How to ensure your CSP is effective against XSS attacks on a WordPress website?

When it comes to the latest core metrics for SEO, Google’s Lighthouse has audits for performance, accessibility, progressive web apps, SEO, and Security. 

If your website is vulnerable you may lose valuable traction in your search engine results. 

Content Security Policy (CSP) is a powerful tool to help mitigate the risk of Cross-Site Scripting (XSS) attacks on your WordPress website. Here are the steps to ensure CSP is effective:

1. Understand CSP Basics

CSP is a security feature that helps prevent various attacks by controlling the sources from which resources (like scripts, styles, and images) can be loaded. A CSP header tells the browser which sources it is allowed to load content from.

2. Implement a Strong CSP

A strong CSP typically disallows inline scripts and styles, and only allows resources from trusted sources. A good starting point for your CSP could look something like this:

Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted.cdn.com; style-src 'self' https://trusted.cdn.com; img-src 'self' data:; connect-src 'self'; font-src 'self' https://trusted.fonts.com; frame-src 'none'; object-src 'none';

This example allows scripts and styles to be loaded only from the site’s own domain ('self') and a trusted CDN. Inline scripts and styles are blocked, as are any frames or objects from other sources.

3. Apply CSP Headers

To apply CSP headers to your WordPress site, you can either use a plugin or modify your .htaccess file (if you’re using Apache) or your server configuration file (for Nginx).

Using a Plugin

  1. CSP plugin: There are several WordPress plugins that can help you manage CSP. A popular one is HTTP Headers.
  2. Install and activate the plugin.
  3. Configure the plugin: Go to the plugin settings and set up your CSP rules.

Using .htaccess (for Apache)

Add the following lines to your .htaccess file:

<IfModule mod_headers.c>
    Header set Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted.cdn.com; style-src 'self' https://trusted.cdn.com; img-src 'self' data:; connect-src 'self'; font-src 'self' https://trusted.fonts.com; frame-src 'none'; object-src 'none';"
</IfModule>

Using Nginx Configuration

Add the following lines to your Nginx configuration file:

add_header Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted.cdn.com; style-src 'self' https://trusted.cdn.com; img-src 'self' data:; connect-src 'self'; font-src 'self' https://trusted.fonts.com; frame-src 'none'; object-src 'none';";

4. Test Your CSP

Before deploying your CSP to production, test it to ensure it does not break your site. Use the Content-Security-Policy-Report-Only header to see what would be blocked by your CSP without actually blocking anything. Monitor the reports and adjust your CSP as needed.

Content-Security-Policy-Report-Only: default-src 'self'; script-src 'self' https://trusted.cdn.com; style-src 'self' https://trusted.cdn.com; img-src 'self' data:; connect-src 'self'; font-src 'self' https://trusted.fonts.com; frame-src 'none'; object-src 'none'; report-uri /csp-report-endpoint;

Set up a reporting endpoint (or use a third-party service) to collect CSP violation reports.

5. Monitor and Maintain Your CSP

Regularly review your CSP reports and logs to ensure no unwanted content is being loaded. Adjust your CSP rules as needed, especially if you add new plugins or make significant changes to your site.

Additional Security Measures

While CSP is effective, it should be part of a broader security strategy:

  • Keep WordPress and all plugins updated.
  • Use a Web Application Firewall (WAF).
  • Sanitize and validate user inputs.
  • Use HTTPS to ensure secure data transmission.
  • Regularly scan your site for vulnerabilities.

By following these steps and integrating CSP into your overall security strategy, you can significantly reduce the risk of XSS attacks on your WordPress website.