Proxy to remote server with CORS support

README.md

cors.py for mitmproxy

Hacking CORS restriction to enable in-browser XHR to any server.

Usage

Say you are running an web app at localhost, and you want to send XHR to http://remote-server:80, but the CORS restriction forbids access because you are sending requests from an origin that remote-server:80 does not allow.

Run:

mitmproxy -s cors.py -R http://remote-server:80 -b localhost -p 8080

Now localhost:8080 is tunnelled to remote-server:80.

And you can XHR to proxied server from localhost:

fetch("http://localhost:8080/api.json")
  .then(function(response) {
    // enjoy the response
  });

Bonus: You can inspect HTTP requests in mitmproxy.

cors.py

from mitmproxy import http

def response(flow):
    flow.response.headers["Access-Control-Allow-Origin"] = "*"
    
    # Use this if the application sends auth info via header
    flow.response.headers["Access-Control-Expose-Headers"] = "Authorization"

def request(flow):
    # Hijack CORS OPTIONS request
    if flow.request.method == "OPTIONS":
        flow.response = http.Response.make(200, b"",
            {"Access-Control-Allow-Origin": "*",
             "Access-Control-Allow-Methods": "GET,POST",
             "Access-Control-Allow-Headers": "Authorization",
             "Access-Control-Max-Age": "1728000"})