> For the complete documentation index, see [llms.txt](https://books.spartan-cybersec.com/web/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://books.spartan-cybersec.com/web/prototype-pollution/lab-1-client-side-prototype-pollution-via-browser-apis.md).

# Lab 1: Client-side prototype pollution via browser APIs

Primero ejecutamos el siguiente script de js en la consola:

```
Object.prototype
```

<figure><img src="/files/XBsmWNYURbWedyPrtQ51" alt=""><figcaption></figcaption></figure>

Luego de lo anterior, se procede a realizar un GET concatenando lo siguiente:

{% code overflow="wrap" %}

```
/?__proto__[variable]=gerhishere
```

{% endcode %}

Y al ejecutar nuevamente el script previo, sale lo siguiente:

<figure><img src="/files/GmJHuPL4nxRSaYwNlGFJ" alt=""><figcaption></figcaption></figure>

Y luego de lo anterior, validamos el codigo fuente para analizarlo:

<figure><img src="/files/es9ZRPZyOczXBcinE5rS" alt=""><figcaption></figcaption></figure>

El ataque de prototype pollution ocurre cuando un atacante puede modificar el prototipo de objetos base en JavaScript, como `Object.prototype`, lo que permite inyectar propiedades maliciosas que pueden ser utilizadas en todo el código que se basa en estos objetos.

En este caso, la función `deparam` podría ser explotada si los parámetros de la URL incluyen claves como `__proto__`. Por ejemplo, un atacante podría pasar una URL como esta:

Teniendo en cuenta lo anterior, se procede a modificar nuestro payload para desplegar una alerta en JS:

<figure><img src="/files/0ohHgLwnmNe2zp8tAsSb" alt=""><figcaption></figcaption></figure>


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