Les Mousquetaires, owner of retailer Intermarché, said fridges at around 150 of its stores were heated to 50 degrees Celsius overnight for six hours.
Hackers raised the temperature of refrigerators and freezers at hundreds of French supermarkets to as high as 50 degrees Celsius overnight, causing fires and spoiling food.
The Paris prosecutor's office said on Monday that a "large-scale" attack was "affecting connected refrigerators in several major food retailers across France," leading to food losses and fires.
A person in his office said Prime Minister Sebastien Lecornu has instructed France's National Cybersecurity Agency (ANSSI) to investigate.
France was coordinating with European counterparts because other countries could also be affected, the person said, adding that authorities were assessing whether other French companies were at risk.
Felix Aime, a senior cyber threat intelligence analyst at Sekoia Threat Detection and Research, said Spain, Portugal, Poland, Germany and Belgium had also been affected.
Carrefour said 80 of its supermarkets were hit, while Monoprix, which operates about 490 stores in France, said 5% of its stores were affected.
Les Mousquetaires, owner of retailer Intermarché, said fridges at around 150 of its stores were heated to 50 degrees Celsius overnight for six hours. A spokesperson said all affected food had been discarded to eliminate health risks.
Paris resident Geert de Clercq witnessed staff at an Intermarché in central Paris throw cartfuls of defrosted frozen food into a giant trash container, with molten ice cream sloshing out of broken tubs onto the street.
Cooperative U CEO Dominique Schelcher said his supermarket group had also been targeted.
"Targeting equipment essential to the preservation of fresh products and to the food supply of the French people is of extreme gravity and is unprecedented," he wrote on X/Twitter.
Food retailers E.Leclerc and Auchan did not respond to requests for comment.
No immediate estimate of industry-wide losses, but Intermarché CEO Thierry Cotillard told BFM TV the "unprecedented" attack would cause losses of "tens of millions of euros nationwide in a single day."
Christel Delberghe, director general at European retail association Eurocommerce, said, "This incident underlines the importance of resilience across the entire supply chain."
Supermarket cold aisles controlled over internet
Supermarket cold aisles are typically run by control panels that manage an entire store's fridges and freezers and can be accessed over the internet, which lets maintenance companies adjust them from anywhere.
The source close to Lecornu said ANSSI had identified a vulnerability in software supplied by Danish company Danfoss.
Danfoss said it was "aware of incidents in France affecting a limited number of refrigeration management systems."
The affected units "were running outdated firmware," Danfoss said, adding that it had "long advised customers to keep supported systems updated with the latest firmware and to migrate legacy systems to current-generation solutions."
On August 9, cybersecurity firm Claroty's Team82 published research detailing vulnerabilities in the Danfoss AK-SM 800A controller. According to the report, Danfoss software updates addressed issues that could have let attackers bypass authentication controls and execute code on compromised devices.
The attacks resemble disruptions reported at about a dozen US military commissaries in late August that led to food losses after fridges were affected.
In a statement, the French retail association Fédération du Commerce et de la Distribution called on Danfoss for "full transparency" about the incident. It said supermarkets aimed to protect consumers and return to normal operations as soon as possible. The FCD said some impacted stores were already back up and running.
Dutch retailer faces security issue, takes several systems offline as precaution
Dutch retailer Ahold Delhaize, which operates the Netherlands' largest supermarket chain Albert Heijn and has major operations in the US, said it was dealing with a "security issue" on its network and had taken several systems offline as a precaution.
A message in the retailer's smartphone app said it couldn't deliver groceries because of a "cybersecurity incident."
Online orders account for about 10% of Albert Heijn sales. Physical stores remained open, while Dutch news agency ANP reported that rival retailers had seen a surge in delivery orders.
Ahold Delhaize said there was no reason to link the incidents in France to its supermarkets.
Ahold is the fourth-largest grocery retailer in the United States. Websites for its Stop & Shop, Giant, Hannaford and Food Lion chains displayed notices saying "site temporarily down."
It was unclear whether other operations had been affected.