The credentials to authenticate a user agent to a proxy server.

Fastly reads this header from requests and writes it into requests. It is defined by an external standard.

PROTECTED:  This header cannot be modified in VCL.

The HTTP Proxy-Authorization request header is usually sent after a server has responded with a 407 Proxy Authentication Required response containing a Proxy-Authenticate response header.

The Proxy-Authorization header field allows the client to identify itself (or its user) to a proxy that requires authentication. Its value consists of credentials containing the authentication information of the client for the proxy and/or realm of the resource being requested.

Unlike Authorization, the Proxy-Authorization header field applies only to the next inbound proxy that demanded authentication using the Proxy-Authenticate field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first inbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.