cache.c in ircd-ratbox before 2.2.9 allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via a HELP command.
http://www.debian.org/security/2010/dsa-1980
http://security.debian.org/pool/updates/main/i/ircd-ratbox/ircd-ratbox_2.2.8.dfsg-2+lenny1.diff.gz
http://secunia.com/advisories/38383
http://secunia.com/advisories/38210
http://lists.ratbox.org/pipermail/ircd-ratbox/2010-January/000891.html