There is not a single incident or accident report yet which would indicate "hacking of aircraft" is a problem; that could help to estimate the probabilities even without knowledge of a/c system design. It's an imaginary problem so far. The scaremongering is a real problem though, some statistician with a better grasp of numbers than me estimated that around 1600 people died following 9/11 because they chose to travel by car, instead of flying. I wonder what the probability is that someone will get killed in a road accident because they read stories about "hacking of aircraft"...probably more than those who get killed by "hacked" aircraft?