Helmut Grohne discovered that denyhosts, a tool preventing SSH
brute-force attacks, could be used to perform remote denial of service
against the SSH daemon. Incorrectly specified regular expressions used
to detect brute force attacks in authentication logs could be exploited
by a malicious user to forge crafted login names in order to make
denyhosts ban arbitrary IP addresses.
SOURCE:
http://seclists.org/oss-sec/2013/q4/535