Nextcloud Deck is a kanban style organization tool aimed at personal planning and project organization for teams integrated with Nextcloud. Prior to 1.14.6 and 1.15.2, a bug in the permission logic allowed users with "Can share" permission to modify the permissions of other recipients. This vulnerability is fixed in 1.14.6 and 1.15.2.
https://hackerone.com/reports/3247499
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-wwr8-hx9g-rjvv
https://github.com/nextcloud/deck/pull/7131
https://github.com/nextcloud/deck/commit/f1da8b30a455f02373d44154da04494c949a95ae