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Skype Ltd. Tuesday patched a critical vulnerability that forced it to dump several features from its VoIP and chat software to prevent attackers from hijacking Windows PCs.

In a security advisory issued today, Skype said it fixed the underlying flaw publicized by Israeli researcher Aviv Raff nearly three weeks ago. The vulnerability, which Raff called a cross-zone scripting bug, could be exploited with rigged video files that leveraged a security flaw in how Skype rendered HTML.

At root, Raff said, was the fact that Skype, which uses IE's Web control to handle internal and external HTML pages, ran the control in a low-security mode. "Skype is running this Web control in Local Zone ... [and] the HTML pages in a not-locked Local Zone mode," Raff said in mid-January.

After Raff and others posted proof-of-concepts, Skype temporarily plugged the hole by first ditching connections to Dailymotion, one of the Internet-calling service's video-sharing partners. Six days later, it severed the line to Metacafe, another partner, when Raff pointed out an even more serious exploit.

More Problems

Last week, Raff spotted yet another Skype problem, this time in the SkypeFind command, which lets users recommend businesses to others and write reviews of those businesses. At the time, Raff said if a hacker crafted a review that included a malicious script, any user who viewed the business via the SkypeFind command would have his PC shanghaied.

Raff traced all three cross-zone scripting vulnerabilities to Skype's poor security model, and said a fix was relatively simple. "To lock the Local Zone, they basically need to change one registry value," he said last Thursday.

Skype hinted that it had done just that. "The core vulnerability has been fixed by setting IE control security context to Internet Zone," said the company in Tuesday's security alert. It also claimed that all three of the exploits -- the two related to Dailymotion and Metacafe and the third connected to SkypeFind -- had been quashed by the patched Skype now available for download.

Raff, however, wasn't willing to give it the thumbs up, at least not yet. After seeing the Skype advisory, he had questions that needed answering before he would give the patch a green light. "I'm still waiting for answers from Skype," Raff said in an instant message interview.

Users can download the patched Skype -- Windows version 3.6.0.248 -- from the service's Web site. Existing Skype users can update by using the software's "Check for Updates" command under the Help menu.