San Francisco uses McKay Guardian XLE meters, Grand said, but because these meters are implemented differently in different cities, his technique may not work outside of San Francisco.

Cities across the U.S. are rolling out computerized parking meter systems designed to be easier to pay and manage. San Francisco's smart meters were rolled out as part of a broader program, known as , which will eventually deploy parking sensors that can detect when a space is empty and transmit that information wirelessly to drivers looking for spots.

But there have been some problems. In May, about 125 smart meters in Chicago stopped working properly, prompting that the machines may have been hacked.

City officials attributed the failure to a computer glitch, and Grand said that the city's explanation sounds about right. "I think personally that the failures were a firmware problem, a bug in the system," he said.

Because they had never looked at parking meters before, Grand and his two co-researchers, Jacob Appelbaum and Chris Tarnovsky, also spent some time taking apart a parking meter they picked up on eBay and payment cards to understand how they work. They will their at the Black Hat security conference in Las Vegas Thursday.