I am using Hetzner's dedicated servers. Today I have got a message:

We have received information that there was an attack from your server.

----- attachment -----

##########################################################################
#               Netscan detected from host      ?.?.?.?                  #
##########################################################################

time                protocol src_ip src_port          dest_ip dest_port
---------------------------------------------------------------------------
Thu Dec  6 09:52:00 2012 TCP         ?.?.?.? 52204 =>     68.64.12.28 21   
Thu Dec  6 09:52:00 2012 TCP         ?.?.?.? 53276 =>     68.64.12.29 21   
Thu Dec  6 09:52:00 2012 TCP         ?.?.?.? 47344 =>     68.64.12.30 21   
...

How can I find out what is causing the problem and how do I fix it?

Thanks in advance, Etam.