New Skype bug opens users to easy hijack, says researcher

Patch the underlying cross-zone scripting flaw already, argues Aviv Raff

Skype plugged yet another critical vulnerability Thursday, but the researcher who reported the bug said that's not enough.

Noted vulnerability researcher Aviv Raff said that another Skype flaw, this one in the software's SkypeFind feature, can be used to inject attack script into systems running the application.

SkypeFind, which was introduced in Skype 3.1 for Windows, lets users recommend businesses to others running the voice-over-IP and chat client and write reviews of those businesses. "Sadly, it could also be used by attackers to own Skype users' machines," Raff said in a blog post.

Specifically, Skype neglects to sanitize reviewers' names, so attacks could replace their Skype names with malicious script. The result is striking, said Raff: "Whenever a victim view[s] a business which was reviewed by the attacker, the malicious script will be executed in an unlocked Local Zone."

Skype issued a security advisory Thursday that confirmed Raff's findings. The company, which has been plagued with multiple cross-zone scripting vulnerabilities this month -- reported by Raff and others -- said it had fixed SkypeFind, but didn't elaborate on what steps it took. "Skype has fixed the vulnerability in SkypeFind," read the bulletin. No software update will be issued.

The company also downplayed the threat. "There is one important precondition for the exploit to work," it said. "[The] victim must receive Skype contact request authorization from the attacker's Skype account."

Not so fast, said Raff. While that's true, there are at least two easy ways for attackers to automate users' contact requests. Both involve relatively basic bots that rely on Skype's own protocol handler or its application programming interfaces (API).

"The victim enters a malicious Web site [that] automatically calls the attacker via Skype," Raff said, describing one vector. "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 Web site opens the malicious SkypeFind dialog, and the victim gets owned!" Raff said.

In an interview via instant messaging this morning, Raff said he isn't satisfied with Skype's fix. "I suspect that there are other ways [to inject a malicious script]," he said. "There is no need for Skype to be running at all, for the attack to take place. You simply visit a malicious Web page, and that's all."

Raff said he had urged Skype to disable the SkypeFind feature entirely until the company comes up with a patch for the overarching cross-zone scripting vulnerability. "For no good reason, they have decided to decline my advice," he said.

Earlier this month, Raff reported a major cross-zone scripting vulnerability in Skype that could be exploited through its video-adding and video-sharing features. While Skype has completely disabled those features in response, it has not yet patched the underlying flaw.

"To lock the Local Zone, they basically need to change one registry value," said Raff. "But the problem is that some [other features] might not work because of that, so I can't tell how long it might take them to fix this."

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Gregg Keizer

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