It was discovered that Dovecot incorrectly handled client certificates. A
remote attacker in possession of a valid certificate with an empty username
field could possibly use this issue to impersonate other users.
Created dovecot tracking bugs for this issue:
Affects: fedora-all [bug 1673416]
This basically affects installation which enable client-side certs. When authenticating with a cert which has empty "user" field, vulnerable versions take the loginame from the user. If no additional password is configured this will allow the user to impersonate anyone else in the system.
Attack can be migitated by having the certificates with proper Extended Key Usage, such as 'TLS Web Server' and 'TLS Web Server Client'. Also client-side certification authentication can be turned off using:
auth_ssl_require_client_cert = no
auth_ssl_username_from_cert = no