Enumeration of subscribed users and unauthenticated email unsubscriptions on https://newsletter.nextcloud.com/?p=unsubscribe

How it was found

Hello,

The mentioned URL contains a form that, when supplied correct user emails, unsubscribes users from the newsletters they're subscribed to. If the user is not subscribed, the form returns a message that says that the user is not subscribed if this is the case.

Regards