In a blog post, researchers Sivagnanam Ganesan Nagarethinam and Sean Gallagher said SystemBC, first seen last year, gave attackers a persistent connection to victims' systems, acting both as a network proxy for hidden communications and also as a remote administration tool.

It was able to run Windows commands, deliver and execute scripts and also malicious executables and DLLs after it was dropped by other malware.

"While SystemBC has been around for over a year, we’ve seen both its use and its features continue to evolve," the two researchers wrote.

"The most recent samples of SystemBC carry code that, instead of acting essentially as a virtual private network via a SOCKS5 proxy, uses the Tor anonymising network to encrypt and conceal the destination of command and control traffic."

The Sophos team found that SystemBC had been used in recent Egregor and Ryuk attacks that they had investigated, often in combination with Cobalt Strike, a took used after exploitation had occurred.

When SystemBC was run on a system, it checked to see if it had been started from a command-line which meant it was started as a scheduled service. If this was not the case, then the backdoor copied itself to a randomly-named directory, within the Program Data directory, using a random filename. Then it scheduled that copy to run as a task.

"However, if SystemBC finds a running process called a2guard.exe — a component of [security firm] Emsisoft’s anti-malware software — it skips the creation of a service," Nagarethinam and Gallagher said. "This behaviour dates back to the first samples of SystemBC found in 2019."

As far as the command and control aspect was concerned, the researchers said there was a beacon connection to a server at one of two domains, both of which were hardcoded, and a Tor client with a small footprint, with most connections being through the latter.

Nagarethinam and Gallagher said the attacks they had studied appeared to have been launched by affiliates of the ransomware operators, or else by the actors themselves who used a number of malware-as-a-service providers.

"They involved days or weeks of time on the targets’ networks and data exfiltration," the duo said. "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."

Others who contributed to this research were Anand Aijan, Syraj Mundalik, Peter Mackenzie, Elida Leite, Syed Shahram and Bill Kearney.