hi guy sif the hacker able to replace your index page what kind of hacking is that? is mybb or host problem?
Host problem - MyBB is only responsible for maintaining the index file.
so he able to penetrate the ftp account and replace the index page with the deface index?
Again HOST problem. Simply put, Get a stronger password
There are no known vulnerabilities in MyBB so your hosting account was compromised directly via a configuration vulnerability or a bad script other than MyBB (that could be a bad plugin as well) or indirectly via another account on the server being hacked and thus the whole server made vulnerable.
If you have a bad plugin or bad MyBB config/perms, then you could be subject to a redirect attack.