The Apache HTTP Server in Apple OS X before 10.12 and OS X Server before 5.2 follows RFC 3875 section 4.1.18 and therefore does not protect applications from the presence of untrusted CGI client data in the HTTP_PROXY environment variable, which might allow remote attackers to redirect an application's outbound HTTP traffic to an arbitrary proxy server via a crafted Proxy header in an HTTP request, aka an "httpoxy" issue, a related issue to CVE-2016-5387.
https://support.apple.com/HT207171
https://support.apple.com/HT207170
http://www.securitytracker.com/id/1036853
http://www.securityfocus.com/bid/93060
http://lists.apple.com/archives/security-announce/2016/Sep/msg00009.html
http://lists.apple.com/archives/security-announce/2016/Sep/msg00006.html