W^{

X: Write XOR Execute}

• {W^{}X} is the general policy: a memory page may be writable or executable, but never both at once.
• NX/DEP are concrete implementations of this principle for the stack and heap.
• Consequence for attackers: you cannot write bytes and then jump to them in the same page.
• This single policy defeats the entire "inject and jump" family of exploits.