all files of website include javascript malware

Hi all,

I have a problem with an account on my vps cpanel and centos, this website have bug but I can not find cause.

the problem is that every day I find a simple <script src="...."></script> in footer of all files php of website.

I see modified date and time of files and analyzed access log hoping to find out which action (for example POST) caused this hacking but I have not found anything, I checked /var/log/messages for proftpd login but resultless.

it is very difficult to discover what is the cause?

the application php that this website use is not opensource because is of private developers, but source is opened