By some estimates, more than 600,000 Macs were infected with Flashback, which spread using a Java vulnerability. Apple began pushing an update for that Java vulnerability less than two weeks after the Mac malware was discovered, on April 4, 2012. By the end of April, the number of reported Flashback infections had significantly decreased.
Still, Apple users were ensnared because of the six-week delay between knowledge of the Java vulnerability becoming public--owing to attackers reverse-engineering a Windows update in February 2012--and Apple releasing its own Java update that patched the flaw. "This window of opportunity helped the Flashback Trojan to infect Macs on a large scale. The Flashback authors took advantage of the gap between Oracle and Apple's patches by exploiting vulnerable websites using WordPress and Joomla to add malicious code snippets," according to a blog post from Symantec Security Response.
Any Mac OS X user visiting a compromised site risked being infected by Flashback. In particular, Symantec said the infected sites would redirect the user's browser to a website hosting multiple Java exploits, which would use the known Java vulnerability to decrypt and install the initial Flashback Java applet. At that point, the applet would install a loader, as well as an ad-clicking component.
The ad-clicking component works with Chrome, Firefox, and Safari, and "can intercept all GET and POST requests from the browser," said Symantec. "Flashback specifically targets search queries made on Google and, depending on the search query, may redirect users to another page of the attacker's choosing, where they receive revenue from the click."
But the intended click would never reach Google. "This ultimately results in lost revenue for Google and untold sums of money for the Flashback gang," Symantec said. How much money? Based on its 2011 study of the Xpaj botnet, Symantec found that 25,000 click-fraud infections could generate up to $450 per day. "Considering the Flashback Trojan [infection] measures in the hundreds of thousands, this figure could sharply rise to the order of $10,000 per day," it said.
Although the number of Flashback infections continues to decline, Russian antivirus vendor Doctor Web, which first discovered the malware, said that older Macs remain at risk.
Overall, 63% of Flashback infections affected Macs running 10.6 (Snow Leopard), while only 11% hit users of 10.7 (Lion), which is the latest Mac operating system and accounts for 40% of all in-use OS X installations, according to NetMarketShare.
But an older Mac operating system, OS X 10.5 (Leopard), which is used by 13% of Mac users, accounted for 25%--the second highest number--of Flashback infections. Although free software can help Leopard users block Flashback, Apple is no longer shipping Leopard security updates. That puts Leopard users at greater risk of being attacked, because the Java vulnerability exploited by Flashback and SabPub will remain unpatched, and thus will likely continue to be targeted by new malware.
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