SSRF as in Server Side Request Forgery is a vulnerability that allows an attacker to force server interfaces into sending packets initiated by the victim server to the local interface or to another server behind the firewall. Consult Web References for more information about this problem.
Properly sanitize user requests or use a special sandboxed host to route requests to remote resources
HTTP/2: The Sequel is Always Worse
Weird proxies/2 and a bit of magic