Nextcloud guests app is a utility to create guest users which can only see files shared with them. In affected versions users were able to load the first page of apps they were actually not allowed to access. Depending on the selection of apps installed this may present a permissions bypass. It is recommended that the Guests app is upgraded to 2.4.1, 2.5.1 or 3.0.1. There are no known workarounds for this vulnerability.
https://hackerone.com/reports/2251074
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-v3qw-7vgv-2fxj