Page History

public class SensitiveClass extends Number {
  // ... Implement abstract methods, such as Number.doubleValue()……

  private static final SensitiveClass INSTANCE = new SensitiveClass();
  public static SensitiveClass getInstance() {
    return INSTANCE;
  }

  private SensitiveClass() {
    // Perform security checks and parameter validation
  }

  private int balance = 1000;
  protected int getBalance() {
    return balance;
  }
}

class Malicious {
  public static void main(String[] args) {
    SensitiveClass sc =
       (SensitiveClass) deepCopy(SensitiveClass.getInstance());
    // Prints false; indicates new instance
    System.out.println(sc == SensitiveClass.getInstance());  
    System.out.println("Balance = " + sc.getBalance());
  }

  // This method should not be used in production code
  static public Object deepCopy(Object obj) {
    try {
      ByteArrayOutputStream bos = new ByteArrayOutputStream();
      new ObjectOutputStream(bos).writeObject(obj);
      ByteArrayInputStream bin =
          new ByteArrayInputStream(bos.toByteArray());
      return new ObjectInputStream(bin).readObject();
    } catch (Exception e) { 
      throw new IllegalArgumentException(e);
    }
  }
}