Is it really that ridiculous? Maybe you should look up Hugh Teso on Google and read about what he did. You might be surprised.
And, for the sceptics, he demonstrated the hack in action with real aircraft computers.
It's actually a pretty simple exploit, using ACARS to substitute data from base with data he sends from the smartphone. He was able to also heading and altitude via the exploit. He also managed to hack the ADS-B transmitter.
Any system that transmits data wirelessly can, eventually, be hacked. All it needs is access to the OS used and, eventually, the hacker will find a way in. It's a matter of 'convincing' the aircraft's systems that you're giving it the real data.
It made an impressive display, but I highly doubt you could pull it off in the real world.
While there is a "master" buss that most everything talks to, most boxes output multiple busses, some of which only go to 1 place. For example an ADIRU may have 1 talk to everyone bus, 1 that only talks to FMS 1, 1 that only talks to ADIRU 2, etc.
If the FMS sees different data from the master buss, and different data from it's protected bus, it will raise its BS flag. I have seen this real world in a Hawker, the MFD symbol generator was spitting out mismatched data, and both Universal FMS's refused to go into approach mode. (I found the fault, by pulling breakers one at a time till they went into approach mode)
Upshot is, in the real world, you could make the avionics throw a temper tantrum and give the pilots a headache, but you couldn't turn the bird into your own personal drone.