NEW YORK (NYTIMES) - Hundreds of businesses around the world, including one of Sweden's largest grocery chains, grappled on Saturday (July 3) with potential cybersecurity vulnerabilities after a software provider that provides services to more than 40,000 organisations, Kaseya, said it had been the victim of a "sophisticated cyberattack."

Security researchers said the attack may have been carried out by REvil, a Russian cybercriminal group that the FBI has said was behind the hacking of the world's largest meat processor, JBS, in May.

In Sweden, grocery retailer Coop was forced to close at least 800 stores on Saturday, according to Sebastian Elfors, a cybersecurity researcher for security company Yubico.

Outside Coop stores, signs turned customers away: "We have been hit by a large IT disturbance and our systems do not work."

Elfors said a Swedish railway and a major pharmacy chain had also been affected by the Kaseya attack. "It's totally devastating," he said.

The attack became public on Friday, when Kaseya said that it was investigating the possibility that it had been the victim of a cyberattack.

The company urged customers that use its systems management platform, called VSA, to immediately shut down their servers to avoid the possibility of being compromised by attackers.

"We are experiencing a potential attack against the VSA that has been limited to a small number of on-premise customers only," Kaseya posted on its website, referring to organisations that keep their software at their own sites rather than housing it with a cloud provider.

"We are in the process of investigating the root cause of the incident with the utmost vigilance."

Fred Voccola, Kaseya's chief executive officer, said in a statement on Saturday that less than 40 customers had been affected by the attack, but those customers include so-called managed service providers, which can each provide security and tech tools to dozens or even hundreds of companies.

That has magnified the attack's severity, said John Hammond, a researcher at cybersecurity company Huntress Labs.

"What makes this attack stand out is the trickle-down effect, from the managed service provider to the small business," Hammond said.

"Kaseya handles large enterprise all the way to small businesses globally, so ultimately, it has the potential to spread to any size or scale business."

Some of the affected companies were being asked for US$5 million (S$6.7 million) in ransom, Hammond said. Thousands of companies were at risk, he said.

The United States Cybersecurity and Infrastructure Security Agency also described the incident in a statement on its website on Friday as a "supply-chain ransomware attack."

It urged Kaseya's customers to shut down their servers and said it was investigating.

Hackers have carried out a slate of prominent cyberattacks against US companies in recent months, including JBS and Colonial Pipeline, which moves fuel along the East Coast.

Both were ransomware attacks, in which hackers try to shut down systems until a ransom is paid.

Video game company Electronic Arts was also recently hacked, but its data was not held for ransom.