Possible WordPress/plugin Vulnerability: wp-loadings.php added on root dir

Recentry we discovered that our site was hacked. We are unaware which exploit is used in order to write files to our server root directory. I am starting this thread in order to find other people which had the same issue, and gain further insight.

Follows a basic description of the hack:

A file is uploaded to the root installation folder (wp-loadings.php) Theme file 404.php is modified. Hack takes place when a googlebot is served a 404 page, a third party page is served instead (from modified 404.php), to add backlinks we think.

Apart from that, the frontend does not seem affected.

A google search, shows that it has affected many other wordpress installations. https://www.google.gr/webhp?q=inurl%3Awp-loadings.php

All google links, lead to a domain like: [removed]

wp-loadings.php source
[removed]