Tapjacking

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Informações Básicas

Tapjacking é um ataque onde uma aplicação maliciosa é lançada e se posiciona sobre uma aplicação vítima. Uma vez que obscurece visivelmente o app vítima, sua interface é projetada de forma a enganar o usuário para interagir com ela, enquanto encaminha a interação para o app vítima.
Em efeito, isso é cegar o usuário quanto ao fato de que ele está realmente realizando ações no app vítima.

Detecção

  • Procure por exported activities no manifest do Android (uma activity com um intent-filter é exportada por padrão). Se uma activity exportada for protegida por uma permission, o app atacante precisará da mesma permission, o que limita a exploitability.
  • Verifique a versão minimum SDK android:minSdkVersion em AndroidManifest.xml. Se for inferior a 30, comportamentos padrão mais antigos podem tornar o tapjacking mais fácil de explorar.
  • Em tempo de execução, use logcat para identificar toques bloqueados no Android 12+: o sistema registra Untrusted touch due to occlusion by <package> quando overlays são filtrados.

Proteção

Bloqueio padrão do Android 12+ e compat flags

Android 12 (API 31) introduziu “Block untrusted touches”: toques vindos de outra janela com UID diferente do tipo TYPE_APPLICATION_OVERLAY (opacity ≥0.8) são descartados. Isso está habilitado por padrão. Durante testes você pode alterná-lo:

# disable blocking for a specific package (for PoC crafting)
adb shell am compat disable BLOCK_UNTRUSTED_TOUCHES com.example.victim
# re‑enable
adb shell am compat reset BLOCK_UNTRUSTED_TOUCHES com.example.victim

Janelas confiáveis (acessibilidade, IME, assistente) ainda recebem eventos. Sobreposições invisíveis ou totalmente transparentes também contornam o bloqueio, que atacantes tentam abusar mantendo alpha < 0.8.

Lidando com obstrução parcial

Sobreposições parciais que deixam a área alvo visível não são bloqueadas automaticamente. Mitigue em views sensíveis rejeitando eventos com a flag FLAG_WINDOW_IS_PARTIALLY_OBSCURED:

@Override
public boolean onFilterTouchEventForSecurity(MotionEvent event) {
if ((event.getFlags() & MotionEvent.FLAG_WINDOW_IS_PARTIALLY_OBSCURED) != 0) {
return false; // drop tap when anything partially obscures us
}
return super.onFilterTouchEventForSecurity(event);
}

filterTouchesWhenObscured

Se android:filterTouchesWhenObscured estiver definido como true, a View não receberá toques sempre que a janela da View for obscurecida por outra janela visível.

setFilterTouchesWhenObscured

O atributo setFilterTouchesWhenObscured definido como true também pode impedir a exploração dessa vulnerabilidade se a versão do Android for mais antiga.
Se definido como true, por exemplo, um botão pode ser automaticamente desabilitado se estiver obscurecido:

<Button android:text="Button"
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:filterTouchesWhenObscured="true">
</Button>

Exploitation

Tapjacking-ExportedActivity

The most recent Android application performing a Tapjacking attack (+ invoking before an exported activity of the attacked application) can be found in: https://github.com/carlospolop/Tapjacking-ExportedActivity.

Follow the README instructions to use it.

FloatingWindowApp

An example project implementing FloatingWindowApp, which can be used to put on top of other activities to perform a clickjacking attack, can be found in FloatingWindowApp (a bit old, good luck building the apk).

Qark

Caution

It looks like this project is now unmaintained and this functionality isn’t properly working anymore

You can use qark with the --exploit-apk –sdk-path /Users/username/Library/Android/sdk parameters to create a malicious application to test for possible Tapjacking vulnerabilities.\

The mitigation is relatively simple as the developer may choose not to receive touch events when a view is covered by another. Using the Android Developer’s Reference:

Sometimes it is essential that an application be able to verify that an action is being performed with the full knowledge and consent of the user, such as granting a permission request, making a purchase or clicking on an advertisement. Unfortunately, a malicious application could try to spoof the user into performing these actions, unaware, by concealing the intended purpose of the view. As a remedy, the framework offers a touch filtering mechanism that can be used to improve the security of views that provide access to sensitive functionality.

To enable touch filtering, call setFilterTouchesWhenObscured(boolean) or set the android:filterTouchesWhenObscured layout attribute to true. When enabled, the framework will discard touches that are received whenever the view’s window is obscured by another visible window. As a result, the view will not receive touches whenever a toast, dialog or other window appears above the view’s window.

Recent overlay-based malware techniques

  • Hook/Ermac variants use nearly transparent overlays (e.g., fake NFC prompts) to capture gestures and lock-screen PINs while forwarding touches underneath, delivered via Accessibility-ATS modules.
  • Anatsa/TeaBot droppers ship overlays for hundreds of banking/crypto apps and show full-screen “maintenance” overlays to stall victims while ATS completes transfers.
  • Hidden-VNC banking RATs briefly display phishing overlays to capture credentials, then rely on covert VNC plus Accessibility to replay taps with fewer on-device artifacts.

Practical takeaway for red teams: mix an alpha < 0.8 overlay to bypass Android 12 blocking, then escalate to a full-screen accessibility overlay once the user toggles the service. Instrument GestureDescription or a headless VNC to keep control after credentials are captured.

Accessibility Overlay Phishing (Banking-Trojan Variant)

Besides classic Tapjacking, modern Android banking malware families (e.g. ToxicPanda, BrasDex, Sova, etc.) abuse the Accessibility Service to place a full-screen WebView overlay above the legitimate application while still being able to forward the user input to the view underneath. This dramatically increases believability and allows attackers to steal credentials, OTPs or even automate fraudulent transactions.

How it works

  1. The malicious APK requests the highly-sensitive BIND_ACCESSIBILITY_SERVICE permission, usually hiding the request behind a fake Google/Chrome/PDF-viewer dialog.
  2. Once the user enables the service, the malware programmatically simulates the taps required to grant additional dangerous permissions (READ_SMS, SYSTEM_ALERT_WINDOW, REQUEST_INSTALL_PACKAGES, …).
  3. A WebView is inflated and added to the window manager using the TYPE_ACCESSIBILITY_OVERLAY window type. The overlay can be rendered totally opaque or semi-transparent and can be flagged as “through” so that the original touches are still delivered to the background activity (thus the transaction really happens while the victim only sees the phishing form).
WebView phishingView = new WebView(getApplicationContext());
phishingView.getSettings().setJavaScriptEnabled(true);
phishingView.loadUrl("file:///android_asset/bank_login.html");

WindowManager wm = (WindowManager) getSystemService(WINDOW_SERVICE);
WindowManager.LayoutParams lp = new WindowManager.LayoutParams(
WindowManager.LayoutParams.MATCH_PARENT,
WindowManager.LayoutParams.MATCH_PARENT,
WindowManager.LayoutParams.TYPE_ACCESSIBILITY_OVERLAY,  // <-- bypasses SYSTEM_ALERT_WINDOW prompt
WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE |
WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL,        // «through» flag → forward touches
PixelFormat.TRANSLUCENT);
wm.addView(phishingView, lp);

Fluxo de trabalho típico usado por banking Trojans

  • Consultar os pacotes instalados (QUERY_ALL_PACKAGES) para identificar qual banking / wallet app está atualmente aberto.
  • Baixar um HTML/JS overlay template do C2 que imita perfeitamente essa aplicação específica (logo, cores, i18n strings…).
  • Exibir o overlay, coletar credenciais/PIN/padrão.
  • Usar a Accessibility API (performGlobalAction, GestureDescription) para automatizar transferências em segundo plano.

Detecção & Mitigação

  • Auditar a lista de apps instalados com adb shell pm list packages -3 -e BIND_ACCESSIBILITY_SERVICE.
  • Do lado da aplicação (bank / wallet):
  • Habilitar android:accessibilityDataSensitive="accessibilityDataPrivateYes" (Android 14+) em views sensíveis para bloquear serviços não provenientes da Play-Store.
  • Combinar com setFilterTouchesWhenObscured(true) e FLAG_SECURE.

Para detalhes adicionais sobre como aproveitar Accessibility Services para controle remoto completo do dispositivo (por exemplo PlayPraetor, SpyNote, etc.) veja:

Accessibility Services Abuse

Referências

Tip

Aprenda e pratique Hacking AWS:HackTricks Training AWS Red Team Expert (ARTE)
Aprenda e pratique Hacking GCP: HackTricks Training GCP Red Team Expert (GRTE) Aprenda e pratique Hacking Azure: HackTricks Training Azure Red Team Expert (AzRTE)

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