We recently reported about a new variant of the Flashback Trojan horse which is using novel techniques to infect Macs. Since then, we have discovered a number of samples of this latest variant, Flashback.G, and have seen evidence that many Mac users have been infected by this malware.

How this malware infects Macs

This new variant of the Flashback Trojan horse uses three methods to infect Macs. The malware first tries to install itself using one of two Java vulnerabilities. If this is successful, users will be infected with no intervention. If these vulnerabilities are not available – if the Macs have Java up to date – then it attempts a third method of installation, trying to fool users through a social engineering trick. The applet displays a self-signed certificate, claiming to be issued by Apple. Most users won’t understand what this means, and click on Continue to allow the installation to continue.
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What this malware does

This malware patches web browsers and network applications essentially to search for user names and passwords. It looks for a number of domains – websites such as Google, Yahoo!, CNN; bank websites; PayPal; and many others. Presumably, the people behind this malware are looking for both user names and passwords that they can immediately exploit – such as for a bank website – as well as others that may be reused on different sites. (Hint: don’t use the same password for all websites!)

One of the clues that a Mac is infected is that certain applications will crash. This is notably the case for web browsers, such as Safari, or other network programs, such as Skype. This is because the injected code interferes with the program making it unstable.

This malware also has an automatic update module that checks a number of websites for new versions.

Means of protection

Most of the cases of infection we are seeing are on Macs running OS X 10.6 Snow Leopard. As we reported in our previous post, OS X Lion does not come with Java pre-installed, but Snow Leopard does. It is therefore essential that anyone running OS X 10.6 update Java immediately. To do this, run Software Update, from the Apple menu; if you do not have the latest version of Java, an update will be available.

Nevertheless, many Macs are getting infected by the social engineering trick of the bogus certificate purporting to be signed by Apple, as shown in our screenshot above. If you see this, don’t trust it, and cancel the process.

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This malware is particularly insidious, as users don’t download anything or double-click any file to launch an installer. Be careful if you see the screenshot above, and check to see if you need to update Java.

If you are infected by this malware, look for a Java applet in ~/Library/Caches and send it to sample@virusbarrier.com before deleting it. We’d like to see as many samples as possible.