April 24, 2012, 3:20 PM — A new, sneakier variant of the Flashback malware was uncovered yesterday by the French security firm Intego.
Flashback.S, which Intego described Monday, uses the same Java vulnerability as an earlier version that has infected an estimated 820,000 Macs since its appearance and still plagues over 600,000 machines.
But unlike Flashback.K, the variant that first surfaced last month and has caused consternation among Mac users, Flashback.S never asks the victim to enter an administrative password for installation, but instead relies only on the silent exploit of the Java bug to sneak onto the system.
"The differences are very subtle," Peter James, a spokesman for Intego, said in an interview Tuesday. "There's no password request [by Flashback.S]."
Flashback.K used different infection tactics: Even though it exploited the same Java vulnerability -- identified as CVE-2012-0507 -- it also displayed the standard OS X password-request dialog. If users entered their password, the malware installed itself in a different location, where it was even harder to detect.
The hackers responsible for Flashback appear to be making money through click fraud, where large numbers of people are redirected to online ads not normally served by the site the user is viewing. The criminals receive kickbacks from shady intermediaries for each ad clicked.
The Java flaw used by both Flashback.S and the earlier Flashback.K was patched by Oracle in mid-February, but Apple, which maintains its own edition of Java for OS X and so is responsible for patching Java bugs, did not issue its fix until April 3, seven weeks later.
Users are infected by Flashback.S when they browse to compromised or malicious sites; the tactic is called a "drive-by" to reflect the lack of required user action beyond steering to a URL.
Some security experts have traced the Flashback infections to tens of thousands of hacked sites and blogs running WordPress.
Because Flashback.S uses different names for the files it drops on a Mac, and installs those files in a different location than Flashback.K, it's possible that the malware seek-and-destroy tool Apple released April 12 won't eradicate the variant.
James said that Intego was not able to confirm whether Apple's tool removes Flashback.S.