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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

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.

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() {
    // Check 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 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;
  }

  // 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 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);
  }
}

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The java.util.Calendar class provides a compareTo() method and an after() method. The after() method is documented in the Java API Reference [API 20062014] 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.

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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.

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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.

Automated Detection

Sound automated detection is not currently feasible.

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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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