MyBB Community Forums

When choosing hosting, can I put my website at "website.com", and the MyBB installation at "forum.website.com", but have them be on two separate machines? So that if one is compromised/hacked, I don't also lose the other? 

Also... Another question could it be possible to disable the "login/registration" in MyBB and have my main website supply the forum with a token/cookie for the user. So the user can only login through my main website, and if the machines are separate the communication of the token/cookie would be the only communication between the two? If that made sense...   

Another question, can I remove "Login" and "Register" from being visible on the forum? 
Can I disable "Register" API on the forum and modify "Login" API so that my main website is the one that's supposed to use it? 
Is there a "cookie refresh API" that my main website can use? So that when a user refreshes their cookie, my main website gets a notification? And when the user does something on my main website, I can send a notification to the forum to refresh their cookie? 

Ty if someone can help

You can have subdomains on different servers, it's not a trouble.
But you can also just separate processes with using different user (like "domain" and "forum") and run each site with its dedicated user, I usually do that (using nginx + php-fpm).

You can remove register (simply disable registration in ACP), and you can remove the login link & form (edit the templates) and use your own form to login users.
Having the cookie global for domain and subdomain might work, but think that you'll have to use the mybb cookie, not yours, unless you want to spend a lot of time to create the good one

I think I need to find where the cookie gets refreshed, and add a "wget" to my server to refresh my cookie there (I think I need to switch from JWT to cookie cause I don't know php to get the user a new JWT thru the forum)

And also add another endpoint in the php/forum to accept a cookie refresh request, from the main server when they do an action there

So if i keep 2 cookies on their browser, and figure out how to call out and accept a request in thru php, maybe it could work..

Also modify Login to take in a 2nd password so that it only works with the main server

.... Just have no idea about how to host on two different machines I've heard you can partition a VM into more VMs or something like that when you buy hosting.... If you say it's not a problem hopefully it's true especially for the myBB installation cause I haven't done that before

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Think maybe I could just delete Login and Registration

Ignore cookies

And find where they authenticate cookies and replace it with signing a token with the same secret and algorithm I use on my main website

So the forum has no sign in... And using the forum will refresh the same token you use on the main website, and vice versa... So I don't even need to communicate between the two ever... Except possibly once to call registration to fill whatever user database there is