Welcome to my series of writeups for n00bzunit3d 2024 capture-the-flag competition. In this post, we look at the web/passwordless challenge. We are given a link to a website that asks for a username field and the backend code of the server that does the verification of the user.

server.py



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#!/usr/bin/env python3
from flask import Flask, request, redirect, render_template, render_template_string
import subprocess
import urllib
import uuid
global leet

app = Flask(__name__)
flag = open('/flag.txt').read()
leet=uuid.UUID('13371337-1337-1337-1337-133713371337')

@app.route('/',methods=['GET','POST'])
def main():
    global username
    if request.method == 'GET':
        return render_template('index.html')
    elif request.method == 'POST':
        username = request.values['username']
        if username == 'admin123':
            return 'Stop trying to act like you are the admin!'
        uid = uuid.uuid5(leet,username) # super secure!
        return redirect(f'/{uid}')

@app.route('/<uid>')
def user_page(uid):
    if uid != str(uuid.uuid5(leet,'admin123')):
        return f'Welcome! No flag for you :('
    else:
        return flag

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=1337)

The server uses uuid5 and the leet object to do authentication. However, it prevents us from entering admin123 in the username field. This is a easy workaround. We can find the uuid5 value of the admin123 username by ourself and enter that directly into the browser searchbar. Opening the correct webpage gives us the flag.