It is very easy to make any computer 100% immune from any cyber attack. Have no network connection. Do aircraft flight control systems have a network connection?
I don't think they currently have a direct one.
However, I wouldn't be surprised if someone came up with the idea that money could be saved if the IFE signals and some other stuff went over the same wire.
I've seen this in some other area, not safety related. What they did was put the devices on the same physical network and just assigned them different IP networks. By default, the devices couldn't see each other (at least, directly). Put the interface into promiscious mode and add a network route to the interface, suddenly you could talk to the "other" network. Security oops.
What about the IFE options for displaying a moving map, flight data, the view the pilots see on the A380 from the camera on top of the stabilizer? Are they truly one-way or is there some bug lurking that just nobody found yet? Are they separate systems that get the data from a different source that isn't used on the flight deck at all? E.g. some GPS mouse feeding data to a simple moving map display which isn't hard to build, it just costs a bit of money...