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Developers often separate program logic across multiple classes or files to modularize code and to increase reusability. When developers modify a superclass (during maintenance, for example), the developer must ensure that changes in superclasses preserve all the program invariants on which the subclasses depend. Failure to maintain all relevant invariants can cause security vulnerabilities.

Noncompliant Code Example

This noncompliant In this code example relies on , a class Account that stores  stores banking-related information without any inherent security. Security is delegated to the subclass BankAccount. The client application is required to use BankAccount because it contains the security mechanism.

private class Account { 
  // Maintains all banking -related data such as account balance
  private double balance = 100;

  boolean withdraw(double amount) {
    if ((balance - amount) >= 0) {
      balance -= amount;
      System.out.println("Withdrawal successful. The balance is : "
                         + balance);
      return true;
    }
    return false;
  }
}

public class BankAccount extends Account { 
  // Subclass handles authentication
  @Override boolean withdraw(double amount) {
    if (!securityCheck()) {
      throw new IllegalAccessException();
    }
    return super.withdraw(amount);
  }

  private final boolean securityCheck() {
    // checkCheck that account management may proceed
  }
}

public class Client {
  public static void main(String[] args) {
    Account account = new BankAccount();
    // Enforce security manager check
    boolean result = account.withdraw(200.0);   
    System.out.println("Withdrawal successful? " + result);
  }
}

At a later date, the maintainer of the class the Account class added a new method called overdraft(). However, the BankAccount class maintainer was unaware of the change. Consequently, the client application became vulnerable to malicious invocations. For example, the overdraft() method could be invoked directly on a BankAccount object, avoiding the security checks that should have been present. The following noncompliant code example shows this vulnerability.:

private class Account { 
  // Maintains all banking -related data such as account balance
  private double balance = 100;

  boolean overdraft() {
    balance += 300;     // Add 300 in case there is an overdraft
    System.out.println("Added back-up amount. The balance is :" 
                       + balance);
    return true;
  }

  // otherOther Account methods
}

public class BankAccount extends Account { 
  // Subclass handles authentication
  // NOTE: unchanged from previous version
  // NOTE: lacks override of overdraft method
}

public class Client {
  public static void main(String[] args) {
    Account account = new BankAccount();
    // Enforce security manager check
    boolean result = account.withdraw(200.0);   
    if (!result) {
      result = account.overdraft();
    }
    System.out.println("Withdrawal successful? " + result);
  }
}

While Although this code works as expected, it adds a dangerous attack vector. Because there is no security check on the overdraft() method has no security check, a malicious client can invoke it without authentication:

public class MaliciousClient {
  public static void main(String[] args) {
    Account account = new BankAccount();
    // No security check performed
    boolean result = account.overdraft(200.0);
    System.out.println("Withdrawal successful? " + result);
  }
}

...

In this compliant solution, the BankAccount class provides an overriding version of the overdraft() method that immediately fails, preventing misuse of the overdraft feature. All other aspects of the compliant solution remain unchanged.

class BankAccount extends Account {
  // ...
  @Override voidboolean overdraft() { // overrideOverride
    throw new IllegalAccessException();
  }
}

...

This noncompliant code example overrides the methods after() and compareTo() of the class java.util.Calendar. The Calendar.after() method returns a boolean value that indicates whether or not the Calendar represents a time after that represented by the specified Object parameter. The programmer wishes to extend this functionality so that the after() method returns true even when the two objects represent the same date. The programmer also overrides the method compareTo() to provide a "comparisons by day" option to clients (for example, comparing today's date with the first day of the week, which differs from country to countryamong countries, to check whether it is a weekday).

class CalendarSubclass extends Calendar {
  @Override public boolean after(Object when) {
    // correctlyCorrectly calls Calendar.compareTo()
    if (when instanceof Calendar && 
        super.compareTo((Calendar) when) == 0) {
      return true;
    }
    return super.after(when);
  }

  @Override public int compareTo(Calendar anotherCalendar) {
    return compareDays(this.getFirstDayOfWeek(),
                       anotherCalendar.getFirstDayOfWeek());
  }

  private int compareDays(int currentFirstDayOfWeek,
                          int anotherFirstDayOfWeek) {
    return (currentFirstDayOfWeek > anotherFirstDayOfWeek) ? 1
           : (currentFirstDayOfWeek == anotherFirstDayOfWeek) ? 0 : -1;
  }

  public static void main(String[] args) {
    CalendarSubclass cs1 = new CalendarSubclass();
    cs1.setTime(new Date());
    // Date of last Sunday (before now)
    cs1.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
    // Wed Dec 31 19:00:00 EST 1969
    CalendarSubclass cs2 = new CalendarSubclass();
    // expectedExpected to print true
    System.out.println(cs1.after(cs2));
  }

  // Implementation of other Calendar abstract methods
}

Such errors generally occur because the developer made assumptions about the implementation-specific details of the superclass. Even when these assumptions are initially correct, implementation details of the superclass may change without warning.

Wiki MarkupThe {{java.util.Calendar}} class provides a {{compareTo()}} method and an {{after()}} method. The {{after()}} method is documented in \[[API 2006|AA. Bibliography#API 06]\] as in the Java API Reference [API 2014] as follows:

The after() method returns whether this Calendar represents a time after the time represented by the specified Object. This method is equivalent to
compareTo(when) > 0
if and only if when is a Calendar instance. Otherwise, the method returns false.

The documentation fails to state whether after() invokes compareTo() or whether compareTo() invokes after(). In the Oracle JDK 1.6 implementation, the source code for after() is as follows:

public boolean after(Object when) {
  return when instanceof Calendar
         && compareTo((Calendar) when) > 0;
}

In this case, the two objects are initially compared using the overriding CalendarSubclass.after() method. This , which invokes the superclass's Calendar.after() method to perform the remainder of the comparison. But the Calendar.after() method internally calls the compareTo() method, which delegates to CalendarSubclass.compareTo(). Consequently, CalendarSubclass.after() actually calls CalendarSubclass.compareTo() and returns false.

The developer of the subclass was unaware of the implementation details of Calendar.after() and incorrectly assumed that the superclass's after() method would invoke only its own the superclass's methods without invoking overriding methods from the subclass. Rule MET05-J. Ensure that constructors do not call overridable methods describes similar programming errors.

Such errors generally occur because the developer made assumptions about the implementation-specific details of the superclass. Even when these assumptions are initially correct, implementation details of the superclass may change without warning.

Compliant Solution Compliant Solution (Calendar)

...

This compliant solution uses a design pattern called composition and forwarding Composition and Forwarding (sometimes also called delegationDelegation) \[ [Lieberman 1986|AA. Bibliography#Lieberman 86]\], \[ [Gamma 1995|AA. Bibliography#Gamma 95], p. 20\]. The compliant solution introduces a new _forwarder_ class that contains a {{ private }} member field of the {{Calendar}} type; this is _composition_ rather than inheritance. In this example, the field refers to {{CalendarImplementation}}, a concrete instantiable implementation of the {{abstract}} {{Calendar}} class. The compliant solution also introduces a wrapper class called {{CompositeCalendar}} that provides the same overridden methods found in the {{CalendarSubclass}} from the preceding noncompliant code CalendarSubclass from the preceding noncompliant code example.

Code Block
bgColor #ccccff
// The CalendarImplementation object is a concrete implementation // of the abstract Calendar class // Class ForwardingCalendar public class ForwardingCalendar { private final CalendarImplementation c; public ForwardingCalendar(CalendarImplementation c) { this.c = c; } CalendarImplementation getCalendarImplementation() { return c; } public boolean after(Object when) { return c.after(when); } public int compareTo(Calendar anotherCalendar) { // CalendarImplementation.compareTo() will be called return c.compareTo(anotherCalendar); } } class CompositeCalendar extends ForwardingCalendar { public CompositeCalendar(CalendarImplementation ci) { super(ci); } @Override public boolean after(Object when) { // This will call the overridden version, i.e. // CompositeClass.compareTo(); if (when instanceof Calendar && super.compareTo((Calendar)when) == 0) { // Return true if it is the first day of week return true; } // DoesNo notlonger comparecompares with first day of week any longer; // Usesuses default comparison with epoch return super.after(when); } @Override public int compareTo(Calendar anotherCalendar) { return compareDays( super.getCalendarImplementation().getFirstDayOfWeek(), anotherCalendar.getFirstDayOfWeek()); } private int compareDays(int currentFirstDayOfWeek, int anotherFirstDayOfWeek) { return (currentFirstDayOfWeek > anotherFirstDayOfWeek) ? 1 : (currentFirstDayOfWeek == anotherFirstDayOfWeek) ? 0 : -1; } public static void main(String[] args) { CalendarImplementation ci1 = new CalendarImplementation(); ci1.setTime(new Date()); // Date of last Sunday (before now) ci1.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY); CalendarImplementation ci2 = new CalendarImplementation(); CompositeCalendar c = new CompositeCalendar(ci1); // expectedExpected to print true System.out.println(c.after(ci2)); } }

...

Modifying a superclass without considering the effect on subclasses can introduce vulnerabilities. Subclasses that are developed without awareness with an incorrect understanding of the superclass implementation can be subject to erratic behavior, resulting in inconsistent data state and mismanaged control flow. Also, if the superclass implementation changes then the subclass may need to be redesigned to take into account these changes.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
OBJ02-J	medium Medium	probable Probable	high High	P4	L3

Automated Detection

Sound automated detection is not currently feasible.

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The introduction of the entrySet() method in the java.util.Hashtable superclass in JDK 1.2 left the java.security.Provider subclass class vulnerable to a security attack. The Provider class extends java.util.Properties, which in turn extends Hashtable. The Provider class maps a cryptographic algorithm name (for example ", RSA") to a class that provides its implementation.

The Provider class inherits the put() and remove() methods from Hashtable and adds security manager checks to each. These checks ensure that malicious code cannot add or remove the mappings. When entrySet() was introduced, it became possible for untrusted code to remove the mappings from the Hashtable because Provider failed to override this method to provide the necessary security manager check [SCG 2009]. This problem situation is commonly known as a the fragile class hierarchy in other object-oriented languages, such as C++ problem.

Related Guidelines

Bibliography

[API 2014]	Class Calendar
[Bloch 2008]	Item 16, "Favor Composition over Inheritance"
[Gamma 1995]	Design Patterns: Elements of Reusable Object-Oriented Software (p. 20)
[Lieberman 1986]	"Using Prototypical Objects to Implement Shared Behavior in Object-Oriented Systems"

 

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Image Added Image Added Image AddedOBJ01-J. Declare data members as private and provide accessible wrapper methods      04. Object Orientation (OBJ)      OBJ03-J. Do not mix generic with nongeneric raw types in new code