sogo_yhn configures SOGo with a parameter "SOGoTrustProxyAuthentication=YES". This causes the password to be bypassed during HTTP Basic authentication. An unauthenticated attacker who provides the username of an existing user and any arbitrary password can successfully log in to that user's account.
This issue was fixed in version 5.8.0~ynh9.
Authorization Bypass Through User-Controlled Key The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.