Kotlin Application Security: Top 10 Vulnerabilities and Protection

Kotlin Application Security: Top 10 Vulnerabilities and Protection Methods

Kotlin is one of the fastest-growing programming languages, used for developing Android applications, server-side solutions, and cross-platform projects. Despite built-in security mechanisms (null safety, data immutability), developers often make mistakes that lead to serious vulnerabilities. In this article, we will break down the top 10 vulnerabilities of Kotlin applications and show how to avoid them.

1. Improper Handling of Nullable Types (Null Safety)

One of Kotlin's strengths is its nullable type system. However, developers often abuse the !! operator (not-null assertion), which leads to NullPointerException on any unexpected null value.

Vulnerability Example:

fun getUserName(user: User?): String {    return user!!.name // If user = null, the application will crash}

How to Fix:

fun getUserName(user: User?): String {    return user?.name ?: "Guest" // Safe call with default value}

Recommendation: Use ?.let, ?: (elvis), and safe casts. Avoid !! in production code.

2. SQL Injections in Server Applications

When working with databases via JDBC or Exposed, developers sometimes concatenate query strings, which opens the door for SQL injections.

Vulnerability Example:

val id = request.getParameter("id")val query = "SELECT * FROM users WHERE id = $id" // Dangerous concatenation

How to Fix:

val id = request.getParameter("id")val stmt = connection.prepareStatement("SELECT * FROM users WHERE id = ?")stmt.setString(1, id) // Use parameterized query

Recommendation: Always use PreparedStatement or ORM libraries (Exposed, Hibernate) with parameterization.

3. Unsafe Data Storage (SharedPreferences)

In Android applications, tokens, passwords, and other confidential information are often stored in SharedPreferences in plain text.

Vulnerability Example:

val prefs = getSharedPreferences("app_prefs", Context.MODE_PRIVATE)prefs.edit().putString("auth_token", "secret123").apply() // Data in plain text

How to Fix:

// Use EncryptedSharedPreferences from AndroidX Securityval masterKey = MasterKey.Builder(context)    .setKeyScheme(MasterKey.KeyScheme.AES256_GCM)    .build()val prefs = EncryptedSharedPreferences.create(    context,    "secure_prefs",    masterKey,    EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,    EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM)prefs.edit().putString("auth_token", "secret123").apply()

Recommendation: For storing sensitive data, use EncryptedSharedPreferences or KeyStore.

4. Data Leakage Through Logs (Logging)

Developers often leave logs in the code that output passwords, tokens, or users' personal data.

Vulnerability Example:

Log.d("Auth", "User password: $password") // Password ends up in logs

How to Fix:

// Use Timber or a custom logger with filteringLog.d("Auth", "User authenticated successfully") // Do not log sensitive data

Recommendation: Enable ProGuard/R8 to remove logs in the release build and use libraries with data masking (e.g., Timber).

5. Unsafe WebView (Android)

WebView is a common entry point for XSS attacks and data leakage if JavaScript is enabled and filters are not configured.

Vulnerability Example: