functions,php files hacked

Hello all,

I was recently contracted out by an entertainment company to do some light web design work. Over the past weekend, no more than three weeks into my contract, I was alarmed to discover all their WordPress websites had ceased to function.

Now, I am not a web developer by any stretch of the imagination. However, I have been able to deduce that the problem lies with a piece of code that is being added to the functions.php file.

[Code moderated. Please do not post hack code blocks in the forums. Please use the pastebin]

This code was added to every functions.php file on our Windows server. If I restore the file (relatively easy), the websites work perfectly fine. However, the malicious code returns within a few hours.

Has anyone encountered this specific attack before? Is there any way I can protect my functions.php files? Or can this only be resolved by generally strengthening server security?

I apologize for any naivete or lack of details in my question. My company unfortunately does not see it fit to hire a full time web developer or IT guy, so I am doing my best with my modest technical skills.