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Note that a private field of a superclass might be accessible to a subclass (for example, if both classes are members of the same class). Nevertheless, a private field is never inherited by a subclass.

Noncompliant Code Example

This noncompliant code example exposes the sensitive (x,y) coordinates through the getPoint() method of the inner class. Consequently, the AnotherClass class that belongs to the same package can access the coordinates.

class Coordinates {
  private int x;
  private int y;

  public class Point {
    public void getPoint() {
      System.out.println("(" + x + "," + y + ")");    
    }
  }
}

class AnotherClass {
  public static void main(String[] args) {
    Coordinates c = new Coordinates();
    Coordinates.Point p = c.new Point();
    p.getPoint();
  }        
}

Compliant Solution

Use the private access specifier for hiding the inner class and all contained methods and constructors. The compiler will refuse to compile AnotherClass because of its attempt to access a private nested class.

class Coordinates {
  private int x;
  private int y;

  private class Point {
    private void getPoint() {
      System.out.println("(" + x + "," + y + ")");    
    }
  }
}

class AnotherClass {
  public static void main(String[] args) {
    Coordinates c = new Coordinates();
    Coordinates.Point p = c.new Point();    // fails to compile
    p.getPoint();
  }        
}

Risk Assessment

The Java Language System weakens the accessibility of sensitive, private entities in inner classes which can result in a security weakness.

Automated Detection

Automated detection of non-private nested classes that define non-private members and constructors is straight-forward. However, this guideline applies only when those classes could potentially expose sensitive data or operations from the outer class. Detection of sensitive data or operations requires programmer assistance.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Related Guidelines

MITRE CWE: CWE-492 "Use of Inner Class Containing Sensitive Data"

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 8.1.3, Inner Classes and Enclosing Instances|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.3] and [Section 8.3|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3] "Field Declarations"
\[[Long 2005|AA. Bibliography#Long 05]\] Section 2.3, Inner Classes
\[[McGraw 2000|AA. Bibliography#McGraw 00]\]

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