Well, before we all set our hair on fire over some half-baked notions we need to think about this, with some understanding of the systems involved and not just ride off in all directions with ill-considered claims. The sky is not falling . . .
1. Neither the ADS nor the ACARS are directly linked / connected to the flight control system, period.
2. The FMS is connected to the flight control system when the autoflight / autothrust systems are being guided by the flight plan data and (to a much lesser extent) the weight-and-balance data entered during the ramp check.
2. The ADS system is an ATC communications system which has no connection to the FMS. ATC cannot control the routing, speed or altitude of the aircraft through ADS.
3. While some operators routinely upload flight plan and weight-and-balance data via ACARS during the ramp flight preparation sequence, many operators' do not have this auto-upload capability and the data is entered manually. In manually-entered circumstances there is no way to upload changes to the flight plan routing via the ACARS to affect aircraft navigation through the FMS which is connected to the autoflight system.
4. Given system and aircraft design, logically the autoflight system must be engaged for this to "work". The FMS has the route data and the autoflight is designed to follow that data.
5. FMS data cannot control altitude and will not command the aircraft to climb or descend even if cruise altitude changes and descent points have previously been entered or otherwise programmed in the FMS. Neither can ACARS nor ADS do this.
6. Within a narrow Mach or CAS range, when routinely engaged, the autoflight / autothrust systems are controlled by the FMS which in turn will control aircraft cruise speed. Cruise speed and speed restrictions at certain waypoints, (oceanic entry and exit points, for example), may be part of the flight plan. As with any FMS entries, there are reasonableness checks which reject incorrect or inappropriate data.
7. Should something like that which is claimed actually succeed, there are at least two human pilots in the cockpit, sometimes three or four depending upon phase of flight, etc who can fly the aircraft manually. When the autoflight system is disconnected none of this works. Also, routine enroute waypoint checks confirm position, speed, altitude, next position and so on and, should immediate but subtle anomalies occur enroute, they would be caught at such waypoints.
This doesn't deny the possibility that ACARS has vulnerabilities, but such potential is not about to take over an airliner in flight as implied by the use of the word, "hijack".
In my view, making claims that it is somehow possible to command an airliner to "dive" or do other untoward maneuvers beyond the crew's ability to counter, using an Android cellphone, is irresponsible.
When the exact method by which the claims in Mr. Teso's article are made is clearly explained and, as such things normally are required to be, peer-reviewed to substantiate serious claims of compromise, then we might take all this seriously. At present, it seems entertainment is where one finds it.
PJ2