Outlaw Group Uses SSH Brute-Force to Deploy Cryptojacking Malware on Linux Servers

Cybersecurity researchers have shed light on an “auto-propagating” cryptocurrency mining botnet called Outlaw (aka Dota) that’s known for targeting SSH servers with weak credentials.

“Outlaw is a Linux malware that relies on SSH brute-force attacks, cryptocurrency mining, and worm-like propagation to infect and maintain control over systems,” Elastic Security Labs said in a new analysis published Tuesday.

Outlaw is also the name given to the threat actors behind the malware. It’s believed to be of Romanian origin. Other hacking groups dominating the cryptojacking landscape include 8220, Keksec (aka Kek Security), Kinsing, and TeamTNT.

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Active since at least late 2018, the hacking crew has brute-forced SSH servers, abusing the foothold to conduct reconnaissance and maintain persistence on the compromised hosts by adding their own SSH keys to the “authorized_keys” file.

The attackers are also known to incorporate a multi-stage infection process that involves using a dropper shell script (“tddwrt7s.sh​”) to download an archive file (“dota3.tar.gz”), which is then unpacked to launch the miner while also taking steps to remove traces of past compromises and kill both the competition and their own previous miners.

A notable feature of the malware is an initial access component (aka BLITZ) that allows for self-propagation of the malware in a botnet-like fashion by scanning for vulnerable systems running an SSH service. The brute-force module is configured to fetch a target list from an SSH command-and-control (C2) server to further perpetuate the cycle.

Some iterations of the attacks have also resorted to exploiting Linux- and Unix-based operating systems susceptible to CVE-2016-8655 and CVE-2016-5195 (aka Dirty COW), as well as attack systems with weak Telnet credentials. Upon gaining initial access, the malware deploys SHELLBOT for remote control via a C2 server using an IRC channel.

SHELLBOT, for its part, enables the execution of arbitrary shell commands, downloads and runs additional payloads, launches DDoS attacks, steals credentials, and exfiltrates sensitive information.

As part of its mining process, it determines the CPU of the infected system and enables hugepages for all CPU cores to increase memory access efficiency. The malware also makes use of a binary called kswap01 to ensure persistent communications with the threat actor’s infrastructure.

“Outlaw remains active despite using basic techniques like SSH brute-forcing, SSH key manipulation, and cron-based persistence,” Elastic said. “The malware deploys modified XMRig miners, leverages IRC for C2, and includes publicly available scripts for persistence and defense evasion.”

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