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Zcrypt Ransomware: Old Wine In A New Bottle

Malware authors have combined old and new approaches to try and sneak Zcrypt past defenses, Check Point says.

Malware typically garners attention for introducing sneaky new tactics and techniques to infiltrate systems and propagate: Zcrypt is getting attention for doing the opposite.

Zcrypt is a newly discovered ransomware sample that security vendor Check Point profiled in a report this week. What sets it apart from the slew of ransomware tools floating in the wild is its use of older techniques along with new ones in order propagate and to evade detection by anti-malware tools optimized to look for newer threats.

In addition to spreading via phishing emails and exploit kits like many reansomware variants, Zcrypt also propagates via USBs. When an infected USB is plugged into a system, Zcrypt automatically launches a file called invoice.exe, which when opened infects the system with the ransomware.

“The Autorun technique for spreading used to be very common about three years ago,” says Tamara Leiderfarb, technology leader at Check Point. “It’s much slower than any other technique that moves through the network, but on the other hand, it is under the radar and does not require credentials.”

The use of USBs also provides a way for Zcrypt to propagate to network segments that would otherwise not be available, Leiderfarb says.

Zcrypt also writes using a dynamic-link library (DLL) file. To unpack the DLL file, Zcrypt uses a process called the Nullsoft Scriptable Install System (NSIS), which is also used by legitimate applications. Running code in DLLs is a relatively older technique, Leiderfarb says. It is designed to evade detection in sandboxes since the malware simply will not run in most cases.

Unlike many ransomware samples, Zcrypt does not attempt to delete shadow copies, delete itself, or copy itself several times before initiating, she says. It is pretty straightforward in that it encrypts and corrupts files to prevent recovery. Zcrypt’s ability to run in different process instances and its use of packers allow it to evade detection by anti-malware tools.

The use of older techniques does not make Zcyrpt any less potent than other sophisticated ransomware threats, however. Zcrypt is designed to overwrite targeted files not once, but twice. It first corrupts the file and then encrypts it to limit the ability of victims to use disk recovery tools. After encrypting target files, Zcrypt monitors for file changes and promptly encrypts any new files on the compromised system.

The mix of older techniques in Zcrypt is interesting and suggests that ransomware purveyors are trying different tacks to sneak their malware past malware defenses, Leiderfarb says. It is the second ransomware tool in recent months—the other being CryptXXX—to use the approach, and it is quite likely there will be other ransomware samples attempting to do the same thing, Leiderfarb says.

“Ransomware attacks have become much more simplistic to evade the security measures put against them,” she says. “In the beginning, ransomware attacks often tried to be innovative and their developers realized these innovations caused their detection. By scaling back, less innovative ransomware attacks have actually become more evasive.”

Security vendor ESET this week warned of a new ransomware family dubbed Crysis that it said has quickly emerged as a significant threat following the recent cessation of the TeslaCrypt ransomware operation.

ESET says Crysis is designed to encrypt files on fixed, removable, and network drives. The malware is being spread via attachments in spam emails, using double file extension. “Using this simple – yet effective – technique, executable files appear as non-executable,” ESET researcher Ondrej Kubovic wrote. The authors of Crysis have also been using malicious files disguised as seemingly harmless installers for legitimate applications to distribute the ransomware, Kubovic said.

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