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The absence of an official patch from Microsoft for a known and widely exploited Internet Explorer flaw has spawned a cottage industry for homegrown fixes.

Security experts say a browser add-on masquerading as a third-party IE patch is being sold on the Internet, but users are being warned to steer clear of software from untrusted sources.

Word of the homegrown fixes come as malicious hackers have begun hijacking ad servers to use as a delivery mechanism for the Bofra/IFrame exploit.

Microsoft Corp. has not yet patched the IFrame buffer overflow vulnerability and, from all indications, a fix won’t be available until Dec. 14, when Redmond is next scheduled to roll out security updates.

The software giant said it had received reports of the homegrown IE patch making the rounds but warned that users should avoid installing or running software from an unknown source.

A Microsoft spokeswoman told that while the add-on may have been developed with good intentions, the company cannot ensure the quality, reliability or safety of the fix.

“[We can’t] assure support for customers who experience problems when installing this add-on. As a security best practice, Microsoft would like to remind customers that they should only install or run software from a source that they trust,” the spokeswoman said.

Gartner security analyst John Pescatore echoed Microsoft’s warning. “Those third-party fixes are more likely to be malicious than helpful,” he said in an interview. “Even someone with good intentions can’t possibly create and test an IE fix properly. It may patch the immediate vulnerability but cause another set of problems with compatibility.”

Marcus Sachs, director of the SANS ISC (Internet Storm Center), said third-party patches had been used in the past by hackers to load spyware on unsuspecting victims.

“If you are going to update your browser, it should be a patch from Microsoft and not someplace else,” Sachs said. “If you use a homemade patch, you could be putting yourself at the mercy of an attacker.”

“I’m not saying the authors of that browser add-on are malicious hackers. But when people use third-party patches, they create opportunities for attackers to take advantage of consumer fear,” he added.

Columnist Jim Rapoza said Microsoft is making it easy to upgrade—to another browser. Click here to read more.

Microsoft said it was actively investigating the new Bofra/IFrame exploits, which are being described as a “criminal attack.”

“Microsoft’s early analysis indicates that the IFrame vulnerability only exists for Internet Explorer 6 and Internet Explorer 6 Service Pack 1,” the company said in a statement.

“It does not exist in Windows XP SP2 or Windows Server 2003; therefore customers who are running Windows XP SP2 or Windows Server 2003 should not be at risk from this attack.”

“[We are] taking this vulnerability very seriously; accordingly an update to correct the vulnerability is currently in development. We will release the security update when the development and testing process is complete, and the update is found to effectively correct the vulnerability.”

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