Is secure boot on the main application processor enough?

This report explains how secure boot and firmware signing provide authenticity and defense-in-depth for embedded devices with multiple processors, noting that not all sub-systems support secure boot and that physical attacks can still bypass protections. It recommends prioritizing secure boot on the main application processor, handling sub-system updates securely, and using safety case methodologies to justify exceptions for lower-risk components while documenting and accepting residual risk.