[neurotech1]

As explained in the linked video above (from DEFCON 22), there is a device called a Network Extension Device (NED) which is a one-way gateway between the FMS and the IFE for showing the progress of the flight to passengers.

[verystealthy]

Yeah, but I think that the main message is that there are pilots in the cockpit who can easily override any weird input. Even if it is theoretically possible (hey, you never know, right?) to impersonate avionics and mess up with the FMS/ECU/EICAS, there's no such thing as a pilot going "holy crap! the plane is going sideways and there's nothing I can do!". I'm pretty comfortable calling this whole "hacking the engines from the in-flight entertainment system" fiction.

[x0054]

Why on earth would you need more than a simple one way API between the FMS and IFE to be able to show inflight progress. For that matter, why would it be more complicated than a simple serial cable with just the O pin on the FMS side, and just the I pin on the IFE side to feed information at regular intervals to the IFE. I have no idea how exactly the systems are setup, but off the top of my head I can come up with several VERY simple methods of insuring that the communication between the FMS and IFE is in deed unidirectional. If this story is true, I would be highly disappointed in Boeing!

[neurotech1]

Its unidirectional in hardware. The exact details are more complicated, but there is only physical wires for a one-way link from the FMS side.

[verystealthy]

Don't you have to call an API?

[x0054]

neurotech1 above says that it's unidirectional in hardware, so it's a moot point. But you could also do a simple relay server with API. FMS pushes the info to a "server" through a one way link. IFE would request the info from the server and displays it as necessary.