A Controller Area Network (CAN) bus is present in nearly all modern cars, and is used by microcontrollers and other devices to talk to each other within the vehicle and carry out the work they are supposed to do. In a CAN injection attack, thieves access the network, and introduce bogus messages as if it were from the car's smart key receiver. These messages effectively cause the security system to unlock the vehicle and disable the engine immobilizer, allowing it to be stolen. To gain this network access, the crooks can, for instance, break open a headlamp and use its connection to the bus to send messages. From that point, they can simply manipulate other devices to steal the vehicle.
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