This makes me miss older, simpler protocols. Sure, a parallel cable could also be compromised to snoop but at least it couldn't pretend to be some other device or install rootkits.
That's because it isn't a cable, it is a computer.
In the 1980's I worked with CAD/CAM software that required hardware dongles on parallel ports as an anti-piracy measure. That probably could have been exploitable: e.g., instead of returning the security response, cram a ton of data and cause buffer overrun on their 80286 + DOS5 application and smash the stack. It would be fun to go back in time and see just how vulnerable that software was.
Sure it can. In the golden days of parport we've used parallel port device autodetection based on Device ID. There was a lot of place in the plug to use for a microcontroller in the end of the parport times.
