Improper Access Control
The net/http package in Go through 1.6 does not attempt to address RFC 3875 section 4.1.18 namespace conflicts and therefore does not protect CGI applications from the presence of untrusted client data in the HTTP_PROXY environment variable, which might allow remote attackers to redirect a CGI application's outbound HTTP traffic to an arbitrary proxy server via a crafted Proxy header in an HTTP request, aka an "httpoxy" issue.
CWE-284 - Improper Access Control
Listed 5th in the 'OWASP Top Ten', improper (or broken) access control attacks are a fundamental type of vulnerability. This includes a broad range of design flaws that enable users to act outside of their intended permissions. They can use these privileges to gain access to restricted files and functionality such as accessing restricted information, falsifying records, destroying data, or executing commands.