The switch was hardcoded into the malware in case the creator wanted to stop it spreading. This involved a very long nonsensical domain name that the malware makes a request to – just as if it was looking up any website – and if the request comes back and shows that the domain is live, the kill switch takes effect and the malware stops spreading.
“I saw it wasn’t registered and thought, ‘I think I’ll have that’,” he is reported as saying. The purchase cost him $10.69. Immediately, the domain name was registering thousands of connections every second.
“They get the accidental hero award of the day,” said Proofpoint’s Ryan Kalember. “They didn’t realize how much it probably slowed down the spread of this ransomware.”