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 stop...
Thank

The UNIX and Linux Forums is not a hackers site. We are not going to give you instructions describing how to break into a Centos Linux box (or any other box).