Update: Microsoft tests fix for IE bug as exploits appear

A recently identified Internet Explorer security hole is now being exploited and Microsoft hopes to include a fix for it in its April 11 patch or maybe even earlier, according to a posting on a Microsoft blog.

So far, attacks are limited in scope, Stephen Toulouse, head of Microsoft's Security Response Center, wrote on the center's blog on Saturday. On Monday however, Web security company Websense Inc. said it has found hundreds of sites taking advantage of the weakness and that the number of such sites is rapidly growing.

The vulnerability, first identified last week before actually being exploited, allows hackers to entice Web surfers to visit Web sites where malicious code can then be automatically run on the visitor's computer. It exploits a vulnerability in the way that IE renders HTML (Hypertext Markup Language) and affects Windows 2000, Windows XP and Windows Server 2003, Toulouse said. Users of IE7 Beta 2, the most recent version of IE, won't be affected by the bug, Microsoft said.

"We're working day and night on development of a cumulative security update for Internet Explorer that addresses the vulnerability," Toulouse wrote. The update is currently being tested and is expected to be released as part of Microsoft's already scheduled April 11 security update, he said. However, Microsoft could release the fix earlier if the threat grows, he said.

The Web sites that Websense have found download different types of malicious code including back doors and Trojan horses onto Web surfers' machines. Back doors bypass regular authentication requirements, enabling unauthorized access to computers, and Trojan horses are malicious programs disguised as legitimate applications.

Microsoft is working with industry partners and law enforcement to remove Web sites that are already exploiting the vulnerability, Toulouse said.

Web surfers can avoid the attack by turning off Active Scripting, Microsoft said. Toulouse's blog posting as well as additional tips for how to prevent being targeted by such attacks can be found here.

The vulnerability is the third such IE bug to surface within the last two weeks and is considered the most serious because it is relatively easy to exploit.