Extension:AutomaticREMOTE USER - MediaWiki
www.mediawiki.org/wiki/Extension:AutomaticREMOTE_USER
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
This extension allows integration with the web server's built-in authentication system via the REMOTE_USER environment variable, which is set through HTTP-Auth, LDAP, CAS, PAM, and other authentication systems. The extension automatically logs-in users using the value of the REMOTE_USER environment variable as the MediaWiki username. If an account of that name does not already exist, one is created.