Hackers have raised refrigerator and freezer temperatures at hundreds of French supermarkets to as high as 50 degrees Celsius, causing fires and spoiling food.
The Paris prosecutor's office said a "large-scale" attack was "affecting connected refrigerators in several major food retailers across France", leading to food losses and fires.
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, a spokesperson for his office said, adding that authorities were assessing whether other French companies were at risk.
A senior cyber threat intelligence analyst says Spain, Portugal, Poland, Germany and Belgium have also been affected.
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 CARR.PA said 80 of its supermarkets were hit, while Monoprix, which operates about 490 stores in France, said 5 per cent of its stores were affected.
Les Mousquetaires, owner of retailer Intermarché, said fridges at about 150 of its stores were heated to 50 C overnight for six hours.
A spokesperson said staff discarded all affected food to eliminate health risks.
Paris resident Geert de Clercq witnessed staff at an Intermarché in central Paris throwing 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.
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 of the European retail association Eurocommerce, said: "This incident underlines the importance of resilience across the entire supply chain."
Cold aisles controlled over internet
Supermarket cold aisles typically run on control panels that manage an entire store's fridges and freezers and can be accessed over the internet, letting 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 stores were already back up and running.
Dutch retailer deals with security issue
Dutch retailer Ahold Delhaize AD.AS, 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 per cent 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 US.
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.
Reuters