exploiting-jwt-algorithm-confusion-attack

name	exploiting-jwt-algorithm-confusion-attack
description	Exploits JWT algorithm confusion vulnerabilities where the server's token verification library accepts the algorithm specified in the JWT header rather than enforcing a fixed algorithm. The tester manipulates the alg header to switch from RS256 to HS256 (using the RSA public key as the HMAC secret), sets alg to none to bypass signature verification, or exploits kid/jku/x5u header injection to supply attacker-controlled keys. Activates for requests involving JWT algorithm confusion, alg none attack, key confusion attack, or JWT signature bypass.
domain	cybersecurity
subdomain	api-security
tags	["api-security","jwt","algorithm-confusion","token-forgery","cryptographic-attack"]
version	1.0.0
author	mahipal
license	Apache-2.0

Exploiting JWT Algorithm Confusion Attack

When to Use

• Testing APIs that use RS256 (asymmetric) JWT tokens for authentication to check for algorithm downgrade to HS256
• Assessing JWT implementations for alg:none bypass where the server skips signature verification
• Evaluating JWT libraries for key confusion vulnerabilities where the public key is used as HMAC secret
• Testing kid (Key ID), jku (JWK Set URL), and x5u (X.509 URL) header parameters for injection
• Validating that the API server enforces a specific algorithm and does not trust the JWT header

Do not use without written authorization. JWT exploitation can lead to authentication bypass and account takeover.

Prerequisites

• Written authorization specifying the target API and JWT-based authentication in scope
• A valid JWT token from the target API (obtained through legitimate authentication)
• The server's RSA public key (obtainable from JWKS endpoint, TLS certificate, or public key endpoint)
• Python 3.10+ with PyJWT, cryptography, and requests libraries
• jwt_tool for automated JWT attack testing
• Burp Suite with JWT Editor extension

Workflow

Step 1: JWT Token Analysis

import base64
import json
import requests
import hmac
import hashlib
import time

BASE_URL = "https://target-api.example.com/api/v1"

# Capture a valid JWT token
login_resp = requests.post(f"{BASE_URL}/auth/login",
    json={"email": "[e-mail]", "password": "TestPass123!"})
valid_token = login_resp.json().get("access_token", "")

# Decode JWT parts
def decode_jwt(token):
    parts = token.split('.')
    if len(parts) != :
         ValueError()

     ():
         s +  * ( - (s) % )

    header = json.loads(base64.urlsafe_b64decode(pad(parts[])))
    payload = json.loads(base64.urlsafe_b64decode(pad(parts[])))
     header, payload, parts[]

header, payload, signature = decode_jwt(valid_token)
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