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Developers often separate program logic across multiple classes or files to modularize code and to increase re-usabilityreusability. When developers modify a superclass (during maintenance, for example), the developer must ensure that changes in superclasses preserve all of 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 , with no 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);   // Enforce security manager check 
    System.out.println("WithdrawWithdrawal 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 is was unaware of the change. The Consequently, the client application subsequently 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 illustrates example shows this vulnerability. :

private class Account { 
  // Maintains all banking -related data such as account balance
  booleanprivate overdraft() {
    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  booleanmanager check
    boolean result = account.withdraw(200.0);   // Enforce security manager check 
    if (!result) {
      result = account.overdraft();
    }
    System.out.println("WithdrawlWithdrawal successful? " + result);
  }
}

While Although this code works as expected, it adds a dangerous attack vector of attack. 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);   // No security check performed
    System.out.println("WithdrawlWithdrawal successful? " + result);
  }
}

Compliant Solution

In this compliant solution, the BankAccount class provides an overriding version of the overdraft() method that immediately fails, thereby 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(); 
  }
}

AlternatelyAlternatively, when 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.

Related Vulnerability: JDK 1.2 java.util.Hashtable.entrySet()

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}} 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 (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 that indicates whether 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 day with the first day of week (which differs from country to country) to check whether it is a weekday.

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 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 among countries, 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) {
      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();
    // Expected to print true
    System.out.println(cs1.after(cs2

class CalendarSubclass extends Calendar {
  @Override public boolean after(Object when) {
    // correctly 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// intImplementation 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());             of other Calendar abstract methods
}

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
          
    cs1.set( Calendar.DAY_OF_WEEK, Calendar.SUNDAY); // Date of last Sunday (before now)
    CalendarSubclass cs2 = new CalendarSubclass();   // Wed Dec 31 19:00:00 EST 1969
    System.out.println(cs1.after(cs2));              // expected to print true
  }

  // Implementation of other Calendar abstract methods 
}
&& compareTo((Calendar) when) > 0;
}

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-Such errors generally occur because the developer has depended on 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.

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

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 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 is delegated to CalendarSubclass.compareTo(). Consequently, CalendarSubclass.after() actually calls CalendarSubclass.compareTo(), and consequently returns false.

Because the developer of the subclass was unaware of the implementation details of Calendar.after(), she incorrectly assumed that the superclass's after() method would invoke only its own methods without invoking overriding methods from the subclass. The guideline MET04-J. Ensure that constructors do not call overridable methods describes similar programming errors.

Compliant Solution (Calendar)

This compliant solution uses a design pattern called composition and forwarding (sometimes also referred to as delegation) \[[Lieberman 1986|AA. Bibliography#Lieberman 86]\] and \[[Gamma 1995|AA. Bibliography#Gamma 95, p. 20]\]. 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;
    }
    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();
    ci1.setTime(new Date());
    ci1.set( Calendar.DAY_OF_WEEK, Calendar.SUNDAY); // Date of last Sunday (before now)

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

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 thus are also unlikely to break CompositeCalendar. Invocations of CompositeCalendar's overriding after() method 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. Consequently, ava.util.Calendar.after() invokes the CalendarImplementation.compareTo() method as required, with the result that the program correctly prints true.

Risk Assessment

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
           : (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);
    // Expected to print true
    System.out.println(c.after(ci2));
  }
}

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 Modifying a superclass without considering the effect on subclasses can introduce vulnerabilities. Subclasses that are unaware developed with an incorrect understanding of the superclass implementation may can be subject to erratic behavior, resulting in inconsistent data state and mismanaged control flow.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

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

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] [Calendar|http://download.oracle.com/javase/6/docs/api/java/util/Calendar.html]
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 16: "Favor composition over inheritance"
\[[Gamma 1995|AA. Bibliography#Gamma 95]\] 
\[[Lieberman 1986|AA. Bibliography#Lieberman 86]\]
\[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 1-3 Understand how a superclass can affect subclass behavior

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

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Image Added Image Added Image AddedOBJ06-J. Compare classes and not class names      04. Object Orientation (OBJ)      OBJ08-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely