GHSA-p2ch-c2c3-4xm5: Winter: CSRF through AJAX handler names reachable as backend page actions

Affected versions of Winter CMS allow a backend AJAX handler to be invoked by a plain top-level GET navigation with no CSRF token. Backend\Classes\Controller::actionExists() accepted any public method on a controller as a page action, so handler-shaped names were never reserved from URL dispatch: an authenticated and authorized request to /backend/system/eventlogs/index_onEmptyLog reached the handler of the same name and truncated the system event log. Backend paths are routed through Route::any, and CSRF validation is skipped for HEAD, GET and OPTIONS requests.

The default SameSite=Lax session cookie is sent on top-level cross-site navigation, so a link on an attacker-controlled page is sufficient — no form, no script and no token. Handler arguments are taken from URL path segments, so the attacker also chooses the target record. Affected handlers across the backend, cms and system modules cover log truncation, resetting settings to their defaults, altering backend user state, and deleting CMS templates. None of them disclose data to the attacker or write attacker-controlled content, so the impact is destructive and state-changing rather than confidential.

To actively exploit this issue, an attacker would need no account of their own, but would need an authenticated backend user holding the relevant permission to follow an attacker-supplied link. The permissions guarding the affected handlers, system.access_logs included, are by default assigned only to the built-in Developer role.