Hardware-Enforced MAVLink Protocol Interposition: Stateful Header Inspection and Physics-Based Interdiction for UAS Cyber Defense

Unmanned aerial systems that rely on the MAVLink protocol are exposed to a class of unauthenticated-command vulnerabilities in which a compromised or spoofing sender reaches the flight controller and drives the actuators — demonstrated by 2026 CVEs in the two dominant open-source autopilots (CVE-2026-1579 in PX4; CVE-2026-38971 in ArduPilot; CVE-2026-32743). This paper presents the IM-1 Sovereign Node, an FPGA hardware interposer on the serial control path between the untrusted control plane and the trusted actuation plane. It performs stateful MAVLink header inspection (system-ID and sequence) at line rate and, on detecting an unauthorized or implausible command, applies a bounded physics-based interdiction of the actuator signal — preserving airframe controllability rather than dropping it — together with hardware-enforced domain isolation and a fail-open safety architecture. The approach is a hardware realization of runtime enforcement / shield synthesis at the actuation boundary, operating deterministically and independently of the software it protects. Verification is reported across unit (22/22), full-system integration (5/5), and live-traffic co-simulation. All results are simulation-based; silicon bring-up on the Artix-7 (Cmod A7) is the next milestone. Five U.S. provisional patents are on file (sole inventor).