Buffer Overflow Attack: How It Works & Practical Example

Buffer overflow attack is a type of security vulnerability that occurs when a program tries to store more data in a buffer than it can handle. The excess data spills over into adjacent memory locations, overwriting the data there, and potentially causing the program to crash or execute arbitrary code. Attackers can exploit this vulnerability by intentionally sending more data to the buffer than it can handle, causing it to overflow.

The following steps illustrate how a buffer overflow attack works:

1. Identify a vulnerable program: The attacker identifies a program that has a buffer overflow vulnerability. This could be a program that is poorly coded or has not been updated with the latest security patches.

2. Determine the size of the buffer: The attacker determines the size of the buffer and the amount of data required to overflow it.

3. Craft the exploit code: The attacker crafts exploit code that contains the excess data required to overflow the buffer. This code is then sent to the vulnerable program.

4. Trigger the vulnerability: The attacker triggers the vulnerability by executing the exploit code. This causes the buffer to overflow and overwrite adjacent memory locations.

5. Execute arbitrary code: If the attacker has crafted the exploit code correctly, it may be possible to execute arbitrary code and gain control of the system.

To demonstrate a buffer overflow attack, I have written a vulnerable C program that accepts user input and stores it in a buffer. Here is the code:

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

void vulnerable(char *input) {
  char buffer[10];
  strcpy(buffer, input);
  printf("Buffer content: %s\n", buffer);
}

int main(int argc, char **argv) {
  char *input = argv[1];
  vulnerable(input);
  return 0;
}

This program accepts user input and stores it in a buffer of size 10. As you can see, there is no check to ensure that the input does not exceed the size of the buffer, making it vulnerable to buffer overflow attacks.

To exploit this vulnerability, I have written a Python script that generates a string of 20 characters and passes it as an argument to the vulnerable program. Here is the code:

import subprocess

payload = 'A' * 20
subprocess.call([" ./vulnerable", payload])

When I run this script, the vulnerable program crashes with a segmentation fault. This is because the buffer overflow caused by the excess data in the input has overwritten adjacent memory locations, causing the program to crash.

This demonstrates how a buffer overflow attack can be used to exploit vulnerable programs and gain control of a system. It is important to note that buffer overflow attacks can be prevented by implementing proper input validation and bounds checking in programs.