In WordPress before 4.7.5, there is improper handling of post meta data values in the XML-RPC API.
https://wpvulndb.com/vulnerabilities/8816
https://wordpress.org/news/2017/05/wordpress-4-7-5/
https://github.com/WordPress/WordPress/commit/3d95e3ae816f4d7c638f40d3e936a4be19724381
https://codex.wordpress.org/Version_4.7.5
http://www.securitytracker.com/id/1038520