The auth component in Dovecot before 2.2.27, when auth-policy is configured, allows a remote attackers to cause a denial of service (crash) by aborting authentication without setting a username.
http://www.securityfocus.com/bid/94639
http://www.openwall.com/lists/oss-security/2016/12/05/12
http://www.openwall.com/lists/oss-security/2016/12/02/4
http://dovecot.org/pipermail/dovecot-news/2016-December/000333.html