Nextcloud Desktop is the desktop sync client for Nextcloud. In versions of Nextcloud Desktop prior to 3.15, 3rdparty applications already installed on a user machine can create link shares for almost all data via the socket API. These shares can then be easily sent off to an external service. Nextcloud Desktop fixes the issue in version 3.15. No known workarounds are available.
https://hackerone.com/reports/1995856
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-qm2f-959g-7p65