Cerber Fights Anti-Ransomware Tools

New variants of the popular Cerber ransomware now try to sniff out decoy image files planted on a targeted machine to help thwart the attack.

Researchers at Cybereason spotted the ransomware searching for .bmp, .jpg, .png, and .tiff image files, which are typically used as decoys or "canary files," by Cybereason's free anti-ransomware tool. When these Cerber variants find a phony file, they don't encrypt that directory - an apparent move to evade detection.

A canary file is placed on a machine as a decoy: if a ransomware program encrypts it, that sounds the alarm to the anti-ransomware tool that then shuts it down. This is a form of so-called deception technology, an evolving approach to being more proactive in defense by placing next-generation honeypots in place to fool the bad guys and trigger alerts.

Lawrence Pingree, a research director with Gartner, says deception technology allows enterprises to deceive, obfuscate, and delay an attacker. "You only need one deception to disrupt the kill chain," he said at the recent Gartner Security & Risk Management Summit in Washington, DC.

The new Cerber variants try to detect deception and then evade anti-ransomware steps. "They are actively trying to evade anti-ransomware measures," says Uri Sternfeld, senior security researcher at Cybereason, which offers the free anti-ransomware tool called RansomFree.

"The Cerber samples were the first [ransomware] assigned or that even addressed our technique" of canary files, he says.

They scan the entire hard drive in search of image files, which they test for validity. If they find a phony/canary file, they flag that directory as one not to encrypt in their attack, he says. "Later when they start encrypting files, they simply skip all the directories that contain the invalid [canary] image files," he says.

So Cybereason turned the tables on these crafty Cerber authors. Their free tool now allows a user to create a phony image file in a sensitive directory: they can copy a non-image file to the directory and use an image file extension in order to appear as a canary/decoy. "Cerber will assume that the file is a canary file installed by an anti-ransomware program on the user’s machine and refuse to encrypt it," Cybereason explains in a blog post on the new foil to combat the cagey Cerber variants rooting out the tool.

This isn't the first time Cerber variants have tried security evasion methods: another version of the ransomware earlier this year was found that evades detection by machine learning algorithms. According to researchers at Trend Micro who discovered it, the ransomware splits the stages of the malware into multiple files and then injects them into the running process, Mark Nunnikhoven, vice president of cloud research for Trend Micro, told Dark Reading in March. "This helps to conceal them from various detection methods."

There are multiple Cerber variants and groups behind the ransomware, Sternfeld notes. The anti-ransomware evasion variants are just certain strains of the malware. "Ransomware is evolving much faster than regular malware," he says. "My opinion is that the monetary incentive is much great, so there's more incentive to change and modify ransomware."

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