HydraIRC 0.3.164 and earlier allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via a long irc:// URI. References ...

Heap-based buffer overflow in HydraIRC 0.3.151 allows remote IRC servers to cause a denial of service (application crash) via a long CTCP request message containing '%' (percent) characters.

That is a good question; just out of interest you may want to check out HydraIRC as a free mIRC alternative. I use this myself, it's not any more difficult than mIRC and a lot of the same ...