Ghost is a Node.js content management system. From 5.18.0 until 6.21.1, a discrepancy in responses from the members signin endpoints made it possible for an unauthenticated attacker to determine whether a given email address belongs to a registered member of a Ghost site. This vulnerability is fixed in 6.21.1.
https://github.com/TryGhost/Ghost/security/advisories/GHSA-chgm-3698-jm42