From the HTTPoxy announcement:
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 now.
This site includes fixes for popular web servers and Guzzle has also released a new version (v6.2.1) to address this.