Securing Kotlin Multiplatform Apps: Essential Practices for Robust Protection

Why Security Matters in KMP

A single vulnerability can lead to:

• Data Breaches: Leaked user credentials, payment info, or personal data.
• Legal Penalties: Fines under GDPR, CCPA, or other privacy laws.
• Reputation Damage: Loss of user trust after security incidents.

KMP-Specific Risks:

• Platform-Specific Flaws: Misconfigured iOS Keychain or Android Keystore.
• Shared Code Exploits: A vulnerability in shared code impacts all platforms.
• Inconsistent Updates: Security patches might lag on certain platforms.

Real-World Impact:

• A travel app using KMP faced a breach when unencrypted SQLite data on iOS was extracted from jailbroken devices.
• A fintech app avoided MITM (man-in-the-middle) attacks by implementing SSL pinning across Android and iOS.

1. Fortify API Communication

1.1 Enforce HTTPS Everywhere

Why It Matters: HTTP exposes data to eavesdropping and tampering.
Implementation:

// Ktor client configuration  
val client = HttpClient(CIO) {  
    expectSuccess = true // Throw exceptions for non-2xx responses  
}  

// Usage  
suspend fun fetchData() = client.get("https://api.secure.com/data")  

Best Practices:

• Use HSTS (HTTP Strict Transport Security) headers on servers.
• Redirect HTTP to HTTPS via server-side rules.

1.2 SSL Pinning

What It Solves: Prevents certificate spoofing in MITM attacks.
Implementation with Ktor:

HttpClient(CIO) {  
    engine {  
        https.pinning = CertificatePinner.Builder()  
            .add("api.secure.com", "sha256/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=")  
            .build()  
    }  
}  

Step-by-Step:

1. Extract your server’s public key hash:openssl s_client -connect api.secure.com:443 | openssl x509 -pubkey | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
2. Update the CertificatePinner with the hash.

Tools: Use SSLPinner for automated pinning.

2. Protect Sensitive Data

2.1 Encrypted Storage

Cross-Platform Solution:

// Using multiplatform-settings-encrypted  
val settings = EncryptedSettings(Settings())  
settings.putString("auth_token", "JWT_TOKEN")  

// Retrieve securely  
val token = settings.getString("auth_token")  

Platform-Specific Best Practices:

• Android: Use EncryptedSharedPreferences or BiometricPrompt for key storage.
• iOS: Store secrets in Keychain with kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly.
• Web: Avoid localStorage for sensitive data; use HTTP-only cookies.

2.2 Secure Key Management

Never Hardcode Secrets:

• Use environment variables for API keys:val apiKey = System.getenv("API_KEY")
• Leverage cloud services like AWS Secrets Manager or Azure Key Vault.

Android Keystore Example:

val keyStore = KeyStore.getInstance("AndroidKeyStore")  
keyStore.load(null)  
val keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore")  
keyGenerator.init(KeyGenParameterSpec.Builder("alias", KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT).build())  
keyGenerator.generateKey()  

3. Authentication Best Practices

3.1 OAuth2 & JWT

Implementation Flow:

1. Login:suspend fun login(username: String, password: String): Token { return HttpClient().post("https://auth.secure.com/token") { headers { append("Authorization", "Basic ${base64Encode("$username:$password")}") } }.body() }
2. Token Refresh:suspend fun refreshToken(refreshToken: String): Token { return HttpClient().post("https://auth.secure.com/refresh") { headers { append("Authorization", "Bearer $refreshToken") } }.body() }

Best Practices:

• Set short-lived access tokens (e.g., 15 minutes).
• Store refresh tokens securely and revoke them on logout.

3.2 Biometric Authentication

Android Implementation:

val biometricPrompt = BiometricPrompt(  
    activity,  
    ContextCompat.getMainExecutor(activity),  
    object : BiometricPrompt.AuthenticationCallback() {  
        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {  
            // Unlock sensitive data  
        }  
    }  
)  

val promptInfo = BiometricPrompt.PromptInfo.Builder()  
    .setTitle("Login")  
    .setSubtitle("Use your fingerprint to authenticate")  
    .setNegativeButtonText("Cancel")  
    .build()  

biometricPrompt.authenticate(promptInfo)  

iOS Implementation:

val context = LAContext()  
context.evaluatePolicy(  
    LAPolicy.LAPolicyDeviceOwnerAuthentication,  
    "Authenticate to access data",  
    completion = { success, error ->  
        if (success) { /* Proceed */ }  
    }  
)  

4. Prevent Reverse Engineering

4.1 Code Obfuscation

Android (ProGuard/R8):

// app/build.gradle  
android {  
    buildTypes {  
        release {  
            minifyEnabled true  
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'  
        }  
    }  
}  

iOS (Bitcode):
Enable Bitcode in Xcode under Build Settings > Enable Bitcode.

Advanced Tools:

• DexGuard: Enhanced obfuscation for Android.
• LLVM Obfuscator: For C/C++ interop code.

4.2 Tamper Detection

Detect Rooted/Jailbroken Devices:

fun isDeviceSecure(): Boolean {  
    // Android  
    val isRooted = File("/system/app/Superuser.apk").exists()  
    // iOS  
    val isJailbroken = NSFileManager.defaultManager.fileExistsAtPath("/Applications/Cydia.app")  
    return !isRooted && !isJailbroken  
}  

Response Strategy:

• Log the event and notify servers.
• Restrict app functionality or force logout.

5. Secure Coding Practices

5.1 Input Validation

Prevent SQL Injection:

// SQLDelight parameterized query  
val query = UserQueries.selectById(userId)  

Sanitize User Input:

fun sanitizeInput(input: String): String {  
    return input.replace("<script>", "", ignoreCase = true)  
}  

5.2 Logging Discipline

Strip Logs in Release Builds:

-assumenosideeffects class android.util.Log {  
    public static int d(...);  
    public static int w(...);  
}  

Secure Logging:

if (BuildConfig.DEBUG) {  
    Log.d("Auth", "Token: $token") // Debug only  
}  

6. Ongoing Security Maintenance

1. Automated Dependency Updates:
   Use Dependabot or Renovate to patch vulnerabilities.
2. Penetration Testing:
   Hire ethical hackers to simulate attacks. Tools: Burp Suite, OWASP ZAP.
3. Monitor Threats:
   Use MobSF for static analysis and Google Play App Signing for tamper detection.

Key Takeaways

1. HTTPS & SSL Pinning: Mandatory for secure API communication.
2. Encrypted Storage: Use platform-native solutions for keys/secrets.
3. OAuth2/JWT: Prefer token-based auth with short expiration times.
4. Code Obfuscation: Deter reverse engineering with ProGuard/R8.
5. Input Validation: Sanitize all user inputs to prevent injection.

• Internal Links: A Comprehensive Guide to Cross-Platform Development, How to Switch Ruby Versions on Mac (M1, M2, M3, M4 Guide), Mastering Lifecycle & Performance in Compose Multiplatform | Cross-Platform Optimization Guide

• External Links: OWASP Mobile Top 10, Ktor Documentation.