Looks like your account is compromised. Probably FTP or your shared host is the issue.
It's happened on 2 of my more active forums, I reported that one to them at CH, and MyBBSource had one that I found. Just MyBB or the website? Who knows..
You should change your host and use more secure passwords
It's only happening with folders that have the permissions set to 777. It could effect CMSs, forums, anything. My site hasn't been affected (not that I know of). I don't think it's a host issue.
But why would the root directory be set to 777 anyway??