Increasing the accessibility of overridden or hidden methods permits a malicious subclass to offer wider access to the restricted method than was originally intended. Consequently, programs must override methods only when necessary and must declare methods final whenever possible to prevent malicious subclassing. When methods cannot be declared final, programs must refrain from increasing the accessibility of overridden methods.
The access modifier of an overriding or hiding method must provide at least as much access as the overridden or hidden method (The Java Language Specification, §184.108.40.206, "Requirements in Overriding and Hiding" [JLS 2015]). The following table lists the allowed accesses.
Overridden/Hidden Method Modifier
Overriding/Hiding Method Modifier
Cannot be overridden
Noncompliant Code Example
This noncompliant code example demonstrates how a malicious subclass
Sub can both override the
doLogic() method of the superclass and increase the accessibility of the overriding method. Any user of
Sub can invoke the
doLogic method because the base class
Super defines it to be
protected, consequently allowing class
Sub to increase the accessibility of
doLogic() by declaring its own version of the method to be public.
This compliant solution declares the
doLogic() method final to prevent malicious overriding:
MET04-J-EX0: For classes that implement the
java.lang.Cloneable interface, the accessibility of the
Object.clone() method should be increased from
public [SCG 2009].
Subclassing allows weakening of access restrictions, which can compromise the security of a Java application.
Detecting violations of this rule is straightforward.
CWE-487, Reliance on Package-Level Scope
Guideline 4-1 / EXTEND-1: Limit the accessibility of classes, interfaces, methods, and fields