Pierluigi Paganini November 25, 2021

Security researchers discovered a new Linux RAT, tracked as CronRAT, that hides in scheduled cron jobs to avoid detection.

Security researchers from Sansec have discovered a new Linux remote access trojan (RAT), tracked as CronRAT, that hides in the Linux task scheduling system (cron) on February 31st.

Threat actors hides the malware in the task names, then the malicious code is constructed using several layers of compression and base64 decoding.

CronRAT is employed in Magecart attacks against online stores web stores and enables attackers to steal credit card data by deploying online payment skimmers on Linux servers.

Researchers explained that CronRAT malware is undetected by many antivirus engines, it leverages the fact that many security products do not scan the Linux cron system.

Below is the list of capabilities implemented by CronRAT:

• Fileless execution
• Timing modulation
• Anti-tampering checksums
• Controlled via binary, obfuscated protocol
• Launches tandem RAT in separate Linux subsystem
• Control server disguised as “Dropbear SSH” service
• Payload hidden in legitimate CRON scheduled task names

“CronRAT’s main feat is hiding in the calendar subsystem of Linux servers (“cron”) on a nonexistant day. This way, it will not attract attention from server administrators. And many security products do not scan the Linux cron system.” reads the post published by Sansec. “https://sansec.io/research/cronrat”

In the attacks investigated by Sansec, CronRAT was used to inject payment skimmers (aka Magecart) in server-side code.

E-skimming attacks are moving from the browser to the server because the back-end is usually unprotected compared with the browser, Sansec director of threat research Willem de Groot explained.

The CronRAT adds a number of tasks to crontab with the date specification “52 23 31 2 3,” which would generate a run time error when executed despite are syntactically valid. However, the researchers pointed out that the run time error will never happen because the tasks are scheduled to run on a day that doesn’t exist.

Once executed, the malware contacts a command and control (C2) server (47.115.46.167) using a feature of the Linux kernel that enables TCP communication via a file.

The malware contacts the server over TCP via port 443 using a fake banner for the Dropbear SSH service.

Sansec found instance of CronRAT on multiple online stores, including a nation’s largest outlet. The experts had to rewrite part of their eComscan algorithm in order to detect this innovative threat.

“CronRAT is currently undetected by other security vendors.” concludes Sansec.

Pierluigi Paganini

(SecurityAffairs – hacking, Magecart)

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