Although his discovery did not fix the damage done by the ransomware, it did stop it spreading to new computers, and he has been hailed an "accidental hero".
Updated windows 10 system are safe now as per the Microsoft latest update. Experts suggested Saturday that the ransomware's progress had been halted, but new attacks could soon follow.
British cybersecurity expert Graham Cluley doesn't want to blame the NSA for the attack.
A huge cyberattack brought disruption to Britain's health system on Friday and infected many Spanish companies with malicious software, and security researchers said a dozen other countries may be affected. The ransomware spreads easily when it encounters unpatched or outdated software.
The 22-year-old man, known by the pseudonym MalwareTech, had taken a week off work, but made a decision to investigate the ransomware after hearing about the global cyber-attack.
But he also placed fault in the governments.
However, a hacker could change the code to remove the domain and try the ransomware attack again.
But MalwareTech now thinks it was not a kill switch at all, 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.
"Once inside the system, the attackers install a rootkit, which enables them to download the software to encrypt the data".
The perpetrators have yet to be identified and the National Crime Agency (NCA), which is also part of the investigation, has said that there is "no indication that United Kingdom policing or other government departments have been infected with the ransomware".
In Britain, patients of the state-funded country-wide service are facing days of chaos as appointments and surgeries were cancelled after almost 45 NHS organisations from London to Scotland were hit in the "ransomware" attack on Friday. "That's not to say that the attacks are new - it's a repercussion of what happened on Friday". A group of hackers spreading massive RansomWare attack all over the world to hack many Windows laptop and computer users.
The virus exploits a vulnerability in Microsoft Windows software, first identified by the US National Security Agency.
Computers around the globe were hacked beginning on Friday using a security flaw in Microsoft's Windows XP operating system, an older version that was no longer given mainstream tech support by the U.S. giant.
Defense minister Michael Fallon told the BBC the government under Prime Minister Theresa May was spending around 50 million pounds on improving the computer systems in the NHS after warning the service that it needed to reduce its exposure to "the weakest system, the Windows XP". The initial demand was for $300 in bitcoins, but it now has gone up to $600 worth of the currency, Gazeley said.
The cyberattack affected 16 organizations that are part of the National Health Service on Friday, causing some surgical procedures to be canceled and ambulances diverted.
NHS bosses are now scrambling to restore systems by tomorrow morning, but insisted no patient data had been compromised.
NHS England said patients needing emergency treatment should go to Accidents & Emergency (A&E) or access emergency services as they normally would.
Jan Op Gen Oorth, spokesman for the Netherlands-based Europol, said the number of individuals who have fallen victim to the cyberextortion attack could be much higher.
"There are other criminals who've launched this attack, and they are ultimately responsible for this", he said.
"I have built myself up for the last two weeks", he told CNN.
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