The bug had only affected Windows systems, as the malware used a Microsoft update to "spread across networks".
Britain's National Cyber Security Center says teams are working "round the clock" to restore hospital computer systems after a global cyberattack that hit dozens of countries forced British hospitals to cancel and delay treatment for patients.
NHS systems were infected with a ransomware virus, which locked down machines and access to patient records, and demanded money to release them.
An worldwide effort is under way to track down the criminals behind an unprecedented global cyber attack that wreaked havoc across the NHS. It has set off fears that the effects of the continuing threat will be felt for months, if not years.
NHS trusts have come under fire for failing to stop using outdated software that left them at risk.
The brunt of the attacks was felt in Russian Federation, including the country's largest mobile phone company. "Despite people's best efforts, this vulnerability still exists, and people will look to exploit it".
Experts said another attack could be imminent and warned people to ensure their security is up to date.
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However, without yet knowing who or which groups are behind the attack, experts are wary of assigning motive beyond extortion.
This is a virus that attacked Windows platforms...
MalwareTech, who wants to remain anonymous, was hailed as an "accidental hero" after registering a domain name to track the spread of the virus, and actually ended up stopping it by finding a hidden "kill switch" in the virus' code. The URL might have been a command and control server or an intentional kill switch. "But it's only temporary".
"I instantly noticed it queried an unregistered domain, which I promptly registered", MalwareTech writes. He said it was "incredibly important that any unpatched systems are patched as quickly as possible".
At least one Australian business has fallen victim to the attack and there are investigations into two other reports, the federal government says. At the same time, national legal systems were not created to handle such global crimes.
The wave of cyber attacks wreaking havoc cross the globe is "at an unprecedented level", European Union's law enforcement agency Europol said yesterday. Microsoft had already released a patch for the vulnerability back in March but many users had not updated their OS.