Nextcloud Desktop Client before 3.3.1 is vulnerable to improper certificate validation due to lack of SSL certificate verification when using the "Register with a Provider" flow.
https://www.debian.org/security/2021/dsa-4974
https://hackerone.com/reports/903424
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-qpgp-vf4p-wcw5