Comment by SAI_Peregrinus

An assembler can also do this same attack. You have to hand-assemble your basic assembler to machine code. In theory a sophisticated-enough hex editor could also be subverted, silently adding in code when it's used to edit executable-looking files. The system dynamic linker can be compromised to subvert any executable. Etc.

If a machine is compromised, it can't be recovered without access to a known-good machine.