Capitalising on spying tools believed to have been developed by the U.S. National Security Agency, the cyber assault has infected tens of thousands of computers in almost 100 countries, with Britain's health system suffering the worst disruptions.
The attack has affected the National Health Service in the United Kingdom, one of Russia's interior ministry and one of its biggest financial institutions, Sberbank, and computers in the Ukraine, Spain, Italy India and China.
Pictures posted on social media showed screens of NHS computers with images demanding payment of 300 (275 euros) in Bitcoin, saying: 'Ooops, your files have been encrypted!'
Britain's National Health Service was badly hit, declaring a "major incident" after the attack, which forced some hospitals to divert ambulances and scrap operations.
A large cyberattack crippled computer systems at hospitals across England on Friday, with appointments canceled, phone lines down and patients turned away. "We are implementing remediation steps as quickly as possible", it said in a statement.
The security firm and others have linked WannaCry to a NSA hacking code known as "Eternal Blue" that was leaked last month by hacking group Shadow Brokers.
Several companies in Spain were hit by ransomware attacks, with telecoms firm Telefonica one of those reporting problems.
Less was known about the scope of the attacks in Spain and Portugal, which affected companies like Telefónica.
On Friday evening, Hampshire Hospitals NHS Trust was warning patients against using A&E unless completely necessary.
The security researcher, on seeing that the ransomware called out to this unregistered domain, immediately registered it so they could monitor the traffic (they could - producing the map above).
The head of Turkey's Information and Communication Technologies Authority or BTK says the nation was among those affected by the ransomware attack.
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There are many types of malware that affect a computer ranging from those that steal your information to those that just delete everything on it. Ransomware, on the other hand, prevents users from accessing their devices and data till a certain amount is paid to its creator as ransom.
British Home Secretary Amber Rudd said that Friday's cyber attack has affected 48 of the 248 NHS trusts in England, with all but six now back to normal.
Only a small number of US -headquartered organizations were hit because the hackers appear to have begun the campaign by targeting organizations in Europe, said Thakur.
Theresa May said the Government is not aware of any evidence that patient records have been compromised in the massive cyber attack.
At the time, some cyber-security experts said some of the malware was real, but old.
But a spokesperson for the West Herts Hospital NHS Trust said the failures are a "separate matter" to the malware attack which has caused disruption at 45 NHS sites across the country over the past couple of days.
The attack was carried out by a Wanna Decryptor virus which took over computers and demanded money payment.
Private security firms identified the ransomware as a new variant of "WannaCry".
Some of the hospitals tweeted that they were taking precautionary measures to protect patients.
The cyberattack was first reported from Sweden, Britain and France.
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