Tapjacking Protection

Tapjacking is a security issue that occurs when the app screen is completely or partially obscured by an overlay window. The user can be tricked into thinking that they are interacting with the overlay. However, their input (a tap) can instead perform actions in the underlying app.

Malwarelytics for Android tries to prevent tapjacking. This is done by preventing click events when a part of the app screen is obscured by an overlay and at least one app that can create such overlays is deemed “problematic”.

Tapjacking protection sensitivity is configurable in the SDK.

Limitations

Identifying Tapjacking Culprit

Android APIs prevent an app from determining specifically which other app is creating an overlay; it is only possible to obtain a list of apps that are capable of doing so. Utilizing data obtained by Anti-Malware feature tapjacking protection can identify which of these apps are indeed “problematic”.

Protecting Dialogs

Tapjacking protection doesn’t work on dialogs because the contents of a dialog receive taps through an overlay regardless of the way the dialog is created.

Configuration

This feature can be configured during the Malwarelytics initialization phase:

val raspConfig = RaspConfig.Builder()
    .blockTapjacking(Boolean)
    .ignoreTapjackingSystemApps(Boolean)
    .blockTapjackingSensitivity(ThreatIndex)
    .allowedTapjackingApps(Collection<RaspConfig.ApkAllowlistItem>)
    // configuration of other RASP features
    .build()

Usage

After initialization, the tapjacking protection feature can be accessed via RaspManager. This can be used to register an observer or to trigger a manual tapjacking detection check.

Registering an Observer

Tapjacking detection can trigger a certain action. To achieve that, an observer needs to be configured and registered.

Observer configuration:

val raspObserver = object : RaspObserver {
    override fun onTapjackingDetected(tapjackingDetection: TapjackingDetection) {
        // handle tapjacking detection
    }
    // handle detection of other RASP features
}

The observer can be registered in RaspManager. When it is no longer needed, it can be unregistered again.

raspManager.registerRaspObserver(raspObserver)
raspManager.unregisterRaspObserver(raspObserver)

Triggering a Manual Check

Tapjacking detection check can be triggered manually in RaspManager. Two methods are available. isBadTapjackingCapableAppPresent() indicates whether any apps deemed “problematic” are installed on the device. getTapjackingDetection() provides more details about tapjacking protection configuration. The returned data depend on the configuration - both on the sensitivity and on ignoring system apps.

val isBadTapjackingCapableAppPresent = raspManager.isBadTapjackingCapableAppPresent()
val tapjackingDetection = raspManager.getTapjackingDetection()

Troubleshooting for App Developers

In case a tapjacking capable app is frequently used by the app’s userbase and its threat index triggers the protection (the threat index is higher than tapjacking protection sensitivity) it can be ignored by configuring raspConfig.allowedTapjackingApps(Collection<RaspConfig.ApkAllowlistItem>).

Allowed tapjacking apps can be specified either by their package name and signature or by their package name only. However, the former is recommended in order to avoid allowing fake (and often malicious) apps.

Obtaining Package Name

Obtaining the app package name is simple – it is listed as a query param id in the URL of the app page on Google Play. For example, the package name of the Android Accessibility Suite is com.google.android.marvin.talkback as can be seen in this Google Play URL: https://play.google.com/store/apps/details?id=com.google.android.marvin.talkback.

Obtaining App Signature

Obtaining the correct app signature is more complicated because a signed APK is necessary. Installing the app from Google Play is recommended. Next, the APK must be pulled from the device.

Find the APK location on the device:

adb shell pm list packages -f | grep "$PACKAGE$" | sed "s/package://" | sed "s/=$PACKAGE//"

Pull the APK from the device:

adb pull "$APK_LOCATION"

Obtain the signature hash in the right format:

apksigner verify --print-certs ${SOME_APK} | grep "SHA-1" | sed "s/.*: //"

The result is the app signature that can be used as the second argument for RaspConfig.ApkAllowlistItem(String, String).

Troubleshooting for Users

The SDK tries to simplify a potential troubleshooting process as much as possible. The app developer is provided with a list of “problematic” apps triggering the protection. Furthermore, the SDK offers a utility method that opens the system settings responsible for granting/removing the system overlay permission for installed apps:

RaspUtilities.openSystemOverlaySettings(Context)

The device user can then adjust the settings to make the app respond to clicks again.