Content Security Policy (CSP) headers are a powerful tool for enhancing the security of your website. They help prevent cross-site scripting (XSS) attacks by controlling which resources can be loaded and executed. One common security concern is inline scripts and styles, which can be exploited by attackers. In this article, we will explore how to use CSP headers to block inline scripts and styles effectively.

Understanding CSP Headers

CSP headers are HTTP response headers that specify allowed sources for content on your website. They can restrict scripts, styles, images, and other resources. By defining strict rules, you reduce the risk of malicious code execution.

Blocking Inline Scripts and Styles

To block inline scripts and styles, you need to set the Content-Security-Policy header with specific directives. The key directives are script-src and style-src. Using 'unsafe-inline' in these directives allows inline content, so to block it, do not include this keyword.

Example CSP Header

Here's an example of a strict CSP header that blocks inline scripts and styles:

Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self';

Implementing CSP Headers

You can implement CSP headers through your web server configuration or via your website's code. For example, in Apache, add the following to your .htaccess file:

Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self';"

For Nginx, include in your server configuration:

add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self';";

Testing Your CSP Policy

After implementing your CSP headers, test your website thoroughly. Use browser developer tools or online CSP testing tools to ensure inline scripts and styles are blocked. Check for errors or blocked resources in the console.

Best Practices and Tips

  • Use 'self' to allow resources from your own domain.
  • Specify trusted external sources explicitly.
  • Regularly review and update your CSP policies.
  • Use nonce or hash-based policies if inline scripts are necessary.
  • Combine CSP with other security measures for comprehensive protection.

By carefully configuring your CSP headers, you can significantly improve your website's security by preventing inline scripts and styles from executing. This proactive approach helps protect your site and its visitors from malicious attacks.