wp-includes/general-template.php in WordPress before 4.9.1 does not properly restrict the lang attribute of an HTML element, which might allow attackers to conduct XSS attacks via the language setting of a site.
https://www.debian.org/security/2018/dsa-4090
https://wpvulndb.com/vulnerabilities/8968
https://wordpress.org/news/2017/11/wordpress-4-9-1-security-and-maintenance-release/
https://lists.debian.org/debian-lts-announce/2017/12/msg00019.html
https://github.com/WordPress/WordPress/commit/3713ac5ebc90fb2011e98dfd691420f43da6c09a