Hack a centos linux box

HI,
I have a Centos linux box and there is interface connect to internet.
I stop the iptables in this box .
After a few day , I find the linux box have been hacked and install some perl script into the box .
Could anyone tell me how the hacker can login into the centos box without knowing the root passwd ???
How can they do it ?? please advise in detailed step...
Thank

What services was your CentOS running? Telnet? OpenSSH? ....
IF your root passwword was too simple, hackers could figure it out in no time!

use john?

john only for you has the password file