Is it possible a theme to have attached some kind of backdoor/virus/shell on it?
Thank you.
Yes. Themes contain templates, templates contain JavaScript (which may grab and forward your login data among other things), in addition to that MyBB uses eval() to evaluate templates so direct PHP execution on your server (and thus direct DB access, shells, etc.) is a possibility.
You should only use plugins/themes from sources you trust.
I dont think so it is possible
