Hi ,
This hack is called HLBrute . The working principle of the hack is like it has a database of most used rcons of server admins and it tries each and every rcon one by one on your server to check the rcon .
To get rid of this hack
1)Use more complex rcon
2) Add the following lines in your server.cfg
sv_rcon_banpenalty 5
sv_rcon_maxfailures 10
sv_rcon_minfailures 5
sv_rcon_minfailuretime 30
It will auto ban the ip after failure with wrong rcon 5 times . Also there is a minfailure time . So the hack will not work efficiently with this settings .
Thanks and regrads,
Anupam
My Site :
http://timepassgamers.co.in