Taken from an NTBugTraq advisory e-mail.

mIRC provides scripting capabilities to allow extension of the
client. A flaw exists in the $asctime identifier, which is used to
format Unix style time stamps. Passing a string of sufficient length
to $asctime will cause a buffer overflow on the stack. This allows
the execution of byte code through calling $asctime with a carefully
constructed string.

The default script included with mIRC does not call $asctime at any
point. However the majority of major scripts available for download
call $asctime to decode data provided by the irc server. Many scripts
call $asctime on data provided from other remote sources. The
exploitation of this flaw therefore depends on the script installed
by the victim.
mIRC has been notified and a new patched version 6.03 is available for download at mIRC.