No, not only mIRC has bugs
For successful exploitation of the security hole the user needs to be tricked to follow a maliciously crafted irc:// link – “Failed exploit attempts may cause denial-of-service conditions.” at least, or might even enable the attacker “to execute arbitrary code with the privileges of the user running the affected application.” - which we all know is Administrator for 95% of all Windows machines.
However, this post on the KVIrc mailing list claims the bug is invalid and KVIrc 3.4.x is not affected but after a short test i can at least confirm that there indeed is an issue that causes a DoS because KVIrc crashes after opening the malformed link.
The usual suggestion to upgrade to the latest version to be not prone to that vulnerability is superfluous at least for the Windows-version of KVIrc, as 3.4.0 is the latest “stable” release that can be obtained from the website.
Update 11/7/08: There is now an update to version 3.4.2 available for download.