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Consider two classes belonging to different protection domains. One of them is malicious whereas the other is trusted. If the malicious class extends the trusted {{public}} non-final class and inherits without overriding a method of the trusted class, the fully qualified invocation of the malicious class's version of the method uses the protection domain of the trusted class. In this case, the trusted class's permissions are examined to execute the method. \[[Gong 2003|java:AA. Java References#Gong 03]\]).

One suggestion is that at all points where the class can be instantiated, there must be checks to ensure that the instance being created has the same type as the class. If the type is found to be that of a subclass instead of the non-final public superclass's type, a security manager check can be performed to ensure that malicious classes cannot misuse the class. This approach is insecure because it allows a malicious class to add a finalizer and obtain a partially initialized instance of the class. (see See guideline OBJ04-J. Do not allow partially initialized objects to be accessed.) . For non-final classes, the method that performs the security manager check must be passed as an argument to a private constructor so that Object's constructor does not exit before the security check is performed.

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This noncompliant code example installs a security manager check in the constructor of the non-final class. Access is denied if the security manager detects that a subclass without the requisite permissions, is trying to instantiate the superclass. \[[SCG 2007|java:AA. Java References#SCG 07]\].

public class NonFinal {
  public NonFinal() {
    // Invoke java.lang.Object.getClass to get class instance
    Class c = getClass();
    // Confirm class type
    if (c != NonFinal.class) {
      // Check the permission needed to subclass NonFinal
      securityManagerCheck(); // throws a security exception if not allowed
    }
  // ...   
  }
}

However, throwing an exception from the constructor is a non-final class is insecure because it allows a finalizer attack. (see See guideline OBJ04-J. Do not allow partially initialized objects to be accessed.).

This noncompliant code example complies with guideline OBJ06-J. Compare classes and not class names because it compares class types and not class names.

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Irrespective of whether it is a trusted instance or an untrusted one, install a security manager check using the technique described in guideline OBJ04-J. Do not allow partially initialized objects to be accessed.

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Allowing a non-final class or method to be inherited without checking the class instance allows a malicious subclass to misuse the privileges of the class.

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