Impact

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.

Patches

The latest version of the Discourse Calendar plugin has the patch.

Workarounds

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.