Nextcloud Calendar is a calendar app for Nextcloud. Prior to 4.7.19, 5.5.6, and 6.0.1, the calendar app allowed blindly booking appointments with a squential ID without known the appointment token. This vulnerability is fixed in 4.7.19, 5.5.6, and 6.0.1.
https://hackerone.com/reports/3275810
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-7x2j-2674-fj95
https://github.com/nextcloud/calendar/pull/7537
https://github.com/nextcloud/calendar/commit/f41650c3681fc4a4130eb883f5c0899c011326b3