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.
https://www.debian.org/security/2020/dsa-4816
https://phabricator.wikimedia.org/T268938
https://lists.wikimedia.org/pipermail/mediawiki-announce/2020-December/000268.html
https://lists.debian.org/debian-lts-announce/2020/12/msg00034.html