Dovecot before version 2.2.29 is vulnerable to a denial of service. When 'dict' passdb and userdb were used for user authentication, the username sent by the IMAP/POP3 client was sent through var_expand() to perform %variable expansion. Sending specially crafted %variable fields could result in excessive memory usage causing the process to crash (and restart), or excessive CPU usage causing all authentications to hang.
https://www.debian.org/security/2017/dsa-3828
https://github.com/dovecot/core/commit/000030feb7a30f193197f1aab8a7b04a26b42735.patch
https://dovecot.org/pipermail/dovecot-news/2017-April/000341.html
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2017-2669