NX / DEP: No Execute

• Problem it kills: classic attack writes shellcode into a buffer (stack/heap) and jumps to it.
• Idea: memory is either writable or executable, never both — the "W X" policy.
• How: the CPU's NX bit (No-eXecute, AMD) / XD bit (Intel) in the page table. Fetch from a non-exec page → fault.
• Stack, heap, and data pages are marked non-executable; only .text and mapped libraries execute.