Test a Content Security Policy with Report-Only Mode

A strict Content Security Policy can block scripts, styles, images or embedded content that a site still needs. Report-only mode lets a team observe violations while the browser continues to load the page. It is a testing phase, not a security policy that blocks those resources.

Start with a narrow candidate

Serve a Content-Security-Policy-Report-Only response header on a representative staging page. Begin with sources you understand, then review which scripts, styles, images, frames and API connections the page actually uses. Avoid adding broad wildcards just to silence reports.

For example, a candidate might start with default-src 'self'; object-src 'none'; base-uri 'self', but this can generate violations for a site that uses external assets or inline code. It is only a starting point for inventory, not a drop-in policy.

Collect and classify violations

Review browser developer tools and configure a supported reporting endpoint if you want centralized reports. Ensure the endpoint is implemented, protected against abuse and does not collect more information than needed. Reports can contain page URLs and other operational details.

Separate expected third-party resources from unexpected ones. Confirm each required domain with the service owner and check whether the resource can be self-hosted. A browser extension can also affect local testing, so compare a clean profile.

Enforce gradually

Test login, payment, forms, embedded media and any administrative workflow before switching to an enforcing header. Roll out to a small representative portion, monitor violations and keep a rollback path. Once enforced, remove obsolete allowances and continue to review the policy when the site changes.

MDN's Report-Only header reference explains the browser behavior. The goal is a policy grounded in observed dependencies, not a long directive copied from an unrelated website.