May hack on worlds' computers cost billions as hackers attempted to blackmail companies and governments.
The Trump administration has publicly blamed North Korea for unleashing the so-called WannaCry cyber attack that crippled hospitals, banks and other companies worldwide in May.
The May cyberattack was for profit, with the authors using ransomware to demand money to unblock infected computers. The attack was widespread and cost billions says Tom Bossert, homeland security adviser to president Donald Trump, in an article in the Wall Street Journal.
British banks, French carmakers, Spanish telecoms companies, American distribution firms, no-one was safe if they were using vulnerable software.
North Korea had already been blamed for the attack in October by the British government, where the National Health Service's computers were particularly badly damaged by the hack.
Some international security experts have agreed, with antivirus editor Symantec naming hacking group Lazarus, suspected of having links with North Korea.