Update: Microsoft tests fix for IE bug as exploits appear

IDG News Service - A recently identified security hole in the Internet Explorer browser is being exploited, and Microsoft Corp. hopes to include a fix for it in its April 11 patch or possibly sooner, 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 Saturday. Today, 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, which was first identified last week, allows hackers to entice Web surfers to visit Web sites where malicious code can be automatically run on the visitor's computer (see "Exploit now publicly available for unpatched IE flaw"). It exploits a vulnerability in the way that IE renders HTML and affects Windows 2000, Windows XP and Windows Server 2003, Toulouse said. Users of IE 7 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 being tested and is expected to be released as part of Microsoft's monthly security update, which is scheduled for April 11, he said. However, Microsoft could release the fix earlier if the threat grows, he said.

The Web sites found by Websense 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 exploiting the vulnerability, Toulouse said.

Web surfers can avoid the attack by turning off Active Scripting, Microsoft said.

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