mitm-injector.py

from bs4 import BeautifulSoup
from mitmproxy import ctx


# Load in the javascript to inject.
with open('extension/injected-javascript.js', 'r') as f:
    injected_javascript = f.read()

def response(flow):
    # Only process 200 responses of HTML content.
    if flow.response.headers['Content-Type'] != 'text/html':
        return
    if not flow.response.status_code == 200:
        return

    # Inject a script tag containing the JavaScript.
    html = BeautifulSoup(flow.response.text, 'lxml')
    container = html.head or html.body
    if container:
        script = html.new_tag('script', type='text/javascript')
        script.string = injected_javascript
        container.insert(0, script)
        flow.response.text = str(html)

        ctx.log.info('Successfully injected the `injected-javascript.js` script.')