In Irssi before 1.0.5, overlong nicks or targets may result in a NULL pointer dereference while splitting the message. This may result in a remote denial of service.
Created irssi tracking bugs for this issue:
Affects: fedora-all [bug 1510963]
As far as I can tell, this was introduced sometime around version 0.8.17-rc2. The versions we ship are older.
This issue did not affect the versions of irssi as shipped with Red Hat Enterprise Linux 6 and 7.