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Skype bug gives attackers access to Mac OS X machines

'Extremely wormable and dangerous'

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Mac users running Skype are vulnerable to self-propagating exploits that allow an attacker to gain unfettered system access by sending a specially manipulated attachment in an instant message, a hacker said.

“The long and the short of it is that an attacker needs only to send a victim a message and they can gain remote control of the victim's Mac,” Gordon Maddern of Australian security consultancy Pure Hacking blogged on Friday. “It is extremely wormable and dangerous.”

The vulnerability, which Maddern said isn't present in the Windows or Linux versions of the popular VoIP program, was confirmed by Skype spokeswoman Brianna Reynaud, who said a fix will be rolled out next week. Its disclosure comes the same week that researchers discovered a new crimekit that streamlines the production of Mac-based malware. It also comes as new malware surfaced for Apple's OS X that masquerades as a legitimate antivirus program.

Reynaud said there are no reports that the Skype vulnerability is being actively exploited.

Maddern said he stumbled on the critical flaw by accident.

“About a month ago I was chatting on skype to a colleague about a payload for one of our clients,” he wrote. “Completely by accident, my payload executed in my colleagues skype client. So I decided to test another mac and sent the payload to my girlfriend. She wasn't too happy with me as it also left the her skype unusable for several days.”

He then set out to write proof-of-concept attack code that used payloads borrowed from the Metasploit exploit framework. The result: a Skype exploit that allows him to remotely gain shell access on a targeted Mac. Because it's sent by instant messages, it might be possible to force each infected machines to send the malicious payload to a whole new set of Macs, causing the attack to grow exponentially.

Maddern didn't say what interaction is required on the part of the victim, and he didn't immediately respond to an email seeking clarification. His blog post says he notified Skype of the vulnerability more than a month ago, and that he will withhold specific details until a patch is released to prevent malicious attacks. ®

Update

According to a post on the Skype Security blog that was published a few hours after this story went live, a hotfix for the vulnerability was released in mid April.

“As there were no reports of this vulnerability being exploited in the wild, we did not prompt our users to install this update, as there is another update in the pipeline that will be sent out early next week,” Skype's Adrian Asher wrote.

He added:

This vulnerability, which they blogged about earlier today, is related to a situation when a malicious contact would send a specifically crafted message that could cause Skype for Mac to crash. Note, this message would have to come from someone already in your Skype Contact List, as Skype's default privacy settings will not let you receive messages from people that you have not already authorized, hence the term malicious contact.

The headline in this article was updated to correct the nature of the vulnerability. It remotely gives shell access.

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