apache2: sets environmental variable based on user supplied Proxy request header (CVE-2016-5387)
The Apache HTTP Server through 2.4.23 follows RFC 3875 section 4.1.18
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, aka an “httpoxy” issue.
(from redmine: issue id 5934, created on 2016-07-20, closed on 2017-04-08)