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 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 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.

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.

» posted by abennett

Computerworld

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