Hacking an aircraft may be possible by a disgruntled LRU manufacturer employee, or a government.
The 'hacker' will have been able to read items such as the ARINC data provided for moving map / flight info features from a secondary bus. It's not that hard to read - a little bit harder than 'hacking' someone's Facebook page (when they leave it open...).
As mentioned, currently, IFE systems are fed by secondary busses, connected on a 'read only' basis.
If you have any understanding of typical aircraft architecture, you will understand why claims to have "taken control" of an aircraft are extremely improbable.
Additionally, if you understand the ARINC standards (there will be very, very few people on this board that have the knowledge I'm alluding to - it's detailed system design level depth of knowledge), and the aircraft systems logic, you'll find the claims even more improbable.
There are not many aircraft types with integrated networks. These have special conditions relating to security attached to them.