BEagle says, "transferred by USB pen drive to the on-board system - which cannot corrupt any aircraft system as it is read-only" but that is not true, never was.
See,
Opinion : An Evil USB Drive Could Take Over Your PC Undetectably. The ubiquitous USB isn't safe at all.
And don't forget that some people supposedly loaded stuxnet onto a USB, dropped it near an Iranian nuclear facility where some unwitting individual then picked it up, carried it inside and then plugged it into the facility's network: centrifuges ruined, job done!
Assumptions of safety are just that.
Regards, Tanimbar