A massive cyber-attack using tools believed to have been developed by the US National Security Agency has struck organisations around the world.
Computers in thousands of locations have been locked by a programme that demands $300 (£230) in Bitcoin.
In April hackers known as The Shadow Brokers claimed to have stolen the tools and released them online.
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.