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.

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.

The unformat_24bit_color function in the format parsing code in Irssi before 0.8.20, when compiled with true-color enabled, allows remote attackers to cause a denial of service (heap corruption and crash) via an incomplete 24bit color code.

The format_send_to_gui function in the format parsing code in Irssi before 0.8.20 allows remote attackers to cause a denial of service (heap corruption and crash) via vectors involving the length of a string.

The nickcmp function in Irssi before 0.8.21 allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via a message without a nick.

Use-after-free vulnerability in Irssi before 0.8.21 allows remote attackers to cause a denial of service (crash) via an invalid nick message.

Irssi before 0.8.21 allows remote attackers to cause a denial of service (out-of-bounds read and crash) via a string containing a formatting sequence (%[) without a closing bracket (]).