Revive Adserver before 3.2.3 suffers from Information Exposure Through Discrepancy. It is possible to check whether or not an email address was associated to one or more user accounts on a target Revive Adserver instance by examining the message printed by the password recovery system. Such information cannot however be used directly to log in to the system, which requires a username.
https://www.revive-adserver.com/security/revive-sa-2016-001/
https://hackerone.com/reports/98612
https://github.com/revive-adserver/revive-adserver/commit/38223a841190bebd7a137c7bed84fbbcb2b0c2a5