Nextcloud Server is a self hosted personal cloud system. Private shared calendar events' recurrence exceptions can be read by sharees. It is recommended that the Nextcloud Server is upgraded to 27.1.10 or 28.0.6 or 29.0.1 and that the Nextcloud Enterprise Server is upgraded to 27.1.10 or 28.0.6 or 29.0.1.
https://hackerone.com/reports/2479325
https://github.com/nextcloud/server/pull/45309
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-h4xv-cjpm-j595