The Nextcloud Desktop Client is a tool to synchronize files from Nextcloud Server with your computer. The Desktop client did not stop with an error but allowed by-passing the signature validation, if a manipulated server sends an empty initial signature. It is recommended that the Nextcloud Desktop client is upgraded to 3.14.2 or later.
https://hackerone.com/reports/2597504
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-r4qc-m9mj-452v
https://github.com/nextcloud/desktop/pull/7333
https://github.com/nextcloud/desktop/commit/97539218e6f63c3a3fd1694cb7d8aef27c5910d7