Companies including Hitachi and Nissan Motor Co. reported problems but said they had not seriously affected their operations.
"It's this constant battle", said Ryan O'Leary, vice president of WhiteHat Security's threat research center.
WannaCry locks up computers, encrypts their data, and demands large Bitcoin payments, which begin at US$300 (S$422) and rise to US$600 before the software destroys files hours later.
As a loose global network of cybersecurity experts fought the ransomware, the attack was disrupting computers that run factories, banks, government agencies and transport systems in scores of countries, including Russia, Ukraine, Brazil, Spain, India and Japan, among others. 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.
On social media, students complained about not being able to access their work, and people in various cities said they hadn't been able to take their driving tests because some local traffic police systems were down.
He also warned hackers could upgrade the virus to remove the "kill switch" that helped to stop it.
When the National Security Agency lost control of the software behind the WannaCry cyberattack, it was like "the USA military having some of its Tomahawk missiles stolen", Microsoft President Brad Smith says, in a message about the malicious software that has created havoc on computer networks in more than 150 countries since Friday.
The WannaCry or WannaCrypt ransomware attack deployed a Windows exploit that the National Security Agency had used for its own purposes until it was leaked in April by the hacking group Shadow Brokers. "This most recent attack represents a completely unintended but disconcerting link between the two most serious forms of cybersecurity threats in the world today - nation-state action and organized criminal action", he added.
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"The numbers are still going up", Wainwright said.
"The massive malware attack that hit multiple countries has caused chaos and has shut down vital institutions such as hospitals", U.S. Representative Ted Lieu (D-CA) said Friday in a statement.
The latest virus attack last week exploits a flaw in a version of Microsoft Windows first identified by USA intelligence.
It was too early to say who was behind the onslaught, which struck 100,000 organizations, and what their motivation was, aside from the obvious demand for money.
Researchers who helped prevent the spread of the malware and cybersecurity firms worked around the clock during the weekend to monitor the situation and install a software patch to block the worm from infecting more computers in corporations across the U.S., Europe and Asia.
"Right now, just about every IT department has been working all weekend rolling this out", said Dan Wire, spokesman at Fireeye Security.
"The ransomware virus is such lethal and smart that "it also drops a file named '!Please Read Me!.txt" which contains the text explaining what has happened (to the computer) and how to pay the ransom".
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