By SYLVIA HUI, SARA BURNETT and ALLEN G.
It appears networks of computers, like schools, companies, hospitals and businesses, are particularly vulnerable.
Darien Huss, a 28-year-old research engineer who helped stop the malware's spread, said he was "still anxious for what's to come in the next few days, because it really would not be so hard for the actors behind this to re-release their code without a kill switch or with a better kill switch".
"I think we need to keep a much closer eye on what government agencies are doing with these cyber weapons because they could've tipped off the government, they could have tipped off users of these operating systems but they didn't, they kept those exploits to themselves". It may have saved governments and companies millions of dollars and slowed the outbreak before US -based computers were more widely infected.
Copycat versions of the malicious software have begun to spread, according to experts.
Darien Huss is a 28-year-old research engineer for the cybersecurity firm Proofpoint. What they didn't realize was that the domain - a random assortment of letters - was actually a kill switch, a way for someone to take control of the ransomware.
A hacking group called Shadow Brokers released the malware in April claiming to have discovered the flaw from the NSA, Kaspersky said.
"All GPs surgeries did open, though some of them had to use pen and paper".
The researcher, identified online only as MalwareTech, explained that he spotted a hidden web address in the "WannaCry" code and made it official by registering its domain name.
"There are other criminals who've launched this attack, and they are ultimately responsible for this", he said.
"We haven't fully dodged this bullet at all until we're patched against the vulnerability itself", Kalember said.
"There's nothing you can do but pay once you're hit", Camacho said in an interview.
That prompted the company to issue another patch on Friday for older and unsupported operating systems such as Windows XP, allowing users to secure their systems without requiring an upgrade to the latest operating software. The tools appeared stolen by hackers, who dumped them on the internet. Britain canceled or delayed treatments for thousands of patients, even people with cancer.
People walk past a Megafon mobile phone shop in Moscow, Russia on May 13. Renault's futuristic assembly line in Slovenia, where rows of robots weld auto bodies together, was stopped cold. Director Dennis So said the systems were not connected to the malls' or tenants' networks.
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So far, the ransomware has hit more than 200,000 computers worldwide.
The U.K.'s National Cyber Security Center was "working round the clock" to restore vital health services, while urging people to update security software fixes, run anti-virus software and back up their data elsewhere.
In a statement provided to HuffPost Australia on Sunday, Special Adviser to the Prime Minister on Cyber Security Alastair MacGibbon said, overall, "Australia has not been significantly affected" by the attack, although there are reports of other businesses being targeted.
Cybersecurity experts said Friday that the Wannacry ransomware shows no signs of slowing down.
The attack held hospitals and other entities hostage by freezing computers, encrypting data and demanding money through online bitcoin payments.
"The assault, which began Friday and was being described as the biggest-ever cyber ransom attack, struck state agencies and major companies around the world - from Russian banks and British hospitals to FedEx and European vehicle factories".
"I don't think it's to do with that preparedness".
The MalwareTech researcher agreed that the threat hasn't disappeared.
However the hackers are widely expected to try again.
Ryan Kalember, senior vice president at Proofpoint Inc. which helped stop its spread, said the version without a kill switch could spread. "They're processing a lot of sensitive data".
Computer experts said the virus spread quickly because it used a digital code believed to have been developed by the U.S. National Security Agency.
A spokesman for Microsoft did not return an email asking for comment, but Microsoft released a patch for the exploit in March. AP reporter Jim Heintz contributed from Moscow.
Burnett reported from Chicago and Breed from Raleigh, North Carolina.