James Martin (also known as uuuppz) released a bug in mIRC which affects all versions prior to 6.0. Its recommended to update your mIRC client! All clients runing version 6.0 are recommended to update to 6.01.
This bug is located in the nick handling system of the popular windows IRC Client. By reconnecting a user to a malicious server and changing the users nick to a size longer then 200 tokens full control can be gained over the victims computer.
For more information visit http://www.uuuppz.com/research/adv-001-mirc.htm
To get the newest mIRC client (6.01) visit http://www.mirc.com