Cash machines have been emptied using USB sticks in a series of real world attacks that hark back to exploits first demonstrated by security researcher Barnaby Jack three years ago.
Crybercrooks have created a strain of malware that creates a backdoor on compromised ATMs using a bootable USB stick. The crooks cut a hole into the plastic chassis of the ATM in order to access the USB port before patching the system up to avoid their tampering being detected.
Once the USB device was in place, the ATMs could be rebooted and the malware would be automatically installed. The malicious code was used to screw with functions normally restricted to engineering diagnostics.
The thieves then used a 12-digit code to access an alternative interface on compromised ATMs. The masterminds behind the scam, not wanting their street crews to go rogue, have built a challenge-response access control into their malicious software so that low-level fraudsters need to contact more senior members of the gang to get the one-time code necessary to withdraw money from compromised machines. The precaution also prevents rival gangs from looting knocked-up cash machines.
Details of the malware-based attack on an unnamed Brazilian bank's cash dispensers* were presented by two security researchers at the Chaos Communication Congress in Hamburg last month. In response to audience questions, Tillman Werner from security upstart CrowdStrike explained that the malware used in the scam was not designed to intercept credit or debit card data.
Video of the fascinating hour-long talk can be found here. The scenario of the attack recalls the ATM Jackpotting exploit against cash machines, as first demonstrated by famed security researcher Barnaby Jack at Black Hat USA 2010.
Werner and his research partner said they were explaining details of the relatively trivial attack against ATMs running versions of Windows for embedded devices in order to encourage vendors and banks to introduce basic security safeguards. Disabling the "boot from USB" functionality on ATMs alone would be enough to frustrate this particular scam.
Cash machine thefts based on the technique surfaced last year and have remained isolated, at least for now. Werner's research is largely based on the analysis of malware found on a USB stick confiscated from a suspected fraudster.
Gunter Ollmann, CTO of computer security services firm IOActive, said in a article for Dark Reading that the ability to write malicious code for ATMs is far from unknown among the denizens of the digital underground.
"While the attack vector - booting from an infected USB stick - will have many security veterans rolling their eyes in disbelief that the targeted bank hadn't already mitigated the threat," Ollmann writes, "I've heard several people argue that writing code (malicious or otherwise) for ATMs is difficult. Unfortunately, it's simpler than most realise. Anyone with an understanding of CEN/XFS, or the time to peruse the online manuals, will quickly master the fundamentals."
"This USB infector process is the low-hanging fruit for criminals targeting ATM machines. Banks that haven't already mitigated the attack vector are, for lack of a better word, negligent. There can be no excuses for not disabling the 'boot from USB' functionality, especially now with the public disclosure of criminal abuse," he adds. ®
* Not stated during the demo but a Portuguese menu plus R$ (Reals) as the currency unit leaves little doubt that the scam hit banks in Brazil.