Uninvited user is able to join and mark the attendance of the the private event

Discourse Calendar adds the ability to create a dynamic calendar in the first post of a topic on Discourse. Uninvited users are able to gain access to private events by crafting a request to update their attendance. This problem is resolved in commit dfc4fa15f340189f177a1d1ab2cc94ffed3c1190. As a workaround, one may use post visibility to limit access.