Understanding Report-Only Mode

Content Security Policy Report-Only mode provides a safe testing environment for CSP policies by monitoring violations without enforcing restrictions. This mode logs what would be blocked under an enforcing policy, providing invaluable data for policy refinement without impacting users. Understanding when and how to use Report-Only mode effectively can mean the difference between a smooth CSP deployment and a problematic one that breaks critical functionality.

The fundamental difference between enforcing and report-only modes:

# Enforcing Mode - Blocks violating resources
Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted.com;

# Report-Only Mode - Logs violations without blocking
Content-Security-Policy-Report-Only: default-src 'self'; script-src 'self' https://trusted.com; report-uri /csp-violations;