Discourse-calendar is a plugin for the Discourse messaging platform which adds the ability to create a dynamic calendar in the first post of a topic. Members of private groups or public groups with private members can be listed by users, who can create and edit post events. This vulnerability only affects sites which have discourse post events enabled.

This issue has been patched in commit ca5ae3e7e which will be included in future releases. Users unable to upgrade should disable the discourse_post_event_enabled setting to fully mitigate the issue. Also, it's possible to prevent regular users from using this vulnerability by removing all groups from the discourse_post_event_allowed_on_groups but note that moderators will still be able to use it.