The intitial investigation is in.

In a “bizarre and unprecedented” cyber-attack, hackers had remotely overridden the temperature settings on thousands of freezers, fridges and chiller cabinets pushing them from as low as -27C (-16.6F) to well over 50C (122F).

If shop managers first assumed it was an isolated failure, it soon became clear up to 700 stores – including those from the Intermarché, Coopérative U, Carrefour, Leclerc and Monoprix chains along with a number of smaller independent shops – had been victims of a cyber-attack.

The hackers had pirated the controllers supplied by the Danish company Danfoss that controls the freezers and fridges. Most supermarket cold aisles are connected to the internet to allow remote maintenance and alerts for system failures. Initial investigations suggest the hackers disabled alarm systems before tampering with the thermostats, or extended the defrost programme by several hours to avoid triggering alarms.

Danfoss said it had identified security flaws last year and had “long recommended” that customers install updates or replace obsolete unsupported systems. The affected refrigeration units were “running outdated firmware”, it said. In the days following the first wave of attacks, France’s cyber-attack alert centre ANSSI confirmed supermarkets using controllers from two other companies were also targeted.