The "thiefproof" Immobilizer car key security system has been broken. From the article:

The systems reduce car theft, because vehicles will not start unless the system recognizes a tiny chip in the authorized key. They are used in millions of Fords, Toyotas and Nissans.

All that would be required to steal a car, the researchers said, is a moment next to the car owner to extract data from the key, less than an hour of computing, and a few minutes to break in, feed the key code to the car and hot-wire it.

More details available here, including the full academic paper (.pdf format).