Skype plugs critical cross-zone scripting hole

Computerworld - Skype Ltd. today 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 Tuesday, 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 the way Skype rendered HTML.

At root, Raff said, was the fact that Skype, which uses Internet Explorer'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-concept code for the exploit, Skype temporarily plugged the hole by first ditching connections to the Dailymotion video-sharing service. Dailymotion is a partner of the Internet-calling service. Six days later, it severed the line to Metacafe, another partner that provides video-sharing services, when Raff pointed out an even more serious exploit.

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 that 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 today'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, which is 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 -- Version 3.6.0.248 for Windows -- from the service's Web site. Existing Skype users can update by using the software's "Check for Updates" command under the Help menu.

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