I heard that the attacks that focus on the bus between chip's blocks (e.g. between CPU and functional blocks / memory blocks) are possible. The buses become the weak parts of hardware security. Then the secret such as secondary keys can be read out from the bus.
Is this kind of attack really achieved by someone or just staying at the theory stage?
If it is already a practical attack, how are the secrets read out from the bus, and is there any effective way to prevent it? What do the chip manufactures do to reduce this risk?