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 (Learn more about VoIP Security products from our VoIP Security Buyer’s Guide).
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.”
In the meantime, Raff recommended that Skype users disable the SkypeFind tab by choosing View/Tab and panels, then uncheck “SkypeFind Tab.” They’ll also need to disable the Skype: URI (Uniform Resource Identifier) protocol handler, which requires an edit of the Windows registry.
“I recommend [that] only for power users,” Raff said. “Other users who don’t want to mess with the registry should uninstall Skype, [because] having Skype installed without using it will not solve the problem, as the Skype: URI handler will automatically open Skype and login.”
Skype has had to fix URI protocol handlers before. In May 2006, for example, Skype fixed a command-line parsing flaw in the URI handler of the Windows client.
“I don’t think Skype is any better or worse when it comes to security than other IM or VoIP clients,” Raff concluded. “The main problem with Skype is that it is very difficult for sysadmins to block Skype at the network level.”
Raff has posted a Flash-based video demonstration of the SkypeFind attack vector on his blog. Guy Mizrahi, another Israeli security researcher, created the video.
