Mercy Wrote: Well there is no way to "check for backdoors" as backdoors are found by hackers. There isn't like a tool where you can run and will tell you all the backdoors and vulnerabilities on your website. You could however deploy a Web Application Firewall or scan your website for vulnerabilties and patch it. By outdated theme I would presume that this theme hasn't been updated for some time. To my understanding of MyBB forums, I think the structure of themes have no changed much so it shouldn't really effect you.
Good luck
Well I mostly use Ransack Agent or whatever it is to scan files. In Garry's Mod (Lua) You mostly search for Run.str or Run.String or something like that so I was asking if there's any specific code which actually lets them to do something bad.