Hackers remotely took over refrigeration systems at about 700 supermarkets in France, spoiling large amounts of fresh and frozen food, CNN and CBC News reported.

The hackers switched on defrost cycles and removed limits that normally stop temperatures from rising, CNN reported, citing a security analysis firm. Display cases reached 50 degrees Celsius (122 degrees Fahrenheit) for about six hours overnight Monday, and some caught fire, CNN reported.

French Finance Minister Roland Lescure told parliament on Tuesday that about 700 of the country's 10,000 supermarkets were affected, CBC News reported. He said there was no risk to France's food supply. "This is a wake-up call for everyone," Lescure said, CNN reported.

As many as 80 Carrefour stores were hit, along with 5% of Monoprix's 490 stores, CNN reported. Fridges at 150 shops of the Les Mousquetaires group were targeted, and about 80 Coopérative U stores were affected, CNN said.

The Paris prosecutor's office is investigating the "large-scale cyberattack," CNN reported. CBC News also reported the investigation. No group has claimed responsibility, and there was no ransom demand, both outlets reported.

The hackers got into controllers made by at least three firms, Danfoss, Copeland and Dixell, CNN reported, citing France's cybersecurity agency, ANSSI. Danfoss said many affected units were "running outdated firmware" or no longer got security updates, CBC News reported. A security firm flagged a flaw in Danfoss controllers in August, CBC News reported.

"There was no business need for the fridge controllers to be accessible on the public internet," Ken Munro of Pen Test Partners told CNN.

A French threat researcher told Reuters that Spain, Portugal, Poland, Germany and Belgium had also been affected, CNN reported.

What we know: Hackers heated fridges at about 700 French supermarkets through controllers left open online, and prosecutors are investigating.
What we don't: Who carried out the attack and why.