A 22-year-old who lives with his parents stopped the worldwide malware hack by registering a domain for $10.69

The "accidental hero" who halted the global spread of an unprecedented ransomware attack by registering a garbled domain name hidden in the malware has warned the attack could be rebooted.

The ransomware used in Friday's attack wreaked havoc on organizations including FedEx and Telefónica, as well as the UK's National Health Service (NHS), where operations were cancelled, X-rays, test results and patient records became unavailable and phones did not work.

But the spread of the attack was brought to a sudden halt when one UK cybersecurity researcher tweeting as @malwaretechblog, with the help of Darien Huss from security firm Proofpoint, found and inadvertently activated a "kill switch" in the malicious software.

The researcher, who identified himself only as MalwareTech, is a 22-year-old from south-west England who works for Kryptos logic, an LA-based threat intelligence company.

"I was out having lunch with a friend and got back about 3 p.m. and saw an influx of news articles about the NHS and various UK organisations being hit," he told the Guardian. "I had a bit of a look into that and then I found a sample of the malware behind it, and saw that it was connecting out to a specific domain, which was not registered. So I picked it up not knowing what it did at the time."