US President Biden orders full investigation, saying "the initial thinking was it was not the Russian government, but we're not sure yet."
A ransomware attack on a US IT company potentially targeted 1,000 businesses, researchers have said, with one of Sweden's biggest supermarket chains revealing it had to temporarily close around 800 stores after losing access to its checkouts.
Russian-based hackers have been blamed for a string of ransomware attacks, and US President Joe Biden recently raised the threat in talks with Russian counterpart Vladimir Putin.
Biden ordered a full investigation on Saturday, while adding "the initial thinking was it was not the Russian government, but we're not sure yet."
"I'll know better tomorrow, and if it is either with the knowledge of and/or a consequence of Russia, then I told Putin we will respond," he said.
The IT company targeted, Kaseya, said on Friday evening it had limited the attack to "a very small percentage of our customers" who use its signature VSA software, "currently estimated at fewer than 40 worldwide."
But cybersecurity firm Huntress Labs said in a Reddit forum that it was working with partners targeted in the attack, and that the software was manipulated "to encrypt more than 1,000 companies."
Scale of attack could be 'without precedent'
Ransomware attacks typically involve locking away data in systems using encryption, making companies pay to regain access.
Brett Callow, an analyst for cybersecurity company Emsisoft, said it remained unknown how many companies were affected and said the scale of attack could be "without precedent."
Kaseya describes itself as a leading provider of IT and security management services to small and medium-sized businesses. VSA is designed to let companies manage networks of computers and printers from a single point.
"One of our subcontractors was hit by a digital attack, and that's why our checkouts aren't working any more," Coop Sweden, which accounts for around 20 percent of the country's supermarket sector, said in a statement.
"We regret the situation and will do all we can to reopen swiftly," the cooperative added.
Coop Sweden did not name the subcontractor or reveal the hacking method used against it.
But the Swedish subsidiary of the Visma software group said the problem was linked to the Kaseya attack.
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Kaseya became aware of a possible incident with VSA at midday Friday on the US East Coast and "immediately shut down" its servers as a "precautionary measure," it said.
It also "notified our on-premises customers via email, in-product notes, and phone to shut down their VSA servers to prevent them from being compromised."
"We believe that we have identified the source of the vulnerability and are preparing a patch to mitigate it," the company said in a statement.
According to the New Zealand government's Computer Emergency Response Team, the attackers were from a hacking group known as REvil.
REvil was also, according to the FBI, behind last month's attack on JBS, one of the world's biggest meat processors, which ended with the Brazil-based company paying bitcoin worth $11 million to the hackers.
UN meeting on cybersecurity
The UN Security Council this week held its first formal public meeting on cybersecurity, addressing the growing threat of hacks to countries' key infrastructure.
Several Security Council members acknowledged the grave dangers posed by cybercrime, notably ransomware attacks on major installations and companies.
Multiple US companies, including the computer group SolarWinds and the Colonial oil pipeline, have also recently been targeted by ransomware attacks.
The FBI has blamed those attacks on hackers based in Russian territory.
But typically, "cybercriminals operate company by company," said Gerome Billois, a cybersecurity expert with Wavestone consultancy.
"In this case, they attacked a company that provides software for managing data systems, allowing them to simultaneously target several dozen, possibly even hundreds, of companies," he said.
Determining exactly how many is difficult, since affected companies lose their communications systems at the same time, Billois said.
And Kaseya, which had urged its clients to shut down servers running its VSA platform, cannot know whether systems were turned off "voluntarily or by force."
"This is one of the largest, most widespread ransomware attacks I've seen in my career," said Alfred Saikali of law firm Shook, Hardy & Bacon.
"I have never seen this many companies hire us in a single day for the same incident. As a general rule, you want to avoid paying the ransom at all costs."