How a Hidden Form Wiped a CDN Cache and Led to Stronger CSRF Protection

A PHP video admin panel was vulnerable to Cross-Site Request Forgery (CSRF) after a hidden HTML form on an unrelated forum page silently triggered a full CDN cache purge by exploiting an active browser session. The incident exposed a common security gap: relying solely on session cookies to authenticate state-changing admin actions. To fix this without disrupting a LiteSpeed page cache or SQLite-backed session storage, the developer adopted a signed double-submit cookie pattern instead of the traditional synchronizer token approach. This stateless method stores a signed HMAC token in both a cookie and a form field, then verifies both match on every unsafe request, without any server-side session writes. The implementation also uses the __Host- cookie prefix and token expiry checks to prevent subdomain or network-level cookie injection attacks.