An input-validation flaw was discovered in the Go programming language built in CGI implementation, which set the environment variable "HTTP_PROXY" using the incoming "Proxy" HTTP-request header. The environment variable "HTTP_PROXY" is used by numerous web clients, including Go's net/http package, to specify a proxy server to use for HTTP and, in some cases, HTTPS requests. This meant that when a CGI-based web application ran, an attacker could specify a proxy server which the application then used for subsequent outgoing requests, allowing a man-in-the-middle attack.