Exploit in WordPress 3.5.1?

Hi guys, my site is under attack and some other sites have suffered the same type of lines. We’re worried about this matter because our site has been demoted from Google and other search engines, they think we’re malware seeds.

In the latter days our sites have been infected in two different places
wp-content/plugins/sharebar/js/default.php
wp-includes/js/jquery/default.php

We change our passwords, usernames and other stuff related to our security but in the last 3 weeks, every other day we receive the same attack. The plugin was removed to avoid future attacks.

Anyone knows how to prevent this?