The apache-auth.conf, apache-nohome.conf, apache-noscript.conf, and apache-overflows.conf files in Fail2ban before 0.8.10 do not properly validate log messages, which allows remote attackers to block arbitrary IP addresses via certain messages in a request.
https://vndh.net/note:fail2ban-089-denial-service
https://raw.github.com/fail2ban/fail2ban/master/ChangeLog
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A17338
http://www.openwall.com/lists/oss-security/2013/06/13/7
http://www.debian.org/security/2013/dsa-2708
http://lists.opensuse.org/opensuse-updates/2014-03/msg00021.html