httpoxy is a set of vulnerabilities that affect application code running in CGI, or CGI-like environments. It comes down to a simple namespace conflict:
- RFC 3875 (CGI) puts the HTTP Proxy header from a request into the environment variables as HTTP_PROXY
- HTTP_PROXY is a popular environment variable used to configure an outgoing proxy
This leads to a remotely exploitable vulnerability. If you're running PHP or CGI, you should block the Proxy header.
The best immediate mitigation is to block Proxy request headers as early as possible, and before they hit your application.