I altered a UBB bulletin board pretty significantly as a favor to a friend. They don't use any standard modules and they share a lot of tricks in common with Matt's Scripts. They also have an odd file-locking system where they open a separate "lock.file" before they open any file... you can figure out what the various pieces do without much difficulty by going through the code.

Verifying users is simple enough. Passwords are stored as plain text in username.cgi files in the "Members/" directory. You just have to parse those files.

But, since their code is only accidentally open source... i.e., they sell their program, I'm not sure how they feel about people digging around in there.