Version 0.8.9 (latest) of Fail2ban allows to perform remote denial of service for arbitrary chosen IP address. Address listed on Fail2ban's whitelist are not affected. The vulnerability exists in Apache rules and it is caused by improper validation of a log file by regular expression. Malicious user can easily inject his own data to analyzed logs and deceive monitoring engine.

Affected files:
/filter.d/apache-auth.conf
/filter.d/apache-nohome.conf
/filter.d/apache-noscript.conf
/filter.d/apache-overflows.conf

Time frames:
01.06.2013 - Cyril Jaquier (contact section) has been informed about the vulnerability (no response)
08.06.2013 - The vulnerability has been released to the public.

More information, including proof of concept and patches is available here:
https://vndh.net/note:fail2ban-089-denial-service