MET04-J. Do not increase the accessibility of overridden or hidden methods

Increasing the accessibility of overridden or hidden methods permits a malicious subclass to offer wider access to the restricted method than was originally intended. The access modifier of an overriding or hiding method must provide at least as much access as the overridden or hidden method [[JLS 2005], §8.4.8.3, "Requirements in Overriding and Hiding"]. The following are the allowed accesses:

Noncompliant Code Example

This noncompliant code example demonstrates how a malicious subclass Sub can both override the doLogic() method of the superclass and also increase the accessibility of the overriding method. Any user of Sub can invoke the doLogic method because the base class BadScope defines it to be protected. Class Sub increases the accessibility of doLogic by declaring its own version of the method to be public.

class BadScope {
  protected void doLogic() {
    System.out.println("Super invoked");
  }
}

public class Sub extends BadScope {
  public void doLogic() {
    System.out.println("Sub invoked");
    // Do sensitive operations
  }
}

Compliant Solution

Override methods only when necessary. Declare methods and fields final whenever possible to avoid malicious subclassing. When methods and fields cannot be declared final, refrain from increasing the accessibility of overridden methods. (See SEC01-J. Minimize the accessibility of classes and their members.)

class BadScope {
  protected final void doLogic() { // declare as final
    System.out.println("Super invoked");
    // Do sensitive operations
  }
}

Exceptions

MET04-EX0: For classes that implement the java.lang.Cloneable interface, the accessibility of the Object.clone() method should be increased from protected to public [SCG 2009].

Risk Assessment

Subclassing allows weakening of access restrictions, which can compromise the security of a Java application.

Automated Detection

Detecting violations of this rule is straightforward.

Related Guidelines

Bibliography

      05. Methods (MET)