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It is common for developers to separate the program logic into different Developers often separate program logic across multiple classes or files to modularize code and to increase re-usability. Unfortunately, this often imposes maintenance hurdles such as having to reusability. When developers modify a superclass (during maintenance, for example), the developer must 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 \[[SCG 2007|AA. Bibliography#SCG 07]\]. 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.

preserve all the program invariants on which the subclasses depend. Failure to maintain all relevant invariants can cause security vulnerabilities.

Noncompliant Code Example

In this code example, a class Account 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.

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

class SuperClass { // SuperClass maintains all banking related data such as account balance
  private double balance = 100;
 
  boolean withdraw(double amount) {
    if (!securityCheck(balance - amount) >= 0) {	
      balancethrow -= amountnew IllegalAccessException();
    }
   System.out.println("Withdrawal successful. The balance is : " + balance return super.withdraw(amount);
  }

  private final boolean return true;securityCheck() {
    }// 
Check that account management returnmay false;proceed
  }
}

public class void overdraft() { // This method is added at a later date
    balance += 300;Client {
  public static void main(String[] args) {
    Account account = new BankAccount();
    // AddEnforce 300security inmanager casecheck
 there is an overdraft
boolean result = account.withdraw(200.0);   
    System.out.println("Added back-up amount. The balance is :Withdrawal successful? " + balanceresult);
  }
}

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.

At a later date, the maintainer of 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:

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;
  }

  // Other 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 class Client {
  public static void main(String[] args) {
    SuperClassAccount scaccount = new SubClassBankAccount();
 // Override
  // Enforce security manager check
    boolean result = if(scaccount.withdraw(200.0));  { 
    if (!result) {
 // Validate and enforce security managerresult check 
= account.overdraft();
    }
    SubClassSystem.out.doLogicprintln(sc, 200.0);  // Withdraw 200.0 from superclass
    } else {
      sc.overdraft(); // Newly added method, has no security manager checks 
    }
  }
"Withdrawal successful? " + result);
  }
}

Although this code works as expected, it adds a dangerous attack vector. Because 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();
    System.out.println("Withdrawal successful? " + result);
  }
}

Compliant Solution

This In this compliant solution is the same as the noncompliant code example, except that it overrides , the BankAccount class provides an overriding version of the overdraft() method and throws an exception to prevent that immediately fails, preventing misuse of the overdraft feature. All other aspects of the compliant solution remain unchanged.

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

Alternatively, install a security manager check in the overridden method if it should be allowable to use it from a subclasswhen the intended design permits the new method in the parent class to be invoked directly from a subclass without overriding, install a security manager check directly in the new method.

Noncompliant Code Example (Calendar)

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 that indicates whether or not the Calendar represents a time after the time that represented by the specified Object parameter. The programmer wishes to extend this functionality and return so that the after() method returns true even when the two objects represent the same date. The programmer also overrides the method compareTo() is also overridden in this example, to to provide a "comparisons by day" option to clients . For (for example, comparing today's day date with the first day of the week (, which differs from country to country) to among 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) { // 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()
  }

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

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

  public static void main(String[] args) {
    CalendarSubclass cs1 = new CalendarSubclass(); 
    CalendarSubclass cs2 = cs1.setTime(new CalendarSubclassDate());
    // WedDate Decof 31 19:00:00 EST 1969last Sunday (before now)
    cs1.setTime(new Date());set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
    // Wed Dec 31 19:00:00 EST 1969
    CalendarSubclass cs2 = new CalendarSubclass();
     // CurrentExpected day'sto dateprint true
    System.out.println(cs1.after(cs2));            // prints false
  }

  // Implementation of other Calendar abstract methods 
}

// The implementation of java.util.Calendar.after() method is shown below

}

The java.util.Calendar class provides a compareTo() method and an after() method. The after() method is documented 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; // 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) \[[Lieberman 1986|AA. Bibliography#Lieberman 86]\] and \[[Gamma 1995|AA. Bibliography#Gamma 95, p. 20]\]. 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.

}

In this case, the two objects are initially compared using the overriding CalendarSubclass.after() method, 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 the superclass's methods without invoking overriding methods from the subclass. 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 (Calendar)

This compliant solution uses a design pattern called Composition and Forwarding (sometimes also called Delegation) [Lieberman 1986], [Gamma 1995]. 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 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 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;
    }
    // No longer compares with first day of week;
    // uses 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

// 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;
                           : (currentFirstDayOfWeek == //anotherFirstDayOfWeek) Uses? default0 comparison with epoch: -1;
  }
	
  @Overridepublic publicstatic intvoid compareTomain(CalendarString[] anotherCalendarargs) {
    return compareDays(super.getCalendarImplementation().getFirstDayOfWeek(),
         CalendarImplementation ci1 = new CalendarImplementation();
    ci1.setTime(new Date());
    // Date of last Sunday anotherCalendar.getFirstDayOfWeek(before now));
  }

  private int compareDays(int currentFirstDayOfWeek, int anotherFirstDayOfWeek) {
    return (currentFirstDayOfWeek > anotherFirstDayOfWeek) ?
           1 : (currentFirstDayOfWeek == anotherFirstDayOfWeek) ? 0 : -1ci1.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);

    CalendarImplementation ci2 = new CalendarImplementation();
    CompositeCalendar c = new CompositeCalendar(ci1);
    // Expected to print true
    System.out.println(c.after(ci2));
  }

  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.

Bibliography

\[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 1-3 Understand how a superclass can affect subclass behavior
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 16: "Favor composition over inheritance"
\[[Gamma 1995|AA. Bibliography#Gamma 95]\] 
\[[Lieberman 1986|AA. Bibliography#Lieberman 86]\]

}

Note that each method of the class ForwardingCalendar redirects to methods of the contained CalendarImplementation class, from which it receives return values; this is the forwarding mechanism. The ForwardingCalendar class is largely independent of the implementation of the class CalendarImplementation. Consequently, future changes to CalendarImplementation are unlikely to break ForwardingCalendar and are also unlikely to break CompositeCalendar. Invocations of the overriding after() method of CompositeCalendar perform the necessary comparison by using the CalendarImplementation.compareTo() method as required. Using super.after(when) forwards to ForwardingCalendar, which invokes the CalendarImplementation.after() method as required. As a result, java.util.Calendar.after() invokes the CalendarImplementation.compareTo() method as required, resulting in the program correctly printing true.

Risk Assessment

Modifying a superclass without considering the effect on subclasses can introduce vulnerabilities. Subclasses that are developed 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.

Automated Detection

Sound automated detection is not currently feasible.

Related Vulnerabilities

The introduction of the entrySet() method in the java.util.Hashtable superclass in JDK 1.2 left the java.security.Provider subclass 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 situation is commonly known as the fragile class hierarchy problem.

Related Guidelines

Bibliography

...

Image Added Image Added Image AddedOBJ06-J. Compare classes and not class names      08. Object Orientation (OBJ)      OBJ08-J. Avoid using finalizers