An HTTP proxy is a server that mediates access between the client browser and the destination web server.
When a browser has been configured to use a proxy server, it makes all its requests to that server. The proxy relays the requests to the relevant web servers and forwards their responses back to the browser. Most proxies also provide additional services, including caching, authentication, and access control.
You should be aware of two differences in how HTTP works when a proxy server is being used:
When a browser issues an unencrypted HTTP request to a proxy server, it places the full URL into the request, including the protocol prefix http://, the server’s hostname, and the port number if this is nonstandard. The proxy server extracts the hostname and port and uses these to direct the request to the correct destination web server.
When HTTPS is being used, the browser cannot perform the SSL hand- shake with the proxy server, because this would break the secure tunnel and leave the communications vulnerable to interception attacks. Hence, the browser must use the proxy as a pure TCP-level relay, which passes all network data in both directions between the browser and the destina- tion web server, with which the browser performs an SSL handshake as normal. To establish this relay, the browser makes an HTTP request to the proxy server using the CONNECT method and specifying the destination hostname and port number as the URL. If the proxy allows the request, it returns an HTTP response with a 200 status, keeps the TCP connection open, and from that point onward acts as a pure TCP-level relay to the destination web server.
By some measure, the most useful item in your toolkit when attacking web applications is a specialized kind of proxy server that sits between your browser and the target website and allows you to intercept and modify all requests and responses, even those using HTTPS.