Modifying websockets message content

Steps to reproduce the problem:

1. Create a simple script to modify the websocket’s message content
2. Run mitmdump like this: “mitmdump.exe --script websockets_com.py --mode “upstream:https://127.0.0.1:8090” --ssl-insecure”
3. The messages are sent unmodified to the upstream proxy and to the server. No errors or exceptions

Any other comments? What have you tried so far?

Example of my code (based on https://github.com/mitmproxy/mitmproxy/blob/master/examples/simple/websocket_messages.py):

def websocket_message(flow):
    message = flow.messages[-1]
    binContent= message.content
    aux = umsgpack.unpackb(binContent)
    finalJSON = json.dumps(decode_msgpack2(aux)) # decode_msgpack is a function that returns a dict
    message.content = re.sub(r'.*', finalJSON, format(strutils.bytes_to_escaped_str(message.content)))
    # I also tried:
    ## message.content = finalJSON
    ## flow.messages[-1].content = finalJSON
    # And to set the content of the new message to plaintext instead of binary data (the original is binary)
    ## message.type = 0x1
    print(message.content) # here the JSON is printed as string, but it is still sent with the original content

I’ve tried without the upstream proxy that I have only for debugging purposes (Burp), and the result is the same.

System information

Windows 10 x64 PS C:\WINDOWS\system32> mitmdump.exe --version Mitmproxy: 4.0.4 Python: 3.6.5 OpenSSL: OpenSSL 1.1.0g 2 Nov 2017 Platform: Windows-10-10.0.15063-SP0

[Kriechi]

If you get the last websocket message, you can check message.from_client which should be True or False.

[Kriechi]

You can call message.kill() to prevent it from being sent to the other endpoint.