Deck is a kanban style organization tool aimed at personal planning and project organization for teams integrated with Nextcloud. In affected versions users could be tricked into executing malicious code that would execute in their browser via HTML sent as a comment. It is recommended that the Nextcloud Deck is upgraded to version 1.9.5 or 1.11.2. There are no known workarounds for this vulnerability.
https://hackerone.com/reports/2058556
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-mg7w-x9fm-9wwc
https://github.com/nextcloud/deck/commit/91f1557362047f8840f53151f176b80148650bcd