And you'd be dangerously overconfident.
A Spy ROM is a physically modified or completely custom ROM chip that looks identical to the original. But when the CPU reads from it, the chip doesn’t just return the expected BASIC interpreter or OS routines. It also executes additional hidden code. spy rom
Similarly, a 1992 CIA internal memo (released partially in 2017) references a "Type-III firmware implant" for the Apple IIe, capable of surviving a full power cycle and disk swap. Its purpose: to monitor the word processor files of a certain Middle Eastern diplomatic mission. The technical brilliance—and horror—of the Spy ROM lies in its constraints. You have, at most, 8KB to 32KB of ROM space. The original OS or BASIC takes up 80% of that. You must squeeze your spy logic into the remaining bytes, without breaking any original function. And you'd be dangerously overconfident