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# lab-3

## JWT Authentication Bypass via Weak Signing Key

**Difficulty:** Practitioner

### Lab Description

This lab uses a JWT-based mechanism for handling sessions. It uses an extremely weak secret key to both sign and verify tokens, which can be brute-forced using a wordlist of common secrets.

**Objective:** Brute-force the website's secret key, use it to sign a modified session token that gives you access to the admin panel at `/admin`, then delete the user `carlos`.

**Credentials:** `wiener:peter`

### Root Cause

When a developer fails to use a strong, unguessable value as the JWT signing secret, an attacker can brute-force the token against a wordlist of common secrets. Once the secret is cracked, the attacker can modify any claim in the payload, generate a valid new signature, and bypass the server's verification step entirely — leading to account takeover, session hijacking, and access control bypass.

### Exploitation

1. Start the login process and capture all packets in Burp Suite.
2. Find `GET /my-account?id=wiener` and send it to Repeater.
3. Copy the JWT token from the request.
4. Crack the secret using Hashcat:

   ```
   hashcat -a 0 -m 16500 <jwt> <wordlist>
   ```
5. Go to [jwt.io](https://jwt.io/) and paste the JWT token into the encoder.
6. Update `"sub"` to `"administrator"`, paste the cracked secret into the **Verify Signature** field, and copy the newly signed JWT.
7. Paste the new JWT into the `GET /my-account?id=wiener` request in Repeater and update the URL to `GET /admin`.
8. Send the request and open the response in the browser.
9. Click **Delete carlos** in the browser and capture the resulting request.
10. Send that request to Repeater, update the JWT token with the new signed token again, and send.
11. The user `carlos` is deleted.

### Lesson Learned

Always use a long, randomly generated, and unguessable secret value for JWT signing. A weak or common secret provides no real protection — it can be cracked with widely available tools and wordlists, completely undermining the cryptographic integrity of the token.


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