Agentless Backup is Not a Myth

Skype has patched a flaw involving its SkypeFind feature. But the security researcher who discovered the flaw said the VoIP platform remains exposed to cross-zone scripting vulnerabilities, like the latest SkypeFind bug and an earlier flaw involving movie files.

SkypeFind lets users recommend businesses, or post reviews, to others running the voice-over-IP client. Problems have arisen because Skype has neglected to sanitise a field designed to pass across reviewers' names (even though it does clean up data provided in the business item entry and text submitted in a review).

"An attacker can inject a malicious script in his Skype's Full Name, and whenever a victim will view a business which was reviewed by the attacker, in the SkypeFind dialog, the malicious script will be executed in an unlocked Local Zone," warns security researcher Aviv Raff, who was also instrumental in revealing previous cross-zone scripting vulnerabilities in Skype.

"Fortunately for the attacker, it is also possible to open the dialog in a specific business details page from the browser, using the skype: URI handler (e.g. skype:?skypefind ). This means that it is possible for the attacker to create a worm," he added.

In a security notice, Skype said it had fixed the SkypeFind feature. It doesn't go into details beyond saying users don't need to update client software. Skype suggested the whole thing was a storm in teacup because the flaw was hard to exploit. "There is one important precondition for the exploit to work. [The] victim must receive Skype contact request authorisation from the attacker's Skype account," it said.

Raff took issue with this assessment, pointing out that hacking techniques to automate users' contact requests would have reduced the difficulty of mounting attacks.

"The victim enters a malicious website [that] automatically calls the attacker via Skype," Raff said, describing one technique. "This can be done by using the Skype: URI handler. The attacker's bot intercept[s] the call and cancels it. Now that the bot has the victim's username, it uses the User.IsAuthorize API call to allow the victim to view the attacker's full name.

"After a few seconds, the malicious website opens the malicious SkypeFind dialog, and the victim gets owned," Raff said.

Raff has posted a Flash-based video demo (created by another security researcher, Guy Mizrahi) of the SkypeFind attack on his blog. Skype's fix fall short in providing adequate safeguards against exploitation, Raff concludes.

"I've contacted Skype security team, and they have provided a quick fix for the full name issue," Raff writes. "Unfortunately, this is not enough. I'm worried that there are probably other ways to inject a script to this dialog."

"I strongly advised Skype to disable this feature until they provide a patch for the cross-zone scripting vulnerability." ®

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