To mitigate these vulnerabilities, it is crucial to implement robust security measures, including using strong encryption and secure algorithms, practicing secure key management, properly validating and verifying tokens, and following best practices for secure token transmission and storage.

```kotlin
import io.jsonwebtoken.JwtException
import io.jsonwebtoken.Jwts
import io.jsonwebtoken.SignatureAlgorithm
import io.jsonwebtoken.security.Keys
import java.security.Key

// Secure JWT validation
fun validateJwt(token: String, secretKey: Key): Boolean {
    try {
        Jwts.parserBuilder()
            .setSigningKey(secretKey)
            .build()
            .parseClaimsJws(token)
        return true // Token is considered valid
    } catch (e: JwtException) {
        return false // Token validation failed
    }
}

fun main() {
    val token = "eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwiaWF0IjoxNTE2MjM5MDIyfQ.4x6fOGYwfFYIQgZepgK1AnbDDr2-TvAp6im0kKk52Es"
    val secretKey = Keys.secretKeyFor(SignatureAlgorithm.HS256) // Generate a secure secret key

    val isValid = validateJwt(token, secretKey)
    if (isValid) {
        println("Token is valid")
        // Proceed with further processing
    } else {
        println("Token validation failed")
        // Handle invalid token
    }
}
```

In this example, the validateJwt function takes a JWT token and a secure secret key as input. The `Keys.secretKeyFor` method from the jjwt library is used to generate a secure secret key using the `HS256` algorithm.

The function attempts to validate the token by parsing its claims using the `Jwts.parserBuilder()` method. The `setSigningKey` method is used to set the secure secret key for signature verification. This ensures that the token's signature is validated securely.

If the token is successfully parsed without any exceptions, it is considered valid. Otherwise, if a `JwtException` occurs during the parsing process, the token validation fails.

By using a secure secret key and following best practices for key management, such as generating keys with sufficient randomness and protecting them from unauthorized access, you can enhance the security of your JWT implementation.