A security researcher has told the BBC how he "accidentally" halted the spread of ransomware affecting hundreds of organisations, including the UK's NHS.
The man, known online as MalwareTech, was analysing the code behind the malware on Friday night when he made his discovery.
He first noticed that the software was trying to contact an unusual web address - iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com - but this address was not connected to a website, because nobody had registered it.
So, every time the malware tried to contact the mysterious website, it failed - and then set about doing its damage.
MalwareTech decided to spend $10.69 (£8) and claimed the web address. By owning the web address, he could also access analytical data and get an idea of how widespread the ransomware was.
But he later realised that registering the web address had also stopped the malware trying to spread itself.
"It was actually partly accidental," he told the BBC, after spending the night investigating. "I have not slept a wink."
Originally it was suggested that whoever created the malware had included a "kill switch" - a way of stopping it from spreading, perhaps if things got out of hand.
If that was the case, the act of registering the mysterious web address would trigger the kill switch.
But MalwareTech now thinks it was not a kill switch, but a way of detecting whether the malware was being run on a "virtual machine" - a secured, disposable environment that researchers use to inspect viruses.
While a real computer would not be able to access iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com, a virtual machine might have artificially responded that it was a genuine website.
"The malware exits to prevent further analysis," MalwareTech wrote in a blog post.
"My registration... caused all infections globally to believe they were inside a [virtual machine] and exit… thus we initially unintentionally prevented the spread and further ransoming of computers."
The researcher has been called an "accidental hero" for slowing the spread of the malware.
"I would say that's correct," he told the BBC.
While the registration of the web address appears to have stopped one strain of the malware spreading, it does not mean the ransomware itself has been defeated.
Any files that were scrambled by the ransomware will still be held to ransom.
Security experts have also warned that new variants of the malware that ignore the "kill switch" will appear.
"This variant shouldn't be spreading any further, however there'll almost certainly be copycats," said security researcher Troy Hunt in a blog post.
MalwareTech warned: "We have stopped this one, but there will be another one coming and it will not be stoppable by us.
"There's a lot of money in this, there is no reason for them to stop. It's not much effort for them to change the code and start over."