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| The ransomware has been identified as WannaCry - here shown in a safe environment on a security researcher's computer |
A massive cyber-attack using tools
believed to have been developed by the US National Security Agency has
struck organisations around the world.
Microsoft released a patch for the vulnerability in March, but many systems may not have been updated.
How big is the attack?
There have been reports of infections in 99 countries, including the UK, US, China, Russia, Spain, Italy and Taiwan.Cyber-security firm Avast said it had seen 75,000 cases of the ransomware - known as WannaCry and variants of that name - around the world. "This is huge," said Jakub Kroustek at Avast.
Many researchers say the incidents appear to be linked, but say it may not be a coordinated attack on specific targets.
Meanwhile wallets for the digital cryptocurrency Bitcoin that were seemingly associated with the ransomware were reported to have started filling up with cash.
Who has been affected?
The UK's National Health Service (NHS) has been hit and screenshots of the WannaCry program were shared by NHS staff.Hospitals and doctors' surgeries were forced to turn away patients and cancel appointments. One NHS worker told the BBC that patients would "almost certainly suffer" as a result.
Some reports said Russia had seen more infections than any other single country. Russia's interior ministry said it had "localised the virus" following an "attack on personal computers using Windows operating system".