Nextcloud supports sharing of the registered users with other Nextcloud servers. Nextcloud supports adding these automated when selecting the "Add server automatically once a federated share was created successfully" setting.
As a public link can be added as federated share, an attacker can trigger this exchange if they have access to a public link. Thus getting access to basic user information.
It is recommended that the Nextcloud Server is upgraded to 19.0.11, 20.0.10 or 21.0.2
Disable "Add server automatically once a federated share was created successfully" in the Nextcloud settings.
If you have any questions or comments about this advisory: