Javascript code injection in nearly all JS files and backdoors

Hi!

We are experiencing problems with hacked wordpress installations with backdoors and javascript code injections in nearly all JS files in all main folders
wp-admin
wp-content
wp-includes

The code which is getting injected always at the end of the files looks like this:

[ Malware redacted, do not post that in these forums. ]

Using the Wordfence and Anti-Malware plugin I have been able to remove the problems and tried to secure the sites as good as possible -.

We have not had problems for a week, but suffered attacks and injections again.

Does somebody have similar problems? I would appreciate help!