The 787 has this chip which has this back door, but the 787 does not have any ability to recieve reprogramming instructions into the FMC while airborne.
Data is transmitted to ground stations for monitoring purposes like ADS/CPDLC etc. but the FMCs do not acccept commands unless the cockpit crew choose to accept it. ie route loading, atc clearances etc.
The chips if hacked may corrupt the software logic of systems but the worst is a systemic shut down of automation.
Those familliar with Boeing system logic will see that when the auto mode is corrupted, it reverts back to primary basic mode; if the system continues to malfunction, the system is isolated!
So the threat may exist, but hackers bringing down a 787 is very isolated.