Modify json body with mitmproxy

Question

I am trying to intercept and modify a graphql response's body. Here is my addon code:

from mitmproxy import ctx
from mitmproxy import http
import json

def response(flow: http.HTTPFlow) -> None:
    if flow.request.pretty_url == "https://my.graphql/endpoint":
        request_data = json.loads(flow.request.get_text())
        if request_data["operationName"] == "MyOperationName":
            data = json.loads(flow.response.get_text())
            data["data"]["product"]["name"] = "New Name"
            flow.response.text = json.dumps(data)

I can see the modified response in mitmproxy console. But the iOS simulator I am using is still getting the original response. Does anyone know how can I pass the modified response to the device?

Answer 1

From the documentation

def response(self, flow: mitmproxy.http.HTTPFlow):
        """
        The full HTTP response has been read.

        Note: If response streaming is active, this event fires after the entire body has been streamed.
        HTTP trailers, if present, have not been transmitted to the client yet and can still be modified.
        """
        ctx.log(f"response: {flow=}")

It appears that you might be streaming the response body which would mean that modifications would be ignored.

Consider using def request event hook instead