This extension to Phabricator performs basic authentication via a web server's REMOTE_USER variable. It should be able to work with a variety of major servers such as Apache and Nginx, but I have only tested it with Apache.

To install this library, simply clone this repository alongside your phabricator installation:

cd /path/to/install
git clone https://github.com/psigen/libphremoteuser.git

Then, simply add the path to this library to your phabricator configuration:

cd /path/to/install/phabricator
./bin/config set load-libraries '["libphremoteuser/src/"]'

When you next log into Phabricator as an Administrator, go to Auth > Add Authentication Provider.
In the list, you should now see an entry called Web Server. Enabling this provider should add a new button to your login screen.

In order to actually log in, your web server needs to populate the $REMOTE_USER variable when the login button is pressed. You can do this by forcing the login URI that Phabricator uses to be restricted, by adding a directive like the following to your web server configuration (this is Apache2):

<Location "/auth/login/RemoteUser:self/">
  Authtype Basic
  AuthName "Phabricator at My Server"
  Require valid-user
  
  Options None
  Order allow,deny
  Allow from all
</Location>

I make no guarantees about this library being totally secure. It's not obviously insecure.
However, please make sure to at least REDIRECT THE LOGIN URI TO SSL, OTHERWISE YOU ARE SENDING PLAIN TEXT PASSWORDS.

If you care about security consider:

• Hosting Phabricator entirely on https/SSL
• Restricting access to the whole Phabricator installation directory, also using SSL.