wp-includes/feed.php in WordPress before 4.9.1 does not properly restrict enclosures in RSS and Atom fields, which might allow attackers to conduct XSS attacks via a crafted URL.
https://www.debian.org/security/2018/dsa-4090
https://wpvulndb.com/vulnerabilities/8967
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/f1de7e42df29395c3314bf85bff3d1f4f90541de