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# lab-1

## JWT Authentication Bypass via Unverified Signature

**Difficulty:** Apprentice

### Lab Description

This lab uses a JWT-based mechanism for handling sessions. Due to an implementation flaw, the server doesn't verify the signature of any JWTs it receives.

**Objective:** Modify your session token to gain access to the admin panel at `/admin`, then delete the user `carlos`.

**Credentials:** `wiener:peter`

### Root Cause

If a developer fails to distinguish between `decode()` and `verify()` in their JWT library and only applies `decode()`, the JWT signature is never validated. The server trusts user-controlled claims directly from the payload without any cryptographic verification, leading to a complete authentication bypass.

Secure flow:

```
Receive JWT
      │
      ▼
Verify Signature
      │
      ▼
Signature Valid?
      │
  Yes │ No
      │
      ▼
Read Claims
      │
      ▼
isAdmin = true
      │
      ▼
Allow Admin
```

Vulnerable flow:

```
Receive JWT
      │
      ▼
Decode JWT
      │
      ▼
Read Claims
      │
      ▼
isAdmin = true
      │
      ▼
Allow Admin
```

### 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. Update the endpoint to `GET /admin`.
4. In the JWT payload, change `"sub"` to `"administrator"`.
5. Send the request and open the response in the browser.
6. Click **Delete carlos** in the browser and capture the resulting request.
7. Send that request to Repeater, update `"sub"` to `"administrator"` in the JWT again, and send.
8. The user `carlos` is deleted.

### Lesson Learned

Always use `verify()` — not just `decode()` — when processing JWTs. The verification step checks that `sign(payload, secret) == signature` on the received token. Never blindly trust a decoded, user-controlled payload without first confirming the signature is valid.


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