In MediaWiki before 1.31.8, 1.32.x and 1.33.x before 1.33.4, and 1.34.x before 1.34.2, private wikis behind a caching server using the img_auth.php image authorization security feature may have had their files cached publicly, so any unauthorized user could view them. This occurs because Cache-Control and Vary headers were mishandled.
https://www.debian.org/security/2020/dsa-4767
https://phabricator.wikimedia.org/T248947
https://lists.wikimedia.org/pipermail/wikitech-l/2020-June/093535.html
https://lists.debian.org/debian-lts-announce/2020/12/msg00034.html
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_34/RELEASE-NOTES-1.34
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_33/RELEASE-NOTES-1.33
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_31/RELEASE-NOTES-1.31