As cars become more like PCs on wheels, what’s to stop a hacker from taking over yours?

In recent demonstrations, hackers have shown they can slam a car’s brakes at freeway speeds, jerk the steering wheel and even shut down the engine — all from their laptop computers.

The hackers are publicising their work to reveal vulnerabilities present in a growing number of car computers. All cars and trucks contain anywhere from 20 to 70 computers. They control everything from the brakes to acceleration to the windows, and are connected to an internal network. A few hackers have recently managed to find their way into these intricate networks.

In one case, a pair of hackers manipulated two cars by plugging a laptop into a port beneath the dashboard where mechanics connect their computers to search for problems. Scarier yet, another group took control of a car’s computers through cellular telephone and Bluetooth connections, the compact disc player and even the tire pressure monitoring system.

To be sure, the “hackers” involved were well-intentioned computer security experts, and it took both groups months to break into the computers. And there have been no real-world cases of a hacker remotely taking over a car. But experts say high-tech hijackings will get easier as automakers give them full Internet access and add computer-controlled safety devices that take over driving duties, such as braking or steering, in emergencies. Another possibility: A tech-savvy thief could unlock the doors and drive off with your vehicle.