Content Security Policy (CSP) is a powerful security feature that helps protect websites from malicious attacks like Cross-Site Scripting (XSS). When implementing new policies, it's crucial to test them safely without risking site functionality. The report-only mode in CSP allows you to do just that.

What is Report-Only Mode in CSP?

Report-only mode enables you to monitor how a CSP policy would behave if enforced, without actually blocking any content. Instead, violations are reported to a specified endpoint, allowing you to review potential issues before applying the policy in enforce mode.

How to Enable Report-Only Mode

To activate report-only mode, add the Content-Security-Policy-Report-Only header to your server configuration. Here's an example of a basic policy:

Example:

Content-Security-Policy-Report-Only: default-src 'self'; report-uri /csp-violation-report

This configuration tells the browser to report violations of the policy to the specified URI without blocking any resources.

Setting Up the Reporting Endpoint

The report-uri directive points to a server endpoint that collects violation reports. You need to create an endpoint that can accept and log these reports for analysis.

For example, in a server-side script, you can log JSON reports sent via POST requests. This helps you understand what resources would have been blocked if CSP enforcement was active.

Analyzing Reports and Adjusting Policies

Once reports start coming in, review them carefully. Look for false positives or resources that need to be whitelisted. Based on this data, refine your CSP policy to balance security and functionality.

Best Practices for Safe Testing

  • Start with a permissive policy and gradually tighten it.
  • Use report-only mode during development and testing phases.
  • Regularly review violation reports to identify issues.
  • Communicate with your team about policy changes and findings.

Using report-only mode is an essential step in deploying effective CSP policies. It helps ensure your website remains secure without disrupting user experience during the testing process.