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.
https://exchange.xforce.ibmcloud.com/vulnerabilities/57790
http://www.vupen.com/english/advisories/2010/1110
http://www.vupen.com/english/advisories/2010/1107
http://www.vupen.com/english/advisories/2010/0987
http://www.vupen.com/english/advisories/2010/0856
http://www.ubuntu.com/usn/USN-929-1
http://slackware.com/security/viewer.php?l=slackware-security&y=2010&m=slackware-security.497301
http://secunia.com/advisories/39797
http://secunia.com/advisories/39620
http://secunia.com/advisories/39365
http://marc.info/?l=oss-security&m=127119240204394&w=2
http://marc.info/?l=oss-security&m=127116251220784&w=2
http://marc.info/?l=oss-security&m=127110132019166&w=2
http://marc.info/?l=oss-security&m=127098845125270&w=2
http://lists.opensuse.org/opensuse-security-announce/2010-05/msg00001.html
http://lists.fedoraproject.org/pipermail/package-announce/2010-May/041054.html
http://irssi.org/news/ChangeLog
http://github.com/ensc/irssi-proxy/commit/85bbc05b21678e80423815d2ef1dfe26208491ab