A nonfinal class or method that is not meant to be inherited can be overridden by an attacker if it is not declared as final \[3\].

If inheritance is to be limited to trusted implementations for a public, nonfinal class, then the class type should be confirmed before creating the instance at each place where an instance of the nonfinal class can be created. A SecurityManager check should be enforced on detecting a subclass (Chapter 6 of \[2\]).

class BankOperation{


	//the account balance has already been retrieved from the database and stored in the foll variable
    private Integer balance = 5000;


    public BankOperation() {

    	//invoke java.lang.Object.getClass to get class instance
    	Class clazz = getClass();
    	//shows the class of the instantiated object
    	System.out.println(clazz);
    }

    public void getBalance() {
        System.out.println("The current balance is: $" + balance);
    }

}

//this class has been written by the attacker
public class SubClass extends BankOperation {

	public void getBalance() {

		//The attacker can change his account balance to any value he wants.
		Integer modifiedBalance = 0;
		//to read the new balance from the attacker
		InputStreamReader input = new InputStreamReader(System.in);
		Field balance=null;
        BufferedReader reader = new BufferedReader(input);
		System.out.print("Enter balance: ");
		try {
			modifiedBalance = Integer.parseInt(reader.readLine());

			//this gets the private field from the superclass
			balance = this.getClass().getSuperclass().getDeclaredField("balance");

			//this changes the accessibility so that field can now be accessed
			if (!Modifier.isPublic(balance.getModifiers())){
				balance.setAccessible(true);
			}
			//retrieve the original balance
			System.out.println("Original Balance: $"+balance.get(this));
			//change the balance
			balance.set(this, modifiedBalance);
			//display the new changed balance
			System.out.println("New Balance: $"+balance.get(this));

		} catch (Exception e) {
			e.printStackTrace();
		}
    }

	public static void main(String[] args) {
        SubClass subclass = new SubClass();
        subclass.getBalance();
    }
}

final class BankOperation{
//normal coding...
}