In Nextcloud Desktop Client 3.13.1 through 3.13.3 on Linux, synchronized files (between the server and client) may become world writable or world readable. This is fixed in 3.13.4.
https://github.com/nextcloud/security-advisories/security/advisories
https://github.com/nextcloud/desktop/pull/7092
https://github.com/nextcloud/desktop/pull/6949
https://github.com/nextcloud/desktop/issues/6863
https://github.com/nextcloud/desktop/compare/v3.13.3...v3.13.4