irssi IRC client 0.8.4, when downloaded after 14-March-2002, could contain a backdoor in the configuration file, which allows remote attackers to access the system.

Irssi before 0.8.15, when SSL is used, does not verify that the server hostname matches a domain name in the subject's Common Name (CN) field or a Subject Alternative Name field of the X.509 certificate, which allows man-in-the-middle attackers to spoof IRC servers via an arbitrary certificate.

The formatsendtogui function in formats.c for irssi before 0.8.9 allows remote IRC users to cause a denial of service (crash).

IRC client irssi in irssi-text before 0.8.4 allows remote attackers to cause a denial of service (crash) via an IRC channel that has a long topic followed by a certain string, possibly triggering a buffer overflow.

core/nicklist.c in Irssi before 0.8.15 allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via vectors related to an attempted fuzzy nick match at the instant that a victim leaves a channel.