03

Write-ups for TCTT2023/Mobile/03

Flag pattern

CTT23{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}

Challenge Files

mflag.zip

Solution

1. Start by opening AVD in Android Studio.

2. Install the app in AVD.

3. After launching the app, we can see that it is a simple app with button. But after we press the button, nothing happens. Only the text is shown The flag was returned but cannot be displayed. Can you help find the flag ???.

This make me think that the flag is hidden outside the app, the app should be using some kind of API to get the flag. So I use burpsuite to intercept the traffic.

4. To intercept the traffic, we need to set the proxy in AVD. Go to Settings > Proxy and set the proxy to your burpsuite proxy. Also we need to start the intercept in burpsuite.

5. After pressing the button, we can see that there is a request to http://18.143.235.72/getmflag

6. I use curl to send the request to the server again.

7. The result is CTT23{c00l_int3rc3pt0r}