Web security firm Trusteer issued a warning this week about the return "with a vengeance" of Shylock, a polymorphic financial malware variant the company discovered last September that is now showing up again in end user machines.
It is aimed primarily at global financial institutions. Trusteer codenamed it Shylock because, "every new build bundles random excerpts from Shakespeare's 'The Merchant of Venice' in its binary," according to Trusteer CTO Amit Klein.
"These are designed to change the malware's file signature to avoid detection by antivirus programs," wrote Klein.
In an interview, Klein said there are hints in Shylock terminology to suggest it comes from Russia or the Ukraine. But who is involved and exactly where it is coming from remain unknown. "These are very difficult to track," he said.
Klein said the authors of the malware are running a surgical operation aimed at specific targets, a dozen or so large banks, some payment card providers and several web mail providers. Shylock amounts to customised financial fraud capabilities for the malware, including an improved methodology for injecting code into additional browser processes to take control of the victim's computer, according to Trusteer.
So far, while it does not appear to have caused widespread damage, Klein said Trusteer has received some reports from banks regarding compromised machines where fraud took place before they cleaned them.
And he suspects the reason Shylock has not been seen much in recent months is because it has been under development and improvement. "It is malware in progress," he said. "They keep throwing in new features, and perhaps have decided it's stable enough to distribute."
Klein said Shylock is distinguished by its ability to almost completely avoid detection by antivirus scanners after installation, using a unique three step process.
First, it doesn't run as a separate process, but embeds itself within applications running on a machine. Second, once it detects antivirus scanning, it deletes its own files and registry entries, and remains active only in memory. That would normally mean it could not survive a system shutdown/reboot. But, Klein says, that is where its third capability comes in to hijack the Windows shutdown.
"It hooks into the Windows shutdown procedure and reinstates the files and registry keys (previously removed) just before the system is completely shut down and after all other applications are closed (including antivirus)," he said.
Beyond that, Klein said Shylock is "pretty sophisticated" malware that not only has its own HTML language, "but appears to have a converter that can take Zeus or SpyEye and turn it into its own format."
Trusteer said machines running its primary product, Rapport, designed to help online banks, brokerages and retailers secure the consumer desktop from financial malware attacks and fraudulent websites, are not vulnerable to Shylock. Klein said machines already infected can get rid of it by installing Rapport. About the only other way to eliminate a Shylock infection, if the machine does not have an internal battery, is to unplug its power source. But that will also clean the memory.
"If you unplug the computer and force a brutal shutdown, the memory will be reset and Shylock will be gone," Klein said. "But Windows is going to whine a bit when it wakes up next. It's tricky to turn off a computer in this way, and you can't be sure it will restart properly."