Hackers planted a Raspberry Pi equipped with a 4G modem in the network of an unnamed bank in an attempt to siphon money out of the financial institution’s ATM system, researchers reported Wednesday.
The researchers with security firm Group-IB said the “unprecedented tactic allowed the attackers to bypass perimeter defenses entirely.” The hackers combined the physical intrusion with remote access malware that used another novel technique to conceal itself, even from sophisticated forensic tools. The technique, known as a Linux bind mount, is used in IT administration but had never been seen used by threat actors. The trick allowed the malware to operate similarly to a rootkit, which uses advanced techniques to hide itself from the operating system it runs on.
End goal: Backdooring the ATM switching network
The Raspberry Pi was connected to the same network switch used by the bank’s ATM system, a position that effectively put it inside the bank’s internal network. The goal was to compromise the ATM switching server and use that control to manipulate the bank’s hardware security module, a tamper-resistant physical device used to store secrets such as credentials and digital signatures and run encryption and decryption functions.
The group behind the attack is tracked in the industry under the name UNC2891. The financially motivated threat group has been active since at least 2017 in targeting the infrastructures of banks. It has earned a well-deserved reputation for proficiency in its use of custom malware in attacks targeting Linux, Unix, and Oracle Solaris systems.
In 2022, Google’s Mandiant division said it had observed UNC2891 spending years inside a targeted network, during which time the intrusion went largely unnoticed. Mandiant researchers went on to identify CakeTap, a custom rootkit for Solaris systems. Among other things, CakeTap manipulated messages passing through an infected ATM switching network, most likely for use in unauthorized cash withdrawals using fraudulent bank cards. Mandiant documented two other custom pieces of malware, which the company named SlapStick and TinyShell.
Group-IB’s report on Wednesday shows that UNC2891 is still active and finding new and advanced ways to burrow into bank networks without detection.
“One of the most unusual elements of this case was the attacker’s use of physical access to install a Raspberry Pi device,” Group-IB Senior Digital Forensics and Incident Response Specialist Nam Le Phuong wrote. “This device was connected directly to the same network switch as the ATM, effectively placing it inside the bank’s internal network. The Raspberry Pi was equipped with a 4G modem, allowing remote access over mobile data.”
To maintain persistence, UNC2891 also compromised a mail server because it had constant Internet connectivity. The Raspberry Pi and the mail server backdoor would then communicate by using the bank’s monitoring server as an intermediary. The monitoring server was chosen because it had access to almost every server within the data center.
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