Step Turn,
All this stuff of SLF hacking aircraft from the confines of their seat was discussed ad-infinitum in the murky depths of the MH370 thread.
Conclusion was that it was speculative drivel by people wearing tin-foil hats in the context of MH370, and there is little reason to suggest its not the same in this context either.
As ExXB says. Flight systems are heavily regulated, carefully programmed and air-gapped from the passenger WiFi network as well as external communications infrastructure.