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.
Exploitability
AV:NAC:HPR:NUI:NScope
S:UImpact
C:NI:NA:L3.7/CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:LInput Validation