I fly A330-340 and with current popular models like 320 or 737 the flight computers are not directly conected to any network. They are conected to the acars a messaging computer (with a very limited protocol) that we use to ask for weather, company messages like how many wheel chairs we have onboard , delay expected , etc... There is also the CPDLC that it's used to send messages to control in the oceanic zones (but is going to be implemented for all flights) Both systems can receive basic flight plan data that we can accept and include in our flight computers (like new way points, wind speeds, etc..) but I guess the comunication protocol is so basic that no mayor owning can happen.

This systems are connected via vhf data channels (dedicated ground stations), or satellite data link.

Even if they manage to crash the flight computer (they sometimes crash on their own) you only have to take manual control and reset everything. But they could never gain control of the autopilot or anything of that kind. They can only (if very very good) manage to impersonate ground controllers, but we are checking via radio comms all the time, so if something doesn't make sense we just have to call to check.

The latest models have more integrated computer systems, like you can open a valve using a trackball in the cockpit (like in the flight simulator) But I don't think they have more connection to the exterior world than older models.

Even wifi access via the inflight entertainment system is going to find a very limited number of ports or protocols to attack.

It's not something that makes even the top 100 things we worry about. We worry more about birds or drones, or dogs running in the runway.

This could change if they keep trying to make more "interactive" the newer models though.