Versions mentioned in the description apply to the upstream
Remediation section below for
Debian:9 relevant versions.
An issue was discovered in MediaWiki before 1.35.1. Missing users (accounts that don't exist) and hidden users (accounts that have been explicitly hidden due to being abusive, or similar) that the viewer cannot see are handled differently, exposing sensitive information about the hidden status to unprivileged viewers. This exists on various code paths.
mediawiki to version 1:1.27.7-1~deb9u7 or higher.