A ROP chain (Return-Oriented Programming chain) is a sequence of small code snippets, called 'gadgets,' that already exist in a program’s memory. Attackers use ROP chains to manipulate a program’s control flow, typically bypassing security mechanisms like non-executable memory protections (DEP/NX). By carefully chaining these gadgets, each ending with a return instruction, attackers can perform complex operations without injecting new code. This technique is commonly used in exploit development, allowing malicious actions even when direct code execution is blocked. ROP attacks are challenging to detect and prevent, making them a significant concern in cybersecurity.