French investigators were hunting the hackers on Tuesday who had seized control of cooling units at hundreds of supermarkets in a cyberattack that underscored the vulnerabilities of connected systems, with no group having publicly claimed responsibility.

The lack of a claim of responsibility from hacktivists or a ransom demand from cybercriminals has puzzled investigators, according to a source familiar with the probe. Such groups typically seek to monetize their hacks or claim credit for them for political purposes.

The cyberattack, which manually overrode the temperatures at fridges in supermarket groups, appears to have been easy to pull off, three cybersecurity experts told Reuters.

Staff at some Paris supermarkets threw boxes of spoiled meat and frozen goods into large waste containers on Tuesday. At least 350 ​supermarkets were caught up in the strike.

The affected refrigerators relied on software from Danish firm Danfoss.

On Monday, Danfoss said many of the affected units had "reached end of life and no longer receive software or security updates" or were "running outdated firmware."

Serious vulnerabilities in older versions of Danfoss' firmware have been known for months.

"This is about basic cyber hygiene," said Ken Munro, a partner at British cybersecurity firm Pen Test Partners. "Get it wrong and you get a smelly mess of defrosted food."

He said the well-known vulnerabilities meant the attackers were essentially pushing at an open door, raising the possibility that the hack was the work of "kids messing around."

Many refrigerators remain accessible via the internet. Using the internet scanning service Shodan, Reuters found at least 4,500 Danfoss interfaces still viewable on Tuesday.

French internet scanning company Onyphe said only a small minority of the devices appeared to be running the latest version of the software.