In the past week anytime a new post goes up, it eventually gets edited, but not by me obviously.
This code is inserted into the post:
<font style="overflow: hidden; width: 0px; position: absolute; height: 0px"><!--4848--><a href="http://www.jtosh.com/temp/hqc.php?blowjob.htm">hardcore blow job</a>
followed by probably a hundred different links, though all starting with "http://www.jtosh.com/temp/hqc.php?". (the link is different on different posts, but the format is the same.)
The links do not show up on the page at all (I'm assuming it's the overflow: hidden; tag), however if someone copys and pastes the text it shows up.
Also, when a post is edited like this the comments are disabled for that post.
What security measures should I take to prevent this from happening?