Security experts say the worldwide cyberattack that hit computer systems in about 150 countries last week appears to have abated.
There had been fears that the ransomware could still impact computers today when office employees who left before the attack commenced Friday logged back on to start the work week.
But a Michigan researcher who helped stop the attack warns that those involved might be able to quickly circumvent the security measures put in place.
The attack affected hundreds of thousands of organizations including Fed Ex, a Spanish telecommunications company and hospitals in Britain.
The ransomware locked files on computers until users made a bitcoin payment to the hackers.
The hackers exploited a vulnerability exposed through documents leaked from the National Security Agency.
Microsoft officials say they patched the flaw months ago.
But it would still affect computers that did not have their software updated.
Russian President Vladimir Putin, for one, said his country had nothing to do with the attack.
Instead Putin blasted the NSA for developing the original code used in the attack, calling it an example of what happens when “a genie (is) let out of a bottle and then (turns on) its authors and creators.”
The spread ended when a Michigan researcher discovered a kill switch in the malware and shared the information with a British counterpart who had stopped the attack by registering a domain name.
Researchers warn, however, that the malware could be quickly re-written to avoid the fixes currently in place and still infect computers that have not had software patched or upgraded.
Ransomware is nothing new.
In just one example, last year officials with the Lansing Board of Water and Light revealed they paid hackers $25,000 to unlock its communications system.
Experts say the best defense against ransomware is to ensure a computer has strong spam filters and anti-virus programs, that all upgrades and patches are current, and that all data is backed up regularly.