Non-registered IRC users using (1) ircd-hybrid 7.0.1 and earlier, (2) ircd-ratbox 1.5.1 and earlier, or (3) ircd-ratbox 2.0rc6 and earlier do not have a rate-limit imposed, which could allow remote attackers to cause a denial of service by repeatedly making requests, which are slowly dequeued.

Integer underflow in the clean_string function in irc_string.c in (1) IRCD-hybrid 7.2.2 and 7.2.3, (2) ircd-ratbox before 2.2.9, and (3) oftc-hybrid before 1.6.8, when flatten_links is disabled, allows remote attackers to execute arbitrary code or cause a denial of service (daemon crash) via a LINKS command.

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.

modules/m_capab.c in (1) ircd-ratbox before 3.0.8 and (2) Charybdis before 3.4.2 does not properly support capability negotiation during server handshakes, which allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via a malformed request.