My guess would be that getting caught doing this could get you federal terrorism charges. I don't even think it's a safe assumption that the network is isolated or properly insulated from pilot instrumentation.
If that assumption isn't safe, then neither is the plane.
Having ANY access AT ALL whether via "hidden" backdoor or authorized login to plane instrumentation from the WiFi would be an insane setup. Just because they're both invisible to you doesn't mean they're connected in some way.
Could you imagine the attack surface?
We'd be hearing about terrorist attacks leveraging that design flaw.
