Nextcloud mail is an email app for the Nextcloud home server platform. In affected versions a missing check of origin, target and cookies allows for an attacker to abuse the proxy endpoint to denial of service a third server. It is recommended that the Nextcloud Mail is upgraded to 2.2.8 or 3.3.0. There are no known workarounds for this vulnerability.
https://hackerone.com/reports/1895874
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-8j9x-fmww-qr37