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According to JLS,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 , or a compile-time error occurs. The allowed accesses are:(The Java Language Specification, §8.4.8.3, "Requirements in Overriding and Hiding" [JLS 2015]). The following table lists the allowed accesses.

This also means that there is potential for some functionality having a restrictive modifier to be overridden by a less restrictive modifier.

Noncompliant

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Code Example

This non-compliant noncompliant code example exemplifies demonstrates how a malicious subclass Sub can both override the doLogic() method of the super classthe superclass and increase the accessibility of the overriding method. Any user of Sub will be able to can invoke the doLogic method even though because the base class BadScope defined it with the private access modifier 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.

class BadScopeSuper {
  privateprotected void doLogic() {
    System.out.println("Super invoked");
  }
}

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

Compliant Solution

Do not override a method unless absolutely necessary. Declare all methods and fields final to avoid malicious subclassing. This is in compliance with <xyz rule>This compliant solution declares the doLogic() method final to prevent malicious overriding:

class BadScopeSuper {
  privateprotected final void doLogic() { // Declare as final
    System.out.println("Super invoked");
    // Do sensitive operations
  }
}

References

Exceptions

MET04-J-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

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