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A 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}} member 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.

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 of a nested class to package-private.

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Also, according to the Java Language Specification \[[JLS 2005\|AA. Bibliography#JLS 05]\], section 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 (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.

Code Block

bgColor	#FFcccc

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.

Code Block

bgColor	#ccccff

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 may result in a security weakness.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
SCP03-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

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\[[JLS 2005|AA. Bibliography#JLS 05]\] [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 8.3 "Field Declarations"
\[[McGraw 2000|AA. Bibliography#McGraw 00]\]
\[[Long 2005|AA. Bibliography#Long 05]\] Section 2.3, Inner Classes
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 492|http://cwe.mitre.org/data/definitions/492.html] "Use of Inner Class Containing Sensitive Data"

SCP02-J. Do not reuse names 05. Scope (SCP) SCP04-J. Reduce the scope of the SuppressWarnings annotation

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