Bobarian Chat Overlay PoC 0.2.1

import com.sun.net.httpserver.HttpServer;

import java.io.*;
import java.net.InetSocketAddress;
import java.net.URL;
import java.net.URLConnection;

public class OverlayServer {

    private static final int PORT = 8357;

    public static void main(final String[] args) throws IOException {
        final HttpServer srv = HttpServer.create(new InetSocketAddress("localhost", PORT), 0);
        srv.createContext("/", ex -> {
            // Request original file from StreamLabs
            final String path = ex.getRequestURI().toString();
            final URLConnection slConn = new URL("https://streamlabs.com" + path).openConnection();
            // StreamLabs doesn't like the Java User Agent, so gotta get rid of it
            slConn.setRequestProperty("User-agent", "");
            final BufferedReader slReader = new BufferedReader(new InputStreamReader(slConn.getInputStream()));

            ex.sendResponseHeaders(200, 0);
            final PrintWriter writer = new PrintWriter(ex.getResponseBody());
            slReader.lines().forEach(line -> {
                // Replace all eventual references to streamlabs with localhost
                // This is necessary to circumvent the Same-Origin Policy
                line = line.replaceAll("https://streamlabs.com", "http://localhost:" + PORT);
                // If we are requesting the chat overlay, insert custom JavaScript
                if (path.startsWith("/widgets/chat-box/")) {
                    // Obviously our actual JS is gonna be more complicated
                    // A Proof of Concept JS file can be found at http://pastebin.com/XyNaxPVR
                    line = line.replace("</body>", "<script>alert('Hello World!');</script></body>");
                }
                writer.print(line);
            });
            writer.flush();
            writer.close();
            ex.close();
        });
        srv.start();
    }
}