A Java vulnerability that hasn't yet been patched by Apple is being exploited by cybercriminals to infect Mac computers with a new variant of the Flashback malware, according to security researchers from antivirus firm F-Secure.
Flashback is a computer Trojan horse for Mac OS that first appeared in September 2011. The first variant was distributed as a fake Flash Player installer, but the malware has been changed significantly since then, both in terms of functionality and distribution methods.
Back in February, several antivirus companies reported that a new Flashback version was being distributed through Java exploits, which meant that the infection process no longer required user interaction.
The Java vulnerabilities targeted by the February exploits dated back to 2009 and 2011, so users with up-to-date Java installations were protected.
However, that's no longer the case with the latest variant of the malware, Flashback.K, which is being distributed by exploiting an unpatched Java vulnerability, security researchers from F-Secure said.
Oracle released a fix for the targeted vulnerability, which is identified as CVE-2012-0507, back in February and it was included in an update for the Windows version of Java.
However, since Apple distributes a self-compiled version of Java for Macs, it ports Oracle's patches to it according to its own schedule, which can be months behind the one for Java on Windows.
Security experts have long warned that this delay in delivering Java patches on Mac OS could be used by malware writers to their advantage, and the new Flashback.K malware confirms that they were right.
After being dropped and executed on the system via the CVE-2012-0507 exploit, the new Trojan horse prompts a dialogue window that asks the user for their administrative password.
F-Secure said that regardless of whether the user inputs the password or not, the malware still infects the system. The Trojan's purpose is to inject itself into the Safari process and modify the contents of certain web pages.
There are rumours that a new exploit for a different unpatched Java vulnerability is currently being sold on the underground market and could be used to target Mac users in a similar way in the future, the F-Secure researchers said.
"If you haven't already disabled your Java client, please do so before this thing really become an outbreak," they said. The antivirus company provides instructions on how to do this.
Apple stopped including Java by default in Mac OS X starting with version 10.7 (Lion). However, if Lion users encounter a web page that requires Java, they are prompted to download and install the runtime and might later forget that they have it on their computers.