Before version 4.8.2, WordPress allowed a Directory Traversal attack in the Customizer component via a crafted theme filename.
https://www.debian.org/security/2017/dsa-3997
https://wpvulndb.com/vulnerabilities/8912
https://wordpress.org/news/2017/09/wordpress-4-8-2-security-and-maintenance-release/
https://core.trac.wordpress.org/changeset/41397