Inherited Orphaned Plone Site

Back in 2003, someone within our company created an intranet information portal using Plone. All known admins for creating new users have since left the company. I'm sure I can eventually find someone with admin privileges on the web server where it's installed, but I suspect they won't know anything about the Plone installation. It continues to run, and existing users who know their passwords can login and make changes to content on the site, but we don't know how to add a new user.

How can I find out what version of Plone is installed?

When I attempt to navigate to internal-domain/directory/of/portal/manage, I get redirected to /insufficient_privileges (I am not logged into the portal, as I don't have an account).

There is a page that allows searching for users, where one can get a list of all users who have ever logged in. The last known admin for the site does not show up in the list of users, so either he was using someone else's account, or he was adding users on the backend without logging into the portal himself.

Are accounts for the Zope Management Interface distinct from Plone user accounts?

I'm guessing that once I find the proper web admin, I need them to follow the instructions given here:
https://docs.plone.org/3/en/manage/deploying/zope.html#adding-users-from-command-line-reset-admin-password

I tried to visit https://plone.org/documentation/faq/locked-out which is referenced in the page above, but that page doesn't exist.

If you have any other tips as to how to regain administrative privileges on an existing Plone site that has been "abandoned" by former admins, I would appreciate it!

if they used company email addresses and you know their loginname (or its the email address) then you can work with your email administrator to get the password reset email for that account from the company catchall. Then you can login as the old admin and create a new admin.

Once you have shell access and find the zserver instance script (which depends on how zope was deployed) then its reasonably simple. There is an adduser command

$ bin/instance help

Documented commands (type help <topic>):
========================================
EOF      debug       help       logtail  restart  show    stop
adduser  fg          kill       quit     run      start   wait
console  foreground  logreopen  reload   shell    status

Miscellaneous help topics:
==========================
startup_command

Undocumented commands:
======================
reopen_transcript  test
  1. create yourself a new admin account using bin/instance adduser <username> <password>

  2. access the ZMI/Zope root over HTTP directly on its HTTP port. By-pass virtual hosting if necessary by using SSH tunneling or whatever.

I would try to add a "Emergency User" to Zope and then i would try to add a Plone Admin to acl_users of your Plone Instance. Add Emergency User

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