Blog | G5 Cyber Security

CMS HTML & XSS: Is it Safe?

G5 Cyber Security

TL;DR

No, CMS-generated HTML is not automatically trusted and remains vulnerable to Cross-Site Scripting (XSS) attacks. Proper output encoding/escaping is crucial at every stage – in the database, when retrieving data, and before rendering it in the browser.

Understanding the Risk

Content Management Systems (CMS) like WordPress, Drupal, or Joomla allow users to create content through an admin interface. This content is then stored (usually in a database) and displayed on the website. Even though you’re using a CMS, malicious code can still be injected if input isn’t handled correctly.

Why CMS HTML Isn’t Inherently Safe

  1. User Input is Key: The core problem is user-supplied data. Even with an admin interface, users are providing the content that ends up in your HTML.
  2. Database Storage: Databases store text as text. They don’t inherently understand or prevent malicious code. A database can hold <script> tags just fine.
  3. Rendering Engine: The browser interprets HTML, and if it encounters a <script> tag, it will execute the JavaScript within it – leading to XSS.

Steps to Protect Against XSS

  1. Input Validation (First Line of Defence):
    • Whitelist: Define exactly what characters and formats are allowed in each field. For example, if a field should only contain numbers, reject anything else.
    • Sanitize: Remove or encode potentially harmful characters before storing data in the database. This is often done server-side.
  2. Output Encoding/Escaping (Crucial Step): This is the most important step.
    • HTML Escaping: Convert potentially dangerous characters into their HTML entities. For example: 
      < becomes &lt;
> becomes &gt;
" becomes &quot;
' becomes &#39;
    • Context-Aware Encoding: Use the correct encoding method based on where the data is being used. For example:
      • HTML context (e.g., within a paragraph): HTML escaping.
      • JavaScript context (e.g., inside a JavaScript variable): JavaScript escaping.
      • URL context (e.g., in a link): URL encoding.
  3. Content Security Policy (CSP): A browser security mechanism that helps prevent XSS attacks by controlling the resources the browser is allowed to load. 
    header("Content-Security-Policy: default-src 'self' ");

    This example only allows loading resources from your own domain.

  4. Regularly Update Your CMS and Plugins: Updates often include security patches that address XSS vulnerabilities.
  5. Use a Template Engine with Auto-Escaping: Many template engines (like Twig, Jinja2) automatically escape variables by default, reducing the risk of forgetting to encode output.

    Example (Twig):

    <p>{{ user_input }}</p>>

    This will automatically HTML-escape user_input.

  6. HTTPOnly Cookies: Set the HTTPOnly flag on cookies to prevent JavaScript from accessing them, mitigating some XSS risks.

Example Scenario

Imagine a CMS allows users to enter their name. If you directly display this name in HTML without encoding, an attacker could enter something like:

<script>alert('XSS Attack!')</script>

This would execute JavaScript code when the page is loaded.

Conclusion

CMS-generated HTML is not inherently safe. You must actively protect against XSS by validating input, encoding output correctly, and implementing other security measures. Treat all user-supplied data as potentially malicious until proven otherwise.

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