That said, I certainly agree that open protocols like ACARS, ADS-B and the like are undoubtedly vulnerable. One scenario that comes to mind would be if a hacker gained access to an airline dispatch communications system - he could then cause a falsified flight plan routing to be uplinked to an aircraft when its crew requested the FP through ACARS. That could certainly have serious real-world consequences.... likewise if an ACARS exploit allowed a hacker to uplink completey false aircraft load manifest data to a crew before departure, and crew made takeoff performance calculations using that false data, again, the outcome could be very serious.
While a false flight plan or load data might be uploaded, it is unlikely that a significantly different version would bypass all the manual checks & balances in the cockpit.
As someone else pointed out, I suspect it is relatively easy to uplink something from an invalid source, but the chance that it could actually do anything malicious is remote at best.