config/filter.d/postfix.conf in the postfix filter in Fail2ban before 0.8.11 allows remote attackers to trigger the blocking of an arbitrary IP address via a crafted e-mail address that matches an improperly designed regular expression.
http://www.kb.cert.org/vuls/id/686662
http://www.debian.org/security/2014/dsa-2979
http://lists.opensuse.org/opensuse-updates/2014-03/msg00021.html