Debian Security Advisory 1456-1 – Daniel B. Cid discovered that fail2ban, a tool to block IP addresses that cause login failures, is too liberal about parsing SSH log files, allowing an attacker to block any IP address.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/62479/dsa-1456-1.txt
Source: https://packetstormsecurity.com/files/62479/Debian-Linux-Security-Advisory-1456-1.html