Spammer or a hacker is uploading a file.....

[romio_58]

Someone is being able to upload to my /var/www/vhosts/domain a file that sends tons of emails which is causing my server almost to crash, i am not able to understand how this is happening? how can i avoid this? anyone could help with this matter its really series since my clients are not able to use anymore webmail.

I have uploaded a sample of the file, the file usually is located on the root of the directory with the one of the following names:

default.php
_default.php
post.php
home.php

I use fedora