MediaWiki before 1.35.1 allows XSS via BlockLogFormatter.php. Language::translateBlockExpiry itself does not escape in all code paths. For example, the return of Language::userTimeAndDate is is always unsafe for HTML in a month value. This affects MediaWiki 1.12.0 and later.
lists.debian.org/debian-lts-announce/2020/12/msg00034.html
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/STT5Z4A3BCXVH3WIPICWU2FP4IPIMUPC/
lists.wikimedia.org/pipermail/mediawiki-announce/2020-December/000268.html
phabricator.wikimedia.org/T268938
www.debian.org/security/2020/dsa-4816