Buffer overflow in mIRC 5.91 and earlier allows a remote server to execute arbitrary code on the client via a long nickname.
The DCC server command in the Mirc 5.5 client doesn't filter characters from file names properly, allowing remote attackers to place a malicious file in a different location, possibly allowing the attacker to execute commands.
The locking feature in mIRC 5.7 allows local users to bypass the password mechanism by modifying the LockOptions registry key.