As a passenger it never ceases to amaze me the vulnerability of the cockpit. If you travel anyway beyond row 7 on a Boeing 737-800 e.g it would not be difficult to get in to the cockpit, when the hosties open the door to give the flight crew tucker. I presume the airlines have some SOPs for when the cockpit door is opened. I hope so anyway.
Video cameras are also used. The door is not opened unless the area is clear and a cabin crew member is monitoring the FD entry.