New Cerber Ransomware Variant Emerges

A new variant of the Cerber ransomware has emerged over the past month, featuring multipart arrival vectors and refashioned file encryption routines, TrendMicro security researchers warn.

An active threat for over a year, Cerber managed to climb at the top of ransomware charts earlier this year, accounting for 87% of attacks in the first quarter of 2017. The rise was fueled mainly by a major decrease in Locky attacks, but Cerber’s popularity among cybercriminals also helped.

Cerber distribution campaigns leverage various methods to increase the rate of infection, including spam email, exploit kits, or newly discovered vulnerabilities, such as the Apache Struts 2 flaw. The malware itself has received numerous improvements, including the ability to evade machine learning security.

Distributed using a ransomware-as-a-service business model, Cerber reached version 6 in April 2017 and is believed to be generating millions of dollars in annual revenue for its operators and developers. The new threat variant packs not only new encryption routines, but also additional defense mechanisms, such as anti-sandbox and anti-AV techniques.

The new variant is being distributed through spam emails containing a zipped attachment with a malicious JavaScript (JS) file inside. After analyzing multiple JS files, TrendMicro found that all have a three-pronged approach: directly download and execute the payload, create a scheduled task to run the malware after two minutes, or run an embedded PowerShell script.

The use of delayed execution allows the ransomware to elude traditional sandbox defenses that feature time-out mechanisms or which wait for the final execution of the malware. The use of PowerShell for Cerber’s delivery isn’t surprising, given the popularity this technique has seen over the past months.

Cerber 6 no longer has a routine for terminating processes, a feature added around October 2016 to terminate database software-related processes to ensure encryption of files. The new threat variant also added another check on file extensions it’s not supposed to encrypt.

“This harks back to how we saw Cerber exhibiting behaviors that foreshadowed its shift to stealth-focused techniques. In February this year, certain variants started checking if the affected system had any firewall, antivirus, and antispyware products installed, ensuring that their associated files aren’t encrypted,” the security researchers say.

Cerber 6 can now be configured to have Windows firewall rules added to block the outbound traffic of executables of firewalls, antivirus, and antispyware products installed in the system, in an attempt to restrict their detection and mitigation capabilities.

The ransomware now uses Cryptographic Application Programming Interface (CryptoAPI) for encryption, and has a separate function that reads and encrypts the contents of the file. Detailed in the beginning of this year, the Spora ransomware also leverages Windows CryptoAPI for encryption.

“Cerber’s developers are noted to implement their own encryption; the abuse of Windows’s CryptoAPI and separation of encryption function for Cerber 6 denote constant efforts from the malware authors to streamline their operations,” TrendMicro notes.

Related: Cerber Dominates Ransomware Charts

Related: Cerber Ransomware Tries to Evade Machine Learning Security