Researchers have found smart meters that are widely used in Spain can be hacked to under-report energy use.
The researchers have since warned that poorly protected credentials inside the devices could let the hackers take control of the gadgets.
The discovery comes as one security expert warns some terror groups may attack critical infrastructure systems.
The utility that deployed the meters is now improving the devices' security to help protect its network.
Independent researchers Javier Vidal and Alberto Illera worked together to find the flaws in the smart meters. Vidal said: "We took them apart to see how they work.
"We suspected there could be some issues with them and we wanted to check.
"We feared the security would be easy to break and we confirmed that."
The pair found keys buried inside the on-board software, that were used to scramble information that the smart meter shares with nodes sitting higher in the power distribution system.
Using the keys and the unique identifier associated with each meter it then became possible for the researchers to spoof messages being sent from the power-watching device to a utility company.
Attackers were able to use what the researchers found to under-report energy meter readings; creating a cheaper bill at the end of the month. The researchers believe with more work it might be possible to seek out meters and cut off the power they are supplying.
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