You’re ready to head to work on a warm summer morning. But when you slip behind the wheel and press the car’s Start button, nothing happens. Nothing, that is, until a message pops up on the touchscreen monitor that tells you that your car has been hacked—and if you want to use it again you’ll have to pay an anonymous hacker.

So-called ransomware is becoming an increasingly common threat for both personal and corporate computer users, and it could soon become an unwelcome reality for motorists, as well. And it’s just one of the many cybersecurity threats facing the auto industry—and motorists around the world.

The automobile has become a “supercomputer on wheels,” according to Mark Rosekind, the head of the National Highway Traffic Safety Administration, and just like any other computer, that’s posing a tempting target for hackers, he said during a day-long cybersecurity conference in Detroit last Friday.

Rosekind isn’t the only one worried. Other speakers at the event warned of a variety of threats hackers could pose as they figure out how to gain access to the many electronic systems going into today’s cars:

As with home computers, some hackers will be looking for personal information, whether credit card numbers or a social security number that can be used to steal your identity

Other hackers might try to take control of your car, especially as autonomous vehicle technology begins coming to market. They could cause a crash, send you to the wrong destination, or even hijack your vehicle, perhaps taking a victim hostage

Hackers could shut down not just your vehicle but perhaps hundreds, even thousands of vehicles. If that happened on a highway, it would bring traffic to a halt

That’s an especially frightening scenario if you’re talking about hackers taking control of a major truck fleet, said Assistant U.S. Attorney General John Carlin, another speaker at the Detroit cybersecurity conference. That’s one of the biggest potential threats to its infrastructure America faces, he said, on a par with shutting down the electric grid.

But there are other serious concerns, according to Carlin. He said terrorists, in particular, may try to take remote control of a car or truck and use the vehicle as a deadly weapons—much as an ISIS disciple drove a truck into a crowd in Nice, France during Bastille Day earlier this month. That attack killed 84 people.

That possibility was made all the more real last year when two security experts hacked into a Jeep Cherokee, took remote control, and drove the vehicle into a ditch.