Testing a CSP Safely: Report-Only Mode Before You Enforce

The Risk of Deploying Blind

Turning on a strict CSP directly in production carries real risk — if the policy is even slightly too restrictive, it can silently break legitimate functionality your users depend on: a payment widget, an analytics script, a font that suddenly fails to load. Finding out about this from angry users, rather than before deployment, is avoidable.

The Report-Only Header

Sending Content-Security-Policy-Report-Only instead of the enforcing Content-Security-Policy header applies the exact same policy logic, but only reports violations rather than actually blocking anything:

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

Every time something on your site would have violated the policy, the browser sends a report to your specified endpoint — but nothing is actually blocked. This lets you see exactly what a policy would break before committing to it.

A Practical Rollout Sequence

1. Deploy in report-only mode with your intended policy.
2. Monitor the reports for a reasonable period — long enough to capture normal traffic patterns and less common user flows.
3. Refine the policy based on legitimate violations you find — adding a genuinely needed source, for instance — while resisting the urge to add overly broad exceptions just to silence noise.
4. Switch to the enforcing header once reports show the policy no longer breaks anything legitimate.
5. Keep monitoring afterward — new features or third-party scripts added later can introduce new violations the original policy didn't anticipate.

Third-Party Scripts Deserve Extra Attention

Analytics, chat widgets, and embedded content frequently load additional scripts from their own domains — sometimes dynamically, which is harder to predict in advance. Report-only testing is especially valuable here, surfacing exactly which third-party origins your site actually depends on before you lock the policy down.