An update to fail2ban 0.8.13 has been released to fix security issues, amongst other bugfixes.
fail2ban versions prior to 0.8.11 would allow a remote unauthenticated attacker to cause arbitrary IP addresses to be blocked by Fail2ban causing legitimate users to be blocked from accessing services protected by Fail2ban. These services are cyrus-imap (CVE-2013-7177) and postfix (CVE-2013-7176).