Attackers, likely working for the same threat group, have looted tens of millions of dollars from at least eight banks in Eastern Europe after gaining initial access to their networks via devices connected directly to a local network.

In some cases, the attackers planted the devices at the banking institution's central office. In others, they were planted in a regional office or even an office in another country, Kaspersky Lab said in a report this week.

They then used the initial foothold to move deeper into the target organization's network, finding and manipulating systems in order to withdraw millions of dollars using ATMs and other services.

The "DarkVishnya" campaign, as Kaspersky Lab has named it, was a series of attacks on financial institutions, says Sergey Golovanov, security researcher at Kaspersky Lab. "What they all had in common was the use of a physical device that was connected to the local network and later scanned in order to access open resources," he says.

The attacks are another reminder that network perimeter defenses alone are not enough, Golovanov notes. "Cybercriminals can connect to the network leaving no trace and no logs in networking gear," he says.

In its report, Kaspersky Lab described the devices used in the DarkVishnya attacks as one of three types: a notebook or cheap laptop, a Raspberry Pi computer, or Bash Bunny, a Linux-based tool that can be plugged into a target computer's USB port to execute malicious payloads.

With each attack, the cybercriminals gained initial access to their target organization's building by pretending to be a courier, job seeker, or some other guise. They then connected their rogue devices to the banks' local networks in meeting rooms or to tables with built-in network sockets.

Each of the planted devices was remote-access-enabled via a built-in or USB-connected modem. The device would show up on the local network as an unknown computer, an external flash drive, or a keyboard. But finding it was hard because the device would typically be hidden or installed in a manner to blend in with the surroundings, Kaspersky Lab said in its report.

The attackers then remotely accessed their rogue devices and used them to scan the network for publicly accessible folders, Web servers, and other open resources. The main goal was to gather as much information as possible on servers and workstations used for making payments.  

Once the attackers discovered such systems, they tried brute-forcing their way in or finding data for logging into the systems using legitimate credentials.

"When a malicious program was installed on one of the computers, this program would not connect to external IP addresses belonging to the threat actors," Golovanov says. Instead, it would open a local TCP-port and let criminals connect to it, he says.

In situations where a firewall prevented the technique from working, the attackers would "use a server of one of the local computers on the network that already had permission to access the target system through the firewall," he says. "So some computers had local ports open, and some computers just had IP addresses of computers from the corporate network, not threat actors' external IP addresses."

Kaspersky Lab researchers estimate that the target banks suffered millions of dollars in direct losses from the attack via fraudulent ATM withdrawals and other services that provide banking clients with funds.

Golovanov says that while the attacks were identical and involved the same kind of rogue devices, Kaspersky Lab is currently not making any claims about the potential identity of the threat actors behind DarkVishnya.

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