He reported he managed to hack into a replica of a Flight Management System (FMS) in a simulator and cause the aircraft to crash. If this is the case, the implications may be great and security upgrades should be implemented fast on the vulnerable systems. In the presentation it says that he used two vulnerable systems which are in wide use: he used ADS-B to find an aircraft, as the ADS-B gives information about the aircraft's 4d position (time, lat, long & altitude), then through ACARS he contacts the aircraft and tries to hack into the FMS (as ACARS has a connection to uplink information to the FMS, so the connection exists).
Once into the FMS he was able to manipulate it.
Well for me this is very worrying, much more worrying than bringing liquids which have not been verified on-board.
The only comfort is that the report is proactive (it is done before something has really happened) and that it concludes with a note that work is being done by EASA to solve the security vulnerabilities.