The "http-client" egg always used a HTTPPROXY environment variable to determine whether HTTP traffic should be routed via a proxy, even when running as a CGI process. Under several web servers this would mean a user-supplied "Proxy" header could allow an attacker to direct all HTTP requests through a proxy (also known as a "httpoxy" attack). This affects all versions of http-client before 0.10.

The "spiffy-cgi-handlers" egg would convert a nonexistent "Proxy" header to the HTTPPROXY environment variable, which would allow attackers to direct CGI programs which use this environment variable to use an attacker-specified HTTP proxy server (also known as a "httpoxy" attack). This affects all versions of spiffy-cgi-handlers before 0.5.