The MAVLink communication protocol does not require cryptographic
authentication by default. When MAVLink 2.0 message signing is not
enabled, any message -- including SERIAL_CONTROL, which provides
interactive shell access -- can be sent by an unauthenticated party with
access to the MAVLink interface. PX4 provides MAVLink 2.0 message
signing as the cryptographic authentication mechanism for all MAVLink
communication. When signing is enabled, unsigned messages are rejected
at the protocol level.