fail2ban 0.7.4 and earlier does not properly parse sshd log files, which allows remote attackers to add arbitrary hosts to the /etc/hosts.deny file and cause a denial of service by adding arbitrary IP addresses to the sshd log file, as demonstrated by logging in via ssh with a login name containing certain strings with an IP address.
https://exchange.xforce.ibmcloud.com/vulnerabilities/30739
http://www.vupen.com/english/advisories/2006/4877
http://www.securityfocus.com/bid/21469
http://security.gentoo.org/glsa/glsa-200702-05.xml
http://secunia.com/advisories/24184