“An unprecedented ‘‘ransomware’’ cyberattack that has already hit tens of thousands of victims in 150 countries could wreak greater havoc as more malicious variations appear and people return to their desks Monday and power up computers at the start of the workweek.

As a loose global network of cybersecurity experts fought a rearguard battle against ransomware hackers, officials and experts on Sunday urged organizations and companies to update operating systems immediately to ensure they aren’t vulnerable to a second, more powerful version of the software — or to future versions that can’t be stopped.

The initial ransomware attack, known as ‘‘WannaCry,’’ paralyzed computers that run Britain’s hospital network, Germany’s national railway and scores of other companies and government agencies worldwide in what was believed to be the biggest online extortion scheme ever recorded.”

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Author: Sylvia Hui and Christopher S. Rugaber, Associated Press