We already include some user 'key' protection, so a single user can't get disconnected and another user 'take over' their session.
This was a work around from an exploit a few years ago.
Without knowing what the root of any exploit is, we can't even guess as to help.
I've looked over a few posts as referenced in original post and linked from some of those posts, and not one has stated how the exploit works.
To 'harden' your server, and ULX for now, I'd recommend some tips.
Disable RCON if you can.
NEVER store your rcon password in ANY cfg file (set it in your servers startup command line, google how.
If you have dedicated host, NEVER use same password for it's cpanel as you do for your rcon and/or ftp.
Remove ulx "rcon, cexec, ent, exec, luarun, adduser, adduserid, userallow, userallowid" from all groups. (meaning, yes, you'll have to add users to your Gmod group users.txt file the old fashioned way)
By no means am I saying ULX isn't exploitable...we just find 99% of most times in past that it's some other gamemode or addon that can be exploited, and ULX makes it just as easy for the 'hacker' to run commands as it does for you as a legitimate admin.
