In WordPress before 4.7.5, there is insufficient redirect validation in the HTTP class, leading to SSRF.
https://www.debian.org/security/2018/dsa-4090
https://wpvulndb.com/vulnerabilities/8815
https://wordpress.org/news/2017/05/wordpress-4-7-5/
https://twitter.com/skansing/status/865362551097393153
https://github.com/WordPress/WordPress/commit/76d77e927bb4d0f87c7262a50e28d84e01fd2b11
https://codex.wordpress.org/Version_4.7.5