Cyberattacks on US banks, New York dam
Hackers demand 20,000 bitcoin units from each lender Reuters

At least three Greek banks have been hit by a team of hackers, who have demanded a ransom to be paid in bitcoin. The hackers, who call themselves Armada Collective, have threatened to paralyse the lenders' electronic systems if they do not heed their demands.

While the Greek central bank remains on high alert, authorities have scrambled a team of experts to deal with the situation. Reports suggest the perpetrators brought down the online banking operations of the three banks on Thursday, 26 November for several hours. A team comprising Greek intelligence agency operatives and cybercrime authorities managed to restore the system after a counter-operation.

"It was a serious threat," an unnamed official from the Bank of Greece told the AFP news agency. The hackers have demanded 20,000 bitcoin units (roughly £5m) of the electronic currency from each lender.

Customer details were not leaked in the last attack, which is thought to be the third such incident in five days. Initial attacks were thought to be Distributed Denial of Service (DDoS) – the rudimentary technique by which a website is flooded with traffic eventually bringing it down – but later they were able to penetrate the banks' websites.

"No bank responded to this extortion, so the same hackers tried again at the weekend and today [30 November]. But we had strengthened our defence in the meantime, so no disruptions took place," a bank official, who did not wish to be identified, told the Financial Times.

Hackers claiming to be from the Armada Collective are suspected to have attacked several private email servers as well in mid-November. Before that, the hackers were thought to have targeted banks in Thailand too.