Among the hardest hit systems are health networks, banks, telecommunications and government agencies. But cybersecurity experts said there are steps that people and organizations can take to avoid becoming victims.
The ransomware has hit more than 150 countries. Officials said WannaCry encrypts files on computers, forcing the owner to pay a ransom in bitcoin to get the files back.
"I think we're going to see more as people log into work this morning and tomorrow as they come into the office," said Jeff Stutzman, CEO of New Hampshire-based Wapack Labs.
Stutzman said ransomware is nothing new, but the speed with which this one struck beginning on Friday was unusual.
"Unbelievable -- the infrastructure that must have been set up to deploy this thing so quickly," he said.
Users of infected computers will see a red screen when they login. Stultzman said that anyone infected will probably have to pay the ransom, adding that the hackers can generally be trusted to deliver. Stutzman said his company paid $30,000 a few weeks ago on behalf of a client.
He said the key to stopping such attacks is prevention. There are three things that computer users should do immediately: Set computers to automatically update, get anti-virus software and back up computer hard drives externally.
"Pick a cloud provider," Stultzman said. "It doesn't matter which one. Don't map your network to it. Don't map your computer so it automatically does it."
Stutzman predicted that such attacks will only grow in number and sophistication.
"We've got botnets with thousands of computers attached," he said. "We've got these ransomware things with computers attached. How do we protect them when they come at us all at once?"
Stutzman, who serves on the governor's cybersecurity task force, said his company will set up a call center if needed, but it's currently offering solutions on its website.