Machine code is the lowest level of software: binary instructions that a processor executes directly. Unlike source code, it is not meant to be read by humans. It represents operations such as loading memory, calling routines, and branching to a new address, all in the exact instruction set a specific CPU understands.
In cyber security, machine code matters because every exploit, payload, and firmware component ultimately becomes it. Attackers may craft shellcode in machine code to run after a memory corruption bug, hide behavior from simple source-level review, or reverse engineer protection logic from compiled binaries. Defenders inspect machine code with disassemblers, emulate it in sandboxes, and audit firmware or embedded systems where source code may not be available. In a custom operating system or bootloader, working at this level gives total control, but it also removes safety nets, so small mistakes can become security flaws.