Resuming:
1 - cups-browsed is able to install printers automatically (without the requirement of user confirmation) by listening to UDP packets on port 631.
2 - Attacker uses this "feature" to install a fake printer with a custom driver (which is also installed without user confirmation and can be downloaded from an arbitrary host) which specifies the "command to run" when a print job is sent.
3 - User prints something in the fake printer and the "command to run" is executed.