New Cross-Platform Backdoor ‘SysJoker’ Used in Targeted Attacks

A backdoor likely used by an advanced persistent threat (APT) actor in targeted attacks was built to target Windows, macOS, and Linux systems, Intezer reports.

Dubbed SysJoker, the backdoor was identified last month in an attack targeting the web server of an educational institution. In addition to the Linux-based variant used in this attack, Intezer’s security researchers identified Mach-O and Windows PE versions of the threat as well.

SysJoker was found on the VirusTotal scanning engine with the suffix .ts, for TypeScript files, which could indicate distribution via an infected npm package, the researchers say.

To evade detection, the threat poses as a system update and its operators are constantly changing the command and control (C&C) server. The C&C is generated by decoding a string fetched from a text file on Google Drive, which the attackers control.

Although written from scratch on all three operating systems, the backdoor features similar behavior across all platforms. It can gather information about the infected machine, achieve persistence, and start communication with the C&C server.

Based on instructions received from the C&C server, SysJoker can drop and run additional executables, as well as run commands and upload the response to the C&C. The malware also includes support for two unimplemented commands apparently related to a self-deletion mechanism.

Intezer believes that SysJoker is operated by an advanced threat actor, because its code hasn’t been observed in previous attacks, because the code is written from scratch for all three platforms, and because the adversary registered multiple domains for it.

Furthermore, no second stage was observed during the incident identified in December, and the attacker did not send a command to the backdoor, suggesting that the malware is used in specific, targeted attacks only.

“Based on the malware’s capabilities we assess that the goal of the attack is espionage together with lateral movement which might also lead to a Ransomware attack as one of the next stages,” Intezer concludes.

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