A ROP (Return-Oriented Programming) chain is a sequence of gadgets that are used to execute arbitrary code in the context of a program. Gadgets are small blocks of code that end in a return instruction and are typically found in compiled programs. By chaining gadgets together, an attacker can execute arbitrary code without actually injecting any new code into the program.
In an ARM 32-bit system, ROP chains are typically used to bypass data execution prevention (DEP) and address space layout randomization (ASLR) defenses. DEP and ASLR are security measures that are designed to make it harder for attackers to execute code by randomizing the locations of code and data in memory. By chaining gadgets together, an attacker can execute code without actually injecting any new code into the program, making it harder for DEP and ASLR to detect the attack.
To create a ROP chain, an attacker must first identify gadgets in the program that can be used to execute the desired code. This can be done using a tool such as ROPgadget. Once the gadgets have been identified, the attacker can then create a chain by overwriting the return address of a function with the address of the first gadget, and using the gadgets to execute the rest of the code.