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Yogeshwar Peela
Yogeshwar Peela

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PicoCTF Web Challenge Writeup: NO FA

#picoctf #cybersecurity #ctf #python

Overview

We're given a Flask web application with a login system and 2FA (Two-Factor Authentication). The goal is to log in as admin and retrieve the flag.

Category: Web Exploitation | Difficulty: Medium | Tools: hashcat, flask-unsign, sqlite3

Files provided:

  • app.py — Flask application source code
  • users.db — SQLite database with user credentials

Step 1 — Recon: Examining the Database 

sqlite3 users.db
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.mode column
.headers on
SELECT * FROM users;
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Key observations:

  • admin is the only account with two_fa = 1
  • Admin uses a different domain (nfs.com vs nfa.com for regular users)
  • The admin password is stored as a SHA-256 hash: c20fa16907343eef642d10f0bdb81bf629e6aaf6c906f26eabda079ca9e5ab67

Step 2 — Cracking the Password Hash

The 64-character hash is SHA-256 (-m 1400 in hashcat):

hashcat -m 1400 c20fa16907343eef642d10f0bdb81bf629e6aaf6c906f26eabda079ca9e5ab67 /usr/share/wordlists/rockyou.txt
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Result — cracked in under 5 seconds:

c20fa169...ab67:apple@123
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Password: apple@123

Step 3 — Analyzing the 2FA Code

Looking at app.py, the OTP generation stood out immediately:

# OTP Generation
otp = str(random.randint(1000, 9999))  # Only 9000 possible values!
session['otp_secret'] = otp            # Stored in SESSION COOKIE
session['otp_timestamp'] = time.time()
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# OTP Verification
if stored_otp and otp == stored_otp and (time.time() - timestamp) < 120:
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Two critical vulnerabilities here:

  1. Tiny keyspace — only 9,000 possible OTP values (1000–9999), trivially brute-forceable
  2. OTP stored in the client-side session cookie — Flask session cookies are base64-encoded and fully readable without the secret key

Step 4 — Reading the OTP from the Session Cookie

After logging in with admin / apple@123, the server redirects to /two_fa and sets a session cookie. Grab it from Browser DevTools → Application → Cookies, then decode it:

pipx install flask-unsign
flask-unsign --decode --cookie "<cookie_value>"
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Output:

{
  "logged": "false",
  "otp_secret": "1149",
  "otp_timestamp": 1779522221.27,
  "username": "admin"
}
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The OTP (1149) is sitting right there in plaintext.

Flask signs cookies to prevent tampering — but it does not encrypt them. Anyone can read the contents without knowing the secret key.

Step 5 — Submitting the OTP

Navigated to /two_fa, entered 1149, and got:

Login successful!
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Flag captured!

Vulnerability Summary

1. Weak password (apple@123) — Present in rockyou.txt, leads to full credential compromise.

2. OTP stored in unencrypted session cookie — The most critical flaw. The OTP is readable by any client without needing the server's secret key.

3. Small OTP keyspace (9,000 values) — Even without cookie access, this is brute-forceable in seconds.

4. No rate limiting on /two_fa — No protection against automated OTP guessing.

Lessons Learned

  • Never store secrets in Flask session cookies. They are signed, not encrypted. Use server-side sessions (Flask-Session with Redis or a database backend) to keep sensitive data off the client.
  • Use cryptographically secure OTP generation. Replace random.randint() with the secrets module — secrets.randbelow() or secrets.token_hex().
  • Enforce strong passwords. apple@123 should never pass any reasonable password policy. Use bcrypt or argon2 for hashing instead of raw SHA-256.
  • Adopt TOTP standards. Rolling your own OTP system is risky. Use TOTP (RFC 6238) with libraries like pyotp, compatible with Google Authenticator and Authy.

Thanks for reading! If you found this helpful, consider following for more CTF writeups.

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