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.
The try_parse_v4_netmask function in hostmask.c in IRCD-Hybrid before 8.0.6 does not properly validate masks, which allows remote attackers to cause a denial of service (crash) via a mask that causes a negative number to be parsed.
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.
Ratbox IRC Daemon (aka ircd-ratbox) 2.2.5 and earlier allows remote attackers to cause a denial of service (resource exhaustion) by making many requests from a single client.
A Denial of Service vulnerability exists in ircd-ratbox 3.0.9 in the MONITOR Command Handler.