Content Security Policy Report Only

Content Security Policy (CSP) and Reporting for Client Scripts

Content Security Policy (CSP) is a powerful browser feature that helps protect web applications from cross-site scripting (XSS) and other code injection attacks. By controlling which resources a browser can load, CSP enhances security and ensures that only trusted scripts are executed on your website.

What is Content Security Policy (CSP)?

CSP is a security standard that defines rules for loading content in web applications. These rules specify what types of content can be loaded and from which sources, thus reducing the risk of malicious attacks.

CSP for First-Party, Second-Party, Third-Party, and Fourth-Party Client Scripts

• First-Party Scripts: These are scripts served directly from the same domain as your application. For example, if your website is https://example.com, any JavaScript file served from this domain, such as https://example.com/script.js, is considered first-party. These scripts are typically trusted, and CSP rules often allow them without restrictions.
• Second-Party Scripts: These scripts come from trusted subdomains or partner domains. For instance, if you have a trusted analytics partner providing services from https://partner.example.com, their scripts would fall under second-party. CSP can be configured to allow such scripts explicitly:

  script-src 'self' https://partner.example.com;

• Third-Party Scripts: These scripts originate from external sources such as ad networks, social media widgets, or analytics providers. For example, scripts from https://cdn.analytics.com or https://ads.provider.com would be classified as third-party. Allowing third-party scripts requires careful consideration to avoid introducing vulnerabilities. CSP rules can whitelist specific domains:

  script-src 'self' https://cdn.analytics.com https://ads.provider.com;

• Fourth-Party Scripts: These are scripts loaded indirectly by third-party scripts. For example, if a script from https://cdn.analytics.com dynamically loads another script from https://another-provider.com, the latter is considered a fourth-party script. These scripts are the most challenging to control and pose significant security risks. CSP cannot directly specify these scripts unless they are explicitly loaded, making it essential to audit and monitor all third-party dependencies.

Using CSP with Report-Only Mode

Report-Only mode in CSP allows you to test policies without enforcing them. Violations are logged, enabling you to refine your rules before applying them. Here’s an example:

Content-Security-Policy-Report-Only: 
  default-src 'self'; 
  script-src 'self' https://trusted-partner.com; 
  report-uri /csp-violation-report-endpoint;

Full CSP Example for Client Scripts

The following is a complete example of a CSP header for managing first-party, second-party, and third-party scripts:

Content-Security-Policy: 
  default-src 'self'; 
  script-src 'self' https://trusted-partner.com https://analytics-provider.com; 
  style-src 'self' 'unsafe-inline'; 
  img-src 'self' https://images-provider.com; 
  connect-src 'self'; 
  report-uri /csp-violation-report-endpoint;

Benefits of Using CSP

• Reduced Attack Surface: By specifying trusted sources, CSP minimizes the risk of malicious code execution.
• Better Visibility: With report-only mode, you gain insights into potential violations and refine your policies.
• Improved User Trust: A secure application boosts user confidence.

Conclusion

Implementing CSP is a critical step towards securing modern web applications. By carefully defining policies for first-party, second-party, third-party, and fourth-party client scripts, you can significantly reduce vulnerabilities and protect your users.