The potential for breaches is escalating as cars transform into Internet-connected computers. A report from Verizon last November found that 14 car manufacturers accounted for 80 percent of the worldwide auto market, and each one has a connected-car strategy. Security experts say one remote hacking of an Uber vehicle could spell disaster for the ride-hailing company.

Mr. Miller and Mr. Valasek have made car hacking a focus. In August, the two demonstrated at the Black Hat and Def Con hacking conferences a way to control hundreds of thousands of vehicles remotely. Over the Internet, they were able to track down cars by their location, see how fast they were traveling and manipulate their blinkers, lights, windshield wipers, radios and navigation and, in some cases, control their brakes and steering.

Mr. Miller, a former “global network exploitation specialist” for the National Security Agency, most recently worked at Twitter. He was hired there after making a name for himself by exploiting Apple- and Android-powered devices.

Two years ago, he and Mr. Valasek turned their attention to cars, because cars were a more tangible target, they said, and because of the increasing momentum behind Internet-connected vehicles.

“I’ve been in security for more than 10 years, and I’ve worked on computers and phones. This time, I wanted to do something that my grandmother would understand. If I tell her, ‘I can hack into your car,’ she understands what that means,” Mr. Miller said in an interview last month.