PHILADELPHIA (CNN)–The biggest cyberattack the world has ever seen is still claiming victims and threatens to create even more havoc on Monday when people return to work.

The head of the European Union’s law enforcement agency Europol, speaking on British TV on Sunday, said Friday’s attack was “unprecedented” in its reach, with more than 200,000 victims in at least 150 countries.

“At the moment, we’re in the face of an escalating threat, the numbers are going up,” Europol director Rob Wainwright told ITV’s Peston on Sunday program. “I’m worried about how the numbers will continue to grow when people go to work and turn their machines on Monday morning.”

Organizations around the world have spent the weekend trying to recover after being hit by a virus that seeks to seize control of computers until victims pay a ransom.

Avoiding Cyber Attacks: What To Do If Ransomware Is In Your Machine

Hospitals, major companies and government offices were among those that were badly affected. Cybersecurity experts have said the majority of the attacks targeted Russia, Ukraine and Taiwan. But U.K. hospitals, Chinese universities and global firms like Fedex also reported they had come under assault.

Security experts said the spread of the virus had been inadvertently stopped late Friday. The ransomware was designed to repeatedly contact an unregistered domain in its code. A 22-year-old security researcher in the U.K, who goes by MalwareTech, registered that domain to analyze the attack, but it turned out the ransomware needed it to remain unregistered to keep spreading.

However, a hacker could change the code to remove the domain and try the ransomware attack again. And it has potential to create much more damage because it’s likely to be lurking on computers in offices around the world that haven’t been used since Friday. Copycat attacks could follow.

U.S. Treasury Secretary Steven Mnuchin, at a meeting in Italy, said Saturday the attack was a reminder of the importance of cybersecurity. “It’s a big priority of mine that we protect the financial infrastructure,” he said.

Europol’s Wainwright underscored the point Sunday. All sectors of the economy were vulnerable and organizations could take lessons from the banking industry, which appeared to have largely escaped the global attack.

“Very few banks if any have been affected because they’ve learned from painful experience of being the number one target for cybercrime,” he said.

The ransomware, called WannaCry, locks down files on an infected computer and asks the computer’s administrator to pay in order to regain control of them. The exploit was leaked last month as part of a trove of NSA spy tools.

The ransomware is spread by taking advantage of a Windows vulnerability that Microsoft released a security patch for in March. But computers and networks that hadn’t updated their systems were still at risk.

In the wake of the attack, Microsoft said it had taken the “highly unusual step” of releasing a patch for computers running older operating systems including Windows XP, Windows 8 and Windows Server 2003.

The patches won’t do any good for machines that have already been hit.

On Saturday, experts said it appeared that the ransomware had made just over $20,000, although they expected that number to pop when people went back into the office Monday.

Security agencies have so far not been able to identify who was behind the attack. Wainwright said Europol did not know the motive. He added that ransomware attacks were normally criminal rather than political in nature. “Remarkably few payments” had so far been made in response to this attack, he added.

WannaCry has already caused massive disruption around the globe.

Sixteen National Health Service organizations in the U.K. were hit, and some of those hospitals canceled outpatient appointments and told people to avoid emergency departments if possible.

Barts Health, which runs five hospitals in London, said Sunday it was still experiencing disruption to its computer systems and it asked for the public to use other NHS services wherever possible.

In China, the internet security company Qihoo360 issued a “red alert” saying that a large number of colleges and students in the country had been affected by the ransomware, which is also referred to as WannaCrypt. State media reported that digital payment systems at some gas stations were offline, forcing customers to pay cash.

Major global companies said they also came under attack. Fedex said Friday it was “experiencing interference with some of our Windows-based systems caused by malware.” Two big telecom companies, Telefónica of Spain and Megafon of Russia, were also hit, as was Japanese carmaker Nissan in the U.K.

Russia’s Interior Ministry acknowledged a ransomware attack on its computers. It said less than 1% of computers were affected, and that the virus was now “localized” and being destroyed.

U.K. defense secretary Michael Fallon said Sunday that Britain’s nuclear submarines were safe from cyberattack.

According to Matthew Hickey, founder of the security firm Hacker House, the attack is not surprising, and it shows many organizations do not apply updates in a timely fashion.

When CNNTech first reported the Microsoft vulnerabilities leaked in April, Hickey said they were the “most damaging” he’d seen in several years, and warned that businesses would be most at risk.

Consumers who have up-to-date software are protected from this ransomware. Here’s how to turn automatic updates on.

It’s not the first time hackers have used the leaked NSA tools to infect computers. Soon after the leak, hackers infected thousands of vulnerable machines with a backdoor called DOUBLEPULSAR.

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