Fail2ban monitors log files for failed authentication attempts and can block hosts responsible for repeated attacks.
A flaw in the method used to parse log entries allows remote, unauthenticated attackers to forge authentication attempts from other hosts.
A remote attacker can add arbitrary hosts to the block list, denying legitimate users access to a resource.
There is no known workaround at this time.
All Fail2ban users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=net-analyzer/fail2ban-0.6.2"