How to Sanitize User Input Strings in Kotlin

Sanitizing user input is crucial to prevent various security vulnerabilities such as SQL injection, cross-site scripting (XSS), and more. In this article, we'll explore how to sanitize user input strings in Kotlin.

Understanding the Need for Sanitization

When creating applications in Kotlin, handling user inputs is inevitable. These inputs can sometimes be malicious, trying to exploit the system. Thus, sanitization is necessary to clean these inputs, ensuring that they do not harm the system.

Basic Sanitization for Strings

The simplest form of input sanitization involves trimming spaces and escaping potentially dangerous symbols.

Trimming Spaces

fun sanitizeInput(input: String): String {
    return input.trim()
}

This function removes leading and trailing spaces, ensuring that accidental space inputs do not cause issues in the application logic.

Escaping HTML Characters

To prevent XSS attacks, it's important to escape HTML entities.

import org.apache.commons.text.StringEscapeUtils

fun escapeHtml(input: String): String {
    return StringEscapeUtils.escapeHtml4(input)
}

The above code uses Apache Commons Text library to escape HTML characters effectively.

Advanced Sanitization Techniques

Removing or Replacing Special Characters

You may want to allow only specific characters. The following function removes all characters except letters and numbers:

fun cleanSpecialCharacters(input: String): String {
    return input.replace(Regex("[^a-zA-Z0-9]"), "")
}

This function uses a regular expression to replace any non-alphanumeric characters with an empty string.

Using Regular Expressions for Validation

Instead of just removing unwanted characters, you may validate input against a pattern:

fun validateInput(input: String): Boolean {
    val pattern = Regex("^[a-zA-Z0-9]+")
    return pattern.matches(input)
}

This function checks if the input contains only letters and numbers.

Using External Libraries

For more robust and comprehensive sanitization, you might consider using libraries like JSoup for HTML sanitization or OWASP Java HTML Sanitizer. These libraries offer more security-tested implementations.

import org.jsoup.Jsoup

fun sanitizeHtml(input: String): String {
    return Jsoup.clean(input, org.jsoup.safety.Whitelist.basic())
}

This example demonstrates using JSoup to clean HTML inputs based on a predefined whitelist.

Conclusion

Sanitizing user input is a critical part of developing secure applications. Whether you handle this with simple Kotlin functions or leverage external libraries, ensuring user inputs are sanitized will protect your application from various vulnerabilities.