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Wiki MarkupA nested class is any class whose declaration occurs within the body of another class or interface \[ [JLS 2005|AA. Bibliography#JLS 05]\]. Nested classes are a broad set of classes that are classified as {{static}} classes and inner classes. "An inner class is a nested class that is not explicitly or implicitly declared {{static}}" \[[JLS 2005|AA. Bibliography#JLS 05]\]. An inner class may be local, anonymous, or non-static.2015]. The use of a nested class is error - prone unless the semantics are well understood. A common notion is that only the outer nested class can may access the contents of the nested outer class. Not only does the nested class have access to the private fields of the outer class, but the same fields can be accessed by another any other class in within the package depending on whether when the nested class is declared public or if it contains public methods or constructors. By defaultAs a result, the javac compiler converts the accessibility of private methods of a nested class to package-private. nested class must not expose the private members of the outer class to external classes or packages.

According to The Java Language Specification (JLS), §8.3, "Field Declarations" [JLS 2015]: Wiki MarkupAlso, according to the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], [Section 8.3|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.3] "Field Declarations"

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 private (x,y) coordinates through the getPoint() method of the inner class. Consequently, the AnotherClass class that belongs to the same package can also 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 to hide 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();    // failsFails to compile
    p.getPoint();
  }        
}

Compilation of AnotherClass now results in a compilation error because the class attempts to access a private nested class.

Risk Assessment

The Java Language System language system weakens the accessibility of sensitive, private entities in inner classes private members of an outer class when a nested inner class is present, which can result in a security weaknessan information leak.

Automated Detection

Automated detection of non-private nested nonprivate inner classes that define non-private nonprivate members and constructors is straight-forward. However, this guideline applies only when those classes could potentially expose sensitive data or operations that leak private data 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.

is straightforward.

Related Guidelines

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Bibliography

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Image Added Image Added Image Added 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 \[[McGraw 2000|AA. Bibliography#McGraw 00]\]OBJ16-J. Do not leak references to inner class objects when the outer class object maintains sensitive data      04. Object Orientation (OBJ)      05. Methods (MET)