A US IT company urged customers to shut down their servers Friday after cyberattackers smuggled ransomware onto its network platform.
Kaseya said Friday evening that 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."
Cybersecurity firm Huntress Labs had earlier said in a Reddit forum that it was working with partners targeted in the attack, and that some 200 businesses "have been encrypted."
Ransomware attacks typically involve locking away data in systems using encryption, making companies pay to regain access.
Kaseya describes itself as a leading provider of IT and security management services to small and medium-sized businesses.
VSA, the company's flagship offering, is designed to let companies manage networks of computers and printers from a single point.
The company became aware of a possible incident with VSA at midday on the US east coast and "immediately shut down" its servers as a "precautionary measure," it said.
Kaseya also "immediately 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.
The US Cybersecurity and Infrastructure Security Agency (CISA) put out word that it was "taking action to understand and address the recent supply-chain ransomware attack" against Kaseya VSA and the service providers using its software.
CISA called on businesses to follow Kaseya's guidance and quickly shut down VSA servers to avoid having systems compromised.
Kaseya lists a US headquarters in Florida and an international headquarters in Ireland.
The UN Security Council this week held its first formal public meeting on cybersecurity, addressing the growing threat of hacks to countries' key infrastructure -- an issue US President Joe Biden recently raised with Russian counterpart Vladimir Putin.
Several Security Council members acknowledged the grave dangers posed by cybercrime, notably ransomware attacks on key 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.