CVE-2026-42613: Grav Vulnerable to Privilege Escalation via Missing Server-Side Validation of groups/access

The Login::register() method in the Login plugin accepts attacker-controlled groups and access fields from the registration POST data without server-side validation. When registration is enabled and groups or access are included in the configured allowed fields list, an unauthenticated user can self-register with admin.super privileges by injecting these fields into the registration request.

This is a missing server-side validation issue — the only defense is a config-level fields allowlist, which is an admin-facing setting, not a hardcoded security boundary.