TK1476 in 2006. Flying from an EU country to Turkey on a major airline.
"Italian Interior Minister Giuliano Amato reported that the hijacker slipped into the cockpit with a package which could have been a bomb when flight attendants opened the cockpit door, and the pilots acted according to the international rules in the matter and did what the hijacker wanted."
There's procedure to leave a tray on corridor and have cabin crew protect it so people cant just jump in cabin.
Pilot also can check camera and deny the open.
So having a successful takeover depends on multiple failures at same time... chernobyl happened so its not impossible but much less likely then before.
At this point id be more worried about pilots or malware on flight computers.
"Italian Interior Minister Giuliano Amato reported that the hijacker slipped into the cockpit with a package which could have been a bomb when flight attendants opened the cockpit door, and the pilots acted according to the international rules in the matter and did what the hijacker wanted."