Nextcloud Calendar is a calendar app for Nextcloud. Authenticated users could create an event with manipulated attachment data leading to a bad redirect for participants when clicked. It is recommended that the Nextcloud Calendar App is upgraded to 4.6.8 or 4.7.2.
https://hackerone.com/reports/2457588
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-2r7q-vfmv-79qf