Nextcloud User Saml is an app for authenticating Nextcloud users using SAML. In affected versions users can be given a link to the Nextcloud server and end up on a uncontrolled thirdparty server. It is recommended that the User Saml app is upgraded to version 5.1.5, 5.2.5, or 6.0.1. There are no known workarounds for this issue.
https://hackerone.com/reports/2263044
https://github.com/nextcloud/user_saml/pull/788
https://github.com/nextcloud/user_saml/commit/b184304a476deeba36e92b70562d5de7c2f85f8a
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-622q-xhfr-xmv7