Using mitmproxy to inject Javascript file.

mitm.py

# Mitmproxy: 4.0.4
# Python:    3.6.5
# OpenSSL:   OpenSSL 1.1.0h-fips  27 Mar 2018
# Platform:  Linux-4.16.3-301.fc28.x86_64-x86_64-with-fedora-28-Twenty_Eight

# usage
# mitmdump -p $LISTEN_PORT -s ./mitm.py

import os
from bs4 import BeautifulSoup
from mitmproxy import http

class Injector:
    def load(self, loader):
        loader.add_option(
            "script", str, "", "My Script Tag"
        )

    def response(self, flow: http.HTTPFlow) -> None:
        if flow.response.headers.get("content-type").find("text/html") != -1:
            html = BeautifulSoup(flow.response.content, "html.parser")
            if html.head:
                script = html.new_tag(
                    "script", id="mitmproxy", src="http://localhost:5000/apply-evasions.browser.js", type="application/javascript")
                html.head.insert(0, script)
                flow.response.content = str(html).encode("utf8")

addons = [Injector()]