I've recently seen a presentation about attacks on a widely used system used in current cars.

There are a couple of different ways that are significantly better than brute force (as in: works against real cars in seconds to a few minutes). (Though since key length is only 48 bit, even brute forcing might be practical.)

This might interest you:

https://www.usenix.org/system/files/conference/usenixsecurit...