Microsoft fixes critical IE flaw

Microsoft Corp. has now fixed a widely reported flaw in its Internet Explorer (IE) browser that had been used by attackers over the past few weeks to take over the PCs of unsuspecting users. The flaw was one of four IE bugs fixed Tuesday in Microsoft’s regularly scheduled software update.

Although attacks based on the vulnerability have not been widespread, it is important that IE users now install the patch, said Neel Mehta, team lead of Internet Security Systems Inc.’s X-Force group. “It’s not of epic proportions,” he said. “But isolated attackers here and there have used it to install malware.”

Security experts had known about the flaw since May, but on Nov. 21 hackers with a U.K. organization called Computer Terrorism Ltd. posted sample code that showed it to be much more serious than originally thought. Within days that sample code was adapted and being used by attackers, prompting many security experts to erroneously predict that Microsoft would rush a patch ahead of its December update.

The bug concerns the way IE processes the “Window()” function in JavaScript, a popular scripting language used by Web developers to make their sites more dynamic. It affects IE users on Windows XP, Windows 2000 and Windows 98. In order to exploit this problem, attackers must first trick users into visiting a maliciously encoded Web site, which has helped prevent the bug from being more widely used.

Microsoft fixed this problem, along with the other three IE bugs, in one of two security updates, released Tuesday. More details on the IE fixes can be found in the MS05-054 Security Bulletin . This update is rated “critical” by Microsoft.

A second update, assigned Microsoft’s less severe security rating of “important,” fixes a problem in the Windows 2000 kernel. That update can be found at this Web site . This bug could help an attacker to circumvent Microsoft’s user privileges mechanism and perform unauthorized tasks on a PC.

Typically, this flaw could not be exploited remotely, as it requires that the attacker gain access to the targeted computer’s keyboard, said Steve Manzuik, security product manager with the company that discovered the bug, eEye Digital Security Inc. Its advisory is at thisWeb site.

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