Ransomware Gangs Use ‘SystemBC’ Tor Backdoor in Attacks

Researchers at Sophos noticed recently that the operators of multiple ransomware families have been using a backdoor named SystemBC, which provides attackers a connection to compromised devices and which uses the Tor anonymity network to hide command and control (C&C) communications.

Initially observed in 2019, SystemBC enables persistent access to the system, providing attackers with means to conceal communications and remotely control the infected devices.

Designed with support for the execution of commands and to allow adversaries to download and execute scripts, executables, and DLLs, the backdoor is continuously evolving, with recent samples having switched from creating a SOCKS5 proxy to using the Tor network for communication purposes.

Over the past months, Sophos researchers identified hundreds of attacks employing SystemBC, some of them being recent Ryuk and Egregor ransomware assaults, in which the backdoor was deployed in combination with other post-exploitation tools, including Cobalt Strike.

In some instances, the backdoor was deployed after the attackers were able to compromise administrative accounts and moved deep into the network.

For persistence, the malware copies itself to a folder in the ProgramData directory and schedules the copy as a task that is launched using the ‘start’ command, as a scheduled service. It searchers for a process associated with Emsisoft’s anti-malware software and skips the creation of the service if the process is found.

Most of the communication with the C&C server is performed over a Tor connection, using an implementation closely resembling the mini-tor open-source library.

When executed from the scheduled task, the malware collects information such as the active Windows user name, OS build number, system architecture (32-bit or 64-bit), and volume serial number, and sends it encrypted to the C&C. It also enables attackers to send and execute files and commands.

“Collectively, these capabilities give attackers a point-and-shoot capability to perform discovery, exfiltration and lateral movement with packaged scripts and executables—without having to have hands on keyboard. These capabilities were originally intended for mass exploitation, but they have now been folded into the toolkit for targeted attacks—including ransomware,” Sophos notes.

The Ryuk and Egregor attacks employing SystemBC were performed either by affiliates of ransomware operators or by the ransomware gangs themselves, through malware-as-a-service (MaaS) service providers. The threat actors lingered for weeks on the victim networks, for data exfiltration.

“SystemBC is an attractive tool in these types of operations because it allows for multiple targets to be worked at the same time with automated tasks, allowing for hands-off deployment of ransomware using Windows built-in tools if the attackers gain the proper credentials,” Sophos explains.

Related: Anatomy of Ryuk Attack: 29 Hours From Initial Email to Full Compromise

Related: HR Giant Randstad Hit by Egregor Ransomware

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Ionut Arghire is an international correspondent for SecurityWeek.

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