Cyber criminals released a virus which encrypted NHS data and demanded a ransom payable in bitcoin - an internet currency - to have them decrypted, with the threat of deletion if the money was not handed over.
How many countries were affected?
Cyber security expert Ollie Taylor told Premier cyber attacks have become more sophisticated, explaining: "It's now possible to take advantage of existing connectivity between computers in business networks and, in this case, it's relating to file-sharing technology that's built into the operating system".
The attack was one of the largest ransomware attacks in history.
United States package delivery giant FedEx, European vehicle factories, Spanish telecoms giant Telefonica, Britain's health service and Germany's Deutsche Bahn rail network were among those hit.
Had it not been for a young cybersecurity researcher's accidental discovery of a so-called "kill switch", the malicious software likely would have spread much farther and faster that it did Friday. The attack is unique, according to Europol, because it combines ransomware with a worm function, meaning once one machine is infected, the entire internal network is scanned and other vulnerable machines are infected.
In the U.S. FedEx was hit.
So far, the culprits are unknown, as is the motivation.
Researchers with security software maker Avast said they had observed 57,000 infections in 99 countries with Russia, Ukraine, and Taiwan the top targets.
Hackers can still gain easy access to personal computers that lack a security update issued in March by Microsoft to fix the vulnerability in its Windows operating system.
"More widely we ask people to use the NHS wisely while we deal with this major incident which is still ongoing", said Dr Anne Rainsberry, NHS England's NHS incident director. His predictions are based on the fact that during the first day of the next working week, when people go back to work and turn on their computer, the virus will be even easier to expand, he lamented.
Europol provides free decryption downloads for most ransomware already detected, though not yet for this particular attack.
"We have not yet seen Friday's attack reoccur, there's been no new wave of attacks".
Forth Valley medical staff are being told to remain vigiliant following last week's cyber attack which hit NHS computers the length and breadth of the UK.
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