Plex Media Server before 0.9.9.3 allows remote attackers to bypass the web server whitelist, conduct SSRF attacks, and execute arbitrary administrative actions via multiple crafted X-Plex-Url headers to system/proxy, which are inconsistently processed by the request handler in the backend web server.
https://forums.plex.tv/index.php/topic/62832-plex-media-server/?p=583250