Improper Access Control
PHP does not attempt to address RFC section namespace conflicts and therefore does not protect applications from the presence of untrusted 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, as demonstrated by (1) an application that makes a getenv('HTTP_PROXY') call or (2) a CGI configuration of PHP, aka an `httpoxy` issue.