According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\]:

A nested class is any class whose declaration occurs within the body of another class or interface.

Nested classes are a broad set of classes that are classified as static member and inner classes. "An inner class is a nested class that is not explicitly or implicitly declared {{static}}." \[[JLS 05|AA. Java References#JLS 05]\]. An inner class may be local, anonymous or non-static.

The use of nested class is error-prone unless the semantics are well understood. A common notion is that only the outer class can access the contents of the nested inner class(es). Not only does the nested class have access to the private fields of the outer class, the same fields can be accessed by another class in the package depending on whether the nested class is declared public or if it contains public methods/constructors. By default, the javac compiler converts the accessibility of private methods to package-private (SCP30-J. Do not define nested classes in classes containing sensitive data if the container is untrusted). 

Also, according to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\]:

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

The code in this noncompliant example illegally exposes the sensitive (x,y) coordinates through the getPoint() method of the inner class. As a result, the AnotherClass class can illegally 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 declaring the inner class(es) 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 access of sensitive, private entities in inner classes which may result in a security weakness.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

SCP02- J

medium

probable

medium

P8

L2

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] [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 8.3 "Field Declarations"
\[[McGraw 00|AA. Java References#McGraw 00]\]
\[[Long 05|AA. Java References#Long 05]\] Section 2.3, Inner Classes
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 492|http://cwe.mitre.org/data/definitions/492.html] "Use of Inner Class Containing Sensitive Data"

SCP01-J. Do not increase the accessibility of overridden or hidden methods      05. Scope (SCP)      SCP03-J. Do not reuse names