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. References: https://phabricator.wikimedia.org/T120883 https://lists.wikimedia.org/pipermail/mediawiki-announce/2020-December/000268.html
Created mediawiki tracking bugs for this issue: Affects: fedora-all [bug 1909241]
External References: https://lists.wikimedia.org/pipermail/mediawiki-announce/2020-December/000268.html