PARIS — France’s cybersecurity authority has warned that a cyberattack targeting supermarkets may still be ongoing as stores restocked Wednesday after the attack sent refrigerator temperatures soaring, raising concerns about the security of vital food supply chains.

The agency said Tuesday night that the attacks were “likely still underway.” It urged stores to reconnect refrigerators to the internet only through a secure VPN connection or protected network, install firmware updates and change passwords.

Attackers remotely took control of refrigeration systems in hundreds of supermarkets and independent stores across France in the early hours of Monday. Temperatures reached 122 degrees Fahrenheit for about six hours, forcing stores to discard fresh and frozen goods. The Intermarché supermarket chain reported losses of “tens of millions of euros” in a single day.

In the Netherlands, a major supermarket chain was restarting systems it had shut down as a precaution amid what parent company Ahold Delhaize called a “cybersecurity incident.” The incident halted home deliveries and disrupted some supplies to its extensive network of stores across the country.

Authorities in both countries were investigating the cyber incidents, which exposed vulnerabilities in some older refrigeration management systems and in the countries’ highly efficient food supply chains.

Authorities investigate

The Dutch National cybersecurity Center said several cyber incidents had affected Europe’s food sector, but it saw “no reason at this time to believe that these incidents are linked.”

The European Union’s police agency, Europol, confirmed to the Associated Press that it was in contact with authorities in the affected countries but declined to give details, citing an ongoing investigation.

No perpetrators have been publicly identified, and no claims of responsibility have been made public.

François Deruty, a former operations director at the French Cybersecurity Agency, said those responsible could range from sophisticated state-sponsored actors to a disgruntled French student. Protests by high school students angry about dilapidated, understaffed schools have roiled France since mid-September.

“I still can’t determine whether it’s being directed by someone or whether it’s an individual trying to take advantage of the prevailing chaos to show off their skills or send a message,” said Deruty, who is now chief intelligence officer at cybersecurity company Sekoia.

“I’m somewhat struck by the timing and the fact that politicians already have enough issues to deal with in France right now,” he added. “This adds another — one that not everyone will necessarily understand technically, but that resonates with all citizens in their everyday lives.”

A wake-up call

French Finance Minister Roland Lescure said that about 700 of the country’s 10,000 supermarkets were affected. He said there was no risk to France’s food supplies but acknowledged that the attack “touches the heart of our food security” and called it a “wake-up call for everyone.”

Deruty said that software vulnerabilities exploited in the hack in France “are an accident waiting to happen” and that “people simply hadn’t patched their equipment” with security updates.

The French cybersecurity agency said Tuesday that it was “aware of numerous compromises” involving refrigeration system remote control modules made by Danish company Danfoss as well as U.S.-based Copeland.

Danfoss has said that most affected systems in France were older products that no longer received software or security updates, and that it is working with affected customers to limit the damage.