known vulnerability with vBulletin?

I belong to a car website ( www.7thgencivic.com ) they use the same vBulletin software that we use here on the O/C forums and they've been hacked twice in the last two days by the same people, they don't do anything to the page, just put there own image as the homepage then disable the forums. Soo... is there a known vulnerability that they are exploiting, they seem to be doing it to prove a point to the pageowner that the page is vulnerable, not much else... OH and BTW at the bottom of the image thay are loading in it says (pardon my french) FVCK the USA and Afghanistan. hmmmm..... it just ****es me of i thought that the vBUlletin software was pretty secure. oh well, sorry for the rant.