Thanks NX bit

2026-02-21 · Source: Hussein Nasser · Field: Technology & Digital — Cybersecurity & Data Privacy, Software Development & Engineering · Depth: Advanced, quick

Summary

The provided content explains how a buffer overflow attack can lead to a page fault, specifically when an attacker overwrites a function's return address on the stack. In such a scenario, malicious code is injected onto the stack, and the return address is manipulated to point to this injected code instead of the legitimate program's text segment. When the function attempts to return, the CPU tries to fetch and execute instructions from the stack. However, the stack memory region is typically marked as read and write only, not executable. Upon attempting to execute from this non-executable region, the CPU raises a "page fault," signaling an unauthorized memory access and transferring control to the operating system kernel. This mechanism prevents the execution of arbitrary code from data segments like the stack.

Key takeaway

For security engineers designing system defenses, understanding the CPU's page fault mechanism is crucial. If you are evaluating exploit mitigation strategies, recognize that the "No-Execute" (NX) bit prevents attackers from executing arbitrary code injected into data segments like the stack. This hardware-level protection is a fundamental barrier against common buffer overflow attacks, ensuring that attempts to run code from non-executable memory regions are immediately halted by the kernel. Prioritize systems with robust Data Execution Prevention (DEP) enabled.

Key insights

The CPU's NX bit prevents execution of injected code from non-executable memory regions like the stack, triggering a page fault.

Principles

• Memory regions are assigned specific access permissions.
• Stacks are typically marked as read/write only, not executable.

Topics

• Buffer Overflow
• Stack Overflow Attack
• Page Fault
• Data Execution Prevention
• Memory Protection
• CPU Architecture

Best for: Security Engineer, Software Engineer

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Editorial summary, takeaway, and curation by AIssential. Original article published by Hussein Nasser.