Where has this javascript come from

Hi,

I have found a lot of javascript embedded in every page, and I am wondering where it has come from. It looks like this:

var SLXSWWSOOK = String.fromCharCode(16 - 6, 123 - 5, and the numbers go on and on and ends with eval(SLXSWWSOOK). There is also quite a lot of spam on some of the pages and posts. Am I right in thinking this has been inserted as the result of a vulnerability somewhere?

The site has Advance Custom Fields, Really Simple Captch, Restrict Content, User Role Editor and Contact Form 7 installed.

Thanks.