Researcher Sergey Golovanov wrote that the attacks which had been collectively named DarkVishnya, led to the loss of tens of millions of dollars.
The devices used in these attacks varied but were one of three tools: a netbook or cheap laptop; a Raspberry Pi computer; or a Bash Bunny, a special tool for carrying out USB attacks.
According to its manufacturer, Hak5, the Bash Bunny "is a simple and powerful multi-function USB attack and automation platform for penetration testers and systems administrators".
Next, the attackers connected remotely and scanned the network, looking for shared folder, Web servers or any other resources that were open. Simultaneously, they also used brute-force methods or sniffed for login data.
Their targets appeared to be machines that were used for making payments, Golovanov said.
"To overcome the firewall restrictions, they planted shellcodes with local TCP servers. If the firewall blocked access from one segment of the network to another, but allowed a reverse connection, the attackers used a different payload to build tunnels," he added.
Once this was done, the attackers logged in to the target system and used remote access software to retain their access. They used fileless attacks and PowerShell to avoid detection.
"If they encountered a whitelisting that could not be bypassed, or PowerShell was blocked on the target computer, the cyber criminals used impacket, and winexesvc.exe or psexec.exe to run executable files remotely," Golovanov said.