If there were a back door to the program so to speak it would be picked up by the script kiddies and used to destroy every UBB out there. The code is open source for a reason and that is for others to be able to modify and create new features and things for the community.

If the code was compiled and no longer available to the community in general then the creators would be tasked with every little aspect of bug finding and development alone without others input causing a decline in the software popularity and distribution.

Well, there is no back door; it'd simply be a matter of setting the flags in your user file to be an administrator... I'm not sure of the line or the data however since it's been an eternity since I've used classic...

You really should upgrade...

<Loginname>
<Password>
<emailaddress>
<WebURL>
WriteAdmin& <------ Need this for being an admin
<morecrap>
<morecrap>
<Post Count>
Administrator <---- user title

Also the original master password was:
username Admin5
password 4tugboat

Give it a try.

You were supposed to change it or delete it after you installed the board.
If you left it intact. I would guess someone used it to delete your members.
Anyway the documentation for your version is
HERE

I totally for got about that. Yes look for that and if it is there get rid of it pronto.

I'd have your web host restore from a backup and then upgrade to at least ubb.classic 6.7.3

There are thousands of the 5-series Ultimate.cgi forums still out there - you'd be amazed at the number/types of sites that still run the old stuff

I'm still running one of my old boards under the old perl UBB. Only cus I never rewrote my firewall to PHP that I need on that board.