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Java Web Tokens (JWT) are widely used for authentication and data exchange in web applications. Understanding their vulnerabilities is crucial for cybersecurity professionals. Below is a detailed breakdown of JWT security, along with practical commands and techniques.
What is JWT?
JWT (JSON Web Token) is a compact, URL-safe means of representing claims between two parties. It consists of three parts:
– Header (algorithm & token type)
– Payload (claims/data)
– Signature (verification)
Example JWT:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
Common JWT Attacks & Exploits
1. None Algorithm Attack
- Some libraries accept tokens with
alg: none, allowing unsigned tokens. - Exploit Command:
jwt_tool <JWT> -X a
2. Weak Secret (Brute Force)
- Crack JWT secrets using tools like
hashcat. - Command:
hashcat -m 16500 -a 0 jwt.txt rockyou.txt
3. Key Confusion Attack
- Trick the server into verifying with a public key when expecting HMAC.
- Exploit:
jwt_tool <JWT> -X k -pk public_key.pem
4. Header Injection (CVE-2015-9235)
- Inject `”kid”: “../../dev/null”` to bypass validation.
You Should Know:
1. Decoding & Modifying JWTs
Use `jq` and `base64` to decode:
echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9" | base64 -d | jq
2. Testing JWT Vulnerabilities
- Using
jwt_tool:python3 jwt_tool.py <JWT> -T
3. Secure JWT Validation in Node.js
const jwt = require('jsonwebtoken');
jwt.verify(token, secretKey, { algorithms: ['HS256'] }, (err, decoded) => {
if (err) throw err;
console.log(decoded);
});
4. Linux Command to Check JWT Expiry
echo $JWT | cut -d '.' -f 2 | base64 -d | jq '.exp'
What Undercode Say
JWT security is critical in modern web apps. Always:
– Use strong secrets (openssl rand -hex 32).
– Restrict algorithms (HS256, RS256).
– Set short expiration times.
– Validate iss, aud, and `sub` claims.
Expected Output:
A secure JWT implementation prevents unauthorized access. Test tokens thoroughly before deployment.
Further Reading:
References:
Reported By: Activity 7313512221039296512 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
