Kanboard's privilege escalation via mass assignment in user invite registration allows any invited user to become admin

Kanboard is project management software focused on Kanban methodology. Prior to 1.2.51, Kanboard's user invite registration endpoint (`UserInviteController::register()`) accepts all POST parameters and passes them to `UserModel::create()` without filtering out the `role` field. An attacker who receives an invite link can inject `role=app-admin` in the registration form to create an administrator account. Version 1.2.51 fixes the issue.