A second variant appears to still be capable of opening up iTunes.
A second variant appears to still be capable of opening up iTunes.

Mac OS users running Safari are falling victim to a tech support scam that can freeze their computer, according to a Thursday post on the MalwareBytes Labs blog.

Similar previous campaigns have used fake alerts notifying victims that something is wrong with their computer, prompting them to reach out for tech assistance. By clicking onto a phony site, or by calling a phony assistance number, the victim can then authorize attackers to gain control of their machines.

One version of this scam, which targeted the browser, was dubbed a browlock. Another which actually loaded malware onto devices was termed a screen locker.

A new version of this strategy that targets Mac OS users running Safari has been detected by Jérôme Segura, lead malware intelligence analyst at Malwarebytes and author of the report. The scam is triggered when a victim visits a phony tech support site. The malicious webpage can determine – via a user agent check – the version of OS X the victim machine is running and then download two different versions of a denial-of-service attack. The first delivers a cascade of email drafts, eventually leading to machines running an older version of MacOS to run out of memory, thus shutting it down. Another version of the attack involves Apple's iTunes.

The flaws were likely addressed with Apple's latest update, macOS Sierra 10.12.2, Segura posited, as it appeared as if Mac users operating the upgraded version have not been affected by the Mail app DoS. However, he added, "the second variant appears to still be capable of opening up iTunes, without any prompt in Safari."

The fraudulent page was most likely pushed by affiliates who sell leads to rogue tech support call centers, Segura told SC Media on Friday.

The attackers are exploiting flaws or abusing HTML5 and JavaScript to trigger unwanted behaviors, such as alerts or denial-of-service attacks, he explained. "Apple has released patches before to prevent webpages or ads to display repetitive alerts or launch the App Store."

Malvertising is the primary vector used to spread those booby-trapped websites, Segura told SC. "Malicious adverts displayed on popular sites or sites with high traffic will automatically redirect your browser to a tech support scam page."

When asked what he believed was the motivation behind these attacks, Segura replied that "the motive is to scare the user and ultimately have them call the toll-free number for assistance."

Mac users, he added, are not immune to tech support scams. "We are seeing some interesting tricks to abuse browsers," he said.