Question
Answer and Explanation
The Content Security Policy (CSP) is a security standard that helps prevent Cross-Site Scripting (XSS) attacks, clickjacking, and other code injection vulnerabilities. Setting `default-src 'self'` as part of your CSP means that the browser should only load resources (such as scripts, images, and styles) from the same origin as the HTML document. When this breaks your website, it’s usually because you are loading resources from different origins or using inline scripts/styles.
Here’s a breakdown of why `default-src 'self'` might break your site and how to fix it:
Reasons for Breakage:
1. External Resources: If your website loads resources from external CDNs (Content Delivery Networks), different domains, or subdomains, `default-src 'self'` will block them. For example, if you're using Google Fonts, a CDN for JavaScript libraries (like jQuery), or images hosted on another server, these will be blocked.
2. Inline Scripts and Styles: CSP, by default, restricts inline JavaScript (<script> tags directly in the HTML) and inline CSS (<style> tags or style attributes). Using `default-src 'self'` without additional directives will block these.
3. `eval()` and `unsafe-inline`: The use of `eval()` or similar functions like `new Function()` is often restricted by CSP, and using `unsafe-inline` to allow inline scripts introduces security risks.
How to Fix It:
1. Identify the Violations: Use your browser's developer console to identify which resources are being blocked due to the CSP. The console will show error messages indicating the CSP violations.
2. Adjust Your CSP Header: Modify your CSP header to explicitly allow the necessary resources. Here are some common scenarios and how to address them:
- Allowing External Resources: Use specific directives to whitelist domains. For example, if you need to load scripts from `https://example.com`, add `script-src 'self' https://example.com;` to your CSP.
Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com; style-src 'self' https://example.com; img-src 'self' https://example.com;
- Allowing Inline Scripts (Use with Caution): If you must use inline scripts, you have a few options, but they come with security trade-offs:
- `unsafe-inline` (Not Recommended): Adds `unsafe-inline` to `script-src`. This is generally discouraged due to security implications.
Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline';
- `nonce` (Better): Add a unique, cryptographically secure nonce to each inline script tag and include the same nonce in your CSP.
HTML:
<script nonce="rAnd0mN0nc3"> ... </script>
CSP Header:
Content-Security-Policy: default-src 'self'; script-src 'self' 'nonce-rAnd0mN0nc3';
- `hash` (Specific): Generate a SHA256, SHA384, or SHA512 hash of the inline script and include it in your CSP.
HTML:
<script> ... </script>
CSP Header:
Content-Security-Policy: default-src 'self'; script-src 'self' 'sha256-HASH_VALUE';
- Refactor Inline Scripts and Styles: The best approach is to move inline scripts and styles to external files. This simplifies CSP configuration and enhances security.
3. Report-Only Mode: Use `Content-Security-Policy-Report-Only` to test your CSP without enforcing it. This allows you to monitor violations and adjust your policy accordingly before deployment.
Content-Security-Policy-Report-Only: default-src 'self'; report-uri /csp-report-endpoint;
4. Be Specific: Avoid using overly broad directives like `unsafe-eval` unless absolutely necessary. The more specific your CSP, the more effective it will be at preventing attacks.
In summary, implementing a CSP with `default-src 'self'` is a good security practice, but it requires careful planning and testing to ensure it doesn't break your website. By identifying the violations and adjusting your CSP accordingly, you can enhance your website's security while maintaining its functionality. Remember to prefer using nonces or hashes over `unsafe-inline` for better security.