OBJ07-J. Understand how a superclass can affect a subclass

It is common for developers to separate the program logic into different classes or files to modularize code and increase re-usability. Unfortunately, this often imposes maintenance hurdles such as having to ensure that changes in superclasses do not indirectly affect subclass behavior.

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

The class Provider inherits the put() and remove() methods from Hashtable and adds security manager checks to each. The security manager 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 java.security.Provider did not override this method to provide the necessary security manager check [[SCG 07]]. This problem is commonly know as a "fragile class hierarchy" in other object-oriented languages such as C++.

Noncompliant Code Example

This noncompliant code example shows a class SuperClass that stores banking related information but delegates the security manager and input validation tasks to the class SubClass. The client application is required to use SubClass as it contains various authentication mechanisms.

class SuperClass { // SuperClass 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;
  }

  void overdraft() { // This method is added at a later date
    balance += 300;  // Add 300 in case there is an overdraft
    System.out.println("Added back-up amount. The balance is :" + balance);
  }
}

class SubClass extends SuperClass { // Subclass handles authentication
  @Override boolean withdraw(double amount) {
    // inputValidation(), securityManagerCheck() and authenticateUser()
    return super.withdraw(amount);
  }	
 		 
  public static void doLogic(SuperClass sc, double amount) {
    if (!sc.withdraw(amount)) {
      throw new IllegalStateException();
    }
    // ...
  }
}

A new method called overdraft is added by the maintainer of the class SuperClass and the extending class SubClass is not aware of this change. This exposes the client application to malicious invocations. One such example is of the overdraft method being used on the currently in-use object. All security checks are deemed useless in this case. The following class shows how a security check may be bypassed if the client and the subclass are unaware of the superclass's implementation changing.

public class Client {
  public static void main(String[] args) {
    SuperClass sc = new SubClass(); // Override
    
    if(sc.withdraw(200.0)) {        // Validate and enforce security manager check 
      SubClass.doLogic(sc, 200.0);  // Withdraw 200.0 from superclass
    } else {
      sc.overdraft(); // Newly added method, has no security manager checks 
    }
  }
}

Compliant Solution

This compliant solution is the same as the noncompliant code example, except that it overrides the overdraft() method and throws an exception to prevent misuse of the overdraft feature.

class SubClass extends SuperClass {
// ...
  @Override void overdraft() { // override
    throw new IllegalAccessException(); 
  }
}

Alternatively, install a security manager check in the overridden method if it should be allowable to use it from a subclass.

Noncompliant Code Example

This noncompliant code example overrides the methods after() and compareTo() of the class java.util.Calendar. The Calendar.after() method returns a boolean value depending on whether the Calendar represents a time after the time represented by the specified Object parameter. The programmer wishes to extend this functionality and return true even when the two objects represent the same date. The method compareTo() is also overridden in this example, to provide a "comparisons by day" option to clients. For example, comparing today's day with the first day of week (which differs from country to country) to check whether it is a weekday.

class CalendarSubclass extends Calendar {
  @Override public boolean after(Object when) {
    // correctly calls Calendar.compareTo()
    if(when instanceof Calendar && super.compareTo((Calendar)when) == 0) { // Note the == operator
      return true;
    }
    return super.after(when); // Calls CalendarSubclass.compareTo() instead of Calendar.compareTo() 
  }
	
  @Override public int compareTo(Calendar anotherCalendar) {
    // This method is erroneously invoked by Calendar.after()
    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(); 
    CalendarSubclass cs2 = new CalendarSubclass(); // Wed Dec 31 19:00:00 EST 1969
    cs1.setTime(new Date());                       // Current day's date 
    System.out.println(cs1.after(cs2));            // prints false
  }

  // Implementation of other abstract methods 
}

// The implementation of java.util.Calendar.after() method is shown below
public boolean after(Object when) {
  return when instanceof Calendar && compareTo((Calendar)when) > 0; // Note the > operator
     // forwards to the subclass's implementation erroneously
}

Typically, errors manifest when assumptions are made about the implementation specific details of the superclass. Here, the two objects are compared in the overriding after() method and subsequently, the superclass's after() method is explicitly called to take over. The issue is that the superclass Calendar's after() method internally uses class Object's compareTo() method. Consequently, the superclass's after() method erroneously invokes the subclass's version of compareTo(). Because the subclass is unaware of the superclass's implementation of after(), it does not expect any of its own overriding methods to get invoked. The guideline MET04-J. Ensure that constructors do not call overridable methods describes similar programming errors.

Compliant Solution

This compliant solution recommends the use of a design pattern called composition and forwarding (sometimes also referred to as delegation) [[Lieberman 86]] and [[Gamma 95]]. A new forwarder class that contains a private member field of the Calendar type is introduced. Such a composite class constitutes composition. In this example, the field refers to CalendarImplementation, a concrete instantiable implementation of the abstract Calendar class. A wrapper class called CompositeCalendar is also introduced. It consists of the same overridden methods that constituted CalendarSubclass in the preceding noncompliant code example.

// 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
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;
    }
    return super.after(when); // Does not compare with first day of week anymore;
                              // Uses default comparison with epoch
  }
	
  @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();
    CalendarImplementation ci2 = new CalendarImplementation();
    CompositeCalendar c = new CompositeCalendar(ci1);
    ci1.setTime(new Date());
    System.out.println(c.after(ci2)); // prints true 
  }
}

Note that each method of the class ForwardingCalendar redirects to methods of the contained class instance (CalendarImplementation), and receives back return values. This is the forwarding mechanism. The ForwardingCalendar class is largely independent of the implementation of the class CalendarImplementation. Consequently, any future changes to the latter will not break ForwardingCalendar and in turn CompositeCalendar, which derives from it. When CompositeCalendar's overriding after() method is invoked, it performs the necessary comparison by using the local version of the compareTo() method as required. Using super.after(when) forwards to ForwardingCalendar which invokes the CalendarImplementation's after() method. In this case, CalendarImplementation's compareTo() method gets called instead of the overriding version in CompositeClass that was inappropriately called in the noncompliant code example.

Risk Assessment

Modifying a superclass without considering the effect on a subclass can introduce vulnerabilities. Subclasses that are unaware of the superclass implementation can be subject to erratic behavior resulting in inconsistent data state and mismanaged control flow.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[SCG 07]] Guideline 1-3 Understand how a superclass can affect subclass behavior
[[Bloch 08]] Item 16: "Favor composition over inheritance"
[[Gamma 95]]
[[Lieberman 86]]

OBJ06-J. Compare classes and not class names      08. Object Orientation (OBJ)      OBJ08-J. Avoid using finalizers