how to fix content security policy default-src 'self' breaking site

Yulia Chive

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07-12-2024

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How to Fix "Content Security Policy: default-src 'self'" Breaking Your Website

The error "Content Security Policy: default-src 'self'" is a common headache for website developers. While it signals a boost in security, it can also break your site if not implemented correctly. This comprehensive guide will walk you through understanding the error, diagnosing the cause, and implementing effective solutions.

Understanding the default-src 'self' Directive

The default-src 'self' directive within a Content Security Policy (CSP) is a powerful security measure. It dictates that only resources originating from the same origin (the same protocol, domain, and port) as the current webpage are allowed to load. This significantly reduces the risk of Cross-Site Scripting (XSS) and other injection attacks. However, if your website relies on external resources like scripts, stylesheets, images, or iframes from other domains, this restrictive policy will block them, leading to broken functionality.

Diagnosing the Problem: Identifying Blocked Resources

The first step is pinpointing precisely what's being blocked. Your browser's developer console is your best friend here.

1. Open your browser's developer tools: This is usually done by pressing F12.

2. Navigate to the "Console" or "Network" tab: Look for error messages related to Content Security Policy violations. These messages will often specify the blocked resource and the directive that caused the blockage. For example, you might see:

   Refused to load the script 'https://example.com/script.js' because it violates the following Content Security Policy directive: "default-src 'self'"

This clearly indicates that the script https://example.com/script.js is blocked because it doesn't originate from your website's domain.

Solutions to Resolve default-src 'self' Issues

The solution depends on which resources your website needs to load from external sources. You'll need to modify your CSP to allow these resources. There are several ways to do this:

1. Specify Allowed Sources:

Instead of default-src 'self', you can explicitly define allowed sources for specific resource types. This offers granular control.

• script-src: Controls where scripts can be loaded from. Example: script-src 'self' https://example.com (allows scripts from your domain and example.com).
• style-src: Controls where stylesheets can be loaded from. Example: style-src 'self' 'unsafe-inline' (allows stylesheets from your domain and inline styles – use cautiously!).
• img-src: Controls where images can be loaded from. Example: img-src 'self' data: (allows images from your domain and data URLs).
• font-src: Controls where fonts can be loaded from. Example: font-src 'self' https://fonts.googleapis.com
• connect-src: Controls where connections (e.g., AJAX requests) can be made.
• frame-src: Controls where iframes can be loaded from.
• object-src: Controls where plugins (like Flash) can be loaded from.

Example of a More Permissive CSP:

Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com; style-src 'self' 'unsafe-inline'; img-src 'self' data:; font-src 'self' https://fonts.googleapis.com;

2. Using unsafe-inline (Use with Extreme Caution!):

The 'unsafe-inline' keyword allows inline JavaScript and CSS. This is a significant security risk and should only be used as a last resort and only for very specific, trusted inline code. Always prefer to use external scripts and stylesheets.

3. Using nonce for Inline Scripts and Styles:

A more secure alternative to 'unsafe-inline' is using a nonce. A nonce is a randomly generated string that's unique to each page load. You include this nonce in the <script> or <style> tag. Your CSP then allows only inline scripts or styles with that specific nonce. This is significantly more secure than 'unsafe-inline'.

Example:

<meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' 'nonce-12345'; style-src 'self' 'nonce-12345';">

<script nonce="12345">
  // Your inline script here
</script>

<style nonce="12345">
  /* Your inline styles here */
</style>

4. Understanding 'unsafe-eval' (Use with Extreme Caution!):

'unsafe-eval' allows the use of eval(), setTimeout with strings, and similar functions. This is a major security vulnerability and should be avoided unless absolutely necessary.

Implementing the Solution:

You can implement your updated CSP in several ways:

• HTTP header: This is the recommended approach. Your web server needs to be configured to add the Content-Security-Policy header to the HTTP response.
• <meta> tag: This is a fallback mechanism. Use it only if you can't set the header directly.

Testing and Iterative Refinement:

After making changes to your CSP, thoroughly test your website to ensure all functionality is restored. You may need to iterate, gradually adding more permissive directives until everything works as expected. Always prioritize security and minimize the use of 'unsafe-inline' and 'unsafe-eval'. Regularly review and update your CSP as your website evolves.

By following these steps and carefully crafting your Content Security Policy, you can resolve the default-src 'self' error while maintaining a strong security posture for your website. Remember, security is an ongoing process, and regular audits and updates are crucial.