Then most probably your hosting provider doesn't perform enough security measures to prevent attack to your website. You may ask their support (also ask it for the owner of that file).
One possible way to break your system is from neighbor webhost (when hosting provider is not
Secure enough). It's possible if, for example every web host is owned by the same user 'nobody' and no separate uids for every web site owner. Ask support is it true about your web site.