Unknown hackers remotely changed the thermostats on fridges and freezers in hundreds of French supermarkets overnight on Sunday to Monday. Food thawed and, in some places, cooked. Up to 700 stores of the Intermarché, Coopérative U, Carrefour, Leclerc and Monoprix chains were hit.

In a few hours, the temperature in freezers jumped from -27C to 50C or more. At one store in Normandy it reached 80C. Fires broke out, the sprinklers came on and the water caused more damage.

At Coopérative U alone, 80 of its 2,000 stores were hit. An employee at an Intermarché store in Issy-les-Moulineaux, near Paris, said staff threw out all the stock. He called what happened a disaster and a huge waste.

Intermarché chief executive Thierry Cotillard put the losses at tens of millions of euros. The exact cost is still unclear. The companies say insurance is unlikely to cover it.

Coopérative U president Dominique Schelcher called the attack unprecedented, unexpected and extremely serious. He said he had never seen anything like it in his whole career.

Early findings from the investigation suggest the hackers broke into controllers made by the Danish firm Danfoss, which set the temperature in refrigeration equipment. To avoid being spotted at once, they switched off the alarms or lengthened the defrost cycles. Danfoss said the weak points have been known since last year. It advised customers to update their software or replace old systems.

Cybersecurity specialist Marta Rybczynska believes many freezers run on old software or factory-set passwords. That makes them easy to get into, she says, and the hackers found the weak spots and used them.

No one has claimed responsibility for the attack. Jérôme Billois, a cybersecurity expert at Wavestone, sees two possibilities: young hackers who want to show off their skills, or sabotage by state actors. Other experts do not rule out links to international players such as Russia or Iran.

Billois says the most likely scenario is an attempt to destabilise France, by making people feel unsafe and that the government has lost control, in order to spread fear. He calls the story "strange" and says the blame is shared between the supermarkets, maintenance firms and software makers.

The problem is freezers linked to the internet without proper protection, and it is a warning to everyone who uses such equipment.

The attack is the latest in a series of hits on French institutions over the past three years. Hackers have broken into the health system, the jobs agency France Travail and the Ministry of the Economy and Finance, from which data on 678,000 taxpayers leaked in summer 2025. According to cybersecurity firm Upguard, France is the country most often attacked by hackers in Europe.