfgang Kandek said in an e-mailed statement. "The workarounds provided by Microsoft were very technical and quite cumbersome to implement, making it imperative for Microsoft to release a fix as quickly as possible."

Kandek suggests that Microsoft is at a disadvantage in updating Internet Explorer because its browser doesn't have a built-in update mechanism like other browser makers. Mozilla, for instance, just released Firefox 3.05 to Firefox users through its auto-update system.

Microsoft is urging users of IE to test and deploy this update as soon as possible.