Criminals cut holes into the machines to plug in USB drives that installed their code onto the ATMs, then patched the holes up to hide the evidence of the hack, the BBC reported Monday.
The thieves would return to the cash machine at a time of their choosing and type in a 12-digit code that launched a special interface and allowed them to withdraw cash from the infected machine.
The hacked interface displayed the amount of money available in each denomination of bill, allowing the attackers to focus on the highest value banknotes to minimize the amount of time they were exposed.
The attacks on the ATMs of an unnamed European bank earlier this year were outlined at the hacker-themed Chaos Computing Congress in Hamburg, Germany.
The thefts were discovered in July when the bank found several its ATMs were being repeatedly emptied despite their use of internal safes to protect the cash inside. No details were provided about the amount of money stolen or number of cash machines involved.
The criminals displayed "profound knowledge of the target ATMs" and had gone to great lengths to make their malware code difficult to analyze, security researchers said.