CHICAGO (CBS) — Companies in the United States and around the world were bracing for a second wave of problems from a weekend cyber attack on Monday, after a “ransomware” attack hit dozens of nations on Friday.

Experts said the WannaCry attack has spread to at least 150 countries.

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When it first surfaced late last week, many overseas businesses already had closed shop for the weekend. As those businesses began coming online again Monday, many were finding the ransomware had infected their systems.

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The hackers responsible for the attack were demanding $300 ransoms, threatening to erase all the data on infected computers if they aren’t paid.

More than 200,000 computers have been infected, and many more were at risk.

“We could have hundreds of thousands, potentially millions of computers that are turned on on Monday that could be vulnerable,” said Proofpoint senior security research engineer Darien Huss.

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It took Huss 10 minutes to disable the malware that hit computers over the weekend, helping contain the ransomware before hackers started spreading new strains. The ransomware been recoded, allowing computers to override the kill switch Huss and a British researcher helped turn on over the weekend to protect countless computers from the attack.