MSC54-J. Carefully design interfaces before releasing them

Interfaces are used to group together all the methods that a class promises to publicly expose. The implementing classes are obliged to provide concrete implementations for all these methods. Interfaces are a necessary ingredient of the public API and once released, it can be very hard to fix any flaws without breaking any code that implements the older version. The security specific repercussions include the following:

• The interface changes resulting from security fixes can severely impair the contracts of the implementing classes. It is even possible that a security fix introduced in a later version is accompanied by modifications to an unrelated interface that must now be implemented by the client. This can prevent the client from implementing the security fix because the new interface may impose additional implementation burden on it.
• If an insider crafts changes to an interface or someone accidentally makes modifications, most of the client code that implements the interface will break resulting in denial of service. This is particularly pernicious in distributed Java based applications.
• Implementers can provide default or skeletal implementations of interface methods that the clients can directly extend, however, such code can adversely affect the behavior of the subclasses. When such default implementations are not provided, the subclasses are forced to provide dummy implementations. This fosters an environment where comments such as 'ignore this code, does nothing', occur incessantly. Such code may never even get tested.
• If there is a security flaw in a public API, (e.g., ThreadGroups, rule THI01-J. Do not invoke ThreadGroup methods) it will persist throughout the lifetime of the application.

Noncompliant Code Example

In this noncompliant code example, an interface User is frozen with two methods authenticate() and subscribe(). Sometime later, the providers release a free service that does not rely on authentication.

public interface User {
  boolean authenticate(String username, char[] password);
  void subscribe(int noOfDays);
  void freeService(); // introduced after the class is publicly released
}

The addition of the freeService() method, unfortunately, breaks all the client code that implements the interface. Moreover, the implementers who wish to use only freeService have to face the onus of also providing the other two methods which pollute the API, for reasons discussed earlier.

Noncompliant Code Example

An alternative idea is to prefer abstract classes for dealing with constant evolution, but this comes at the cost of flexibility that interfaces offer (a class may implement multiple interfaces but extend only one class). One notable pattern is for the provider to distribute an abstract skeletal class that implements the evolving interface. The skeletal class can selectively implement a few methods and force the extending classes to provide concrete implementations of the others. If a new method is added to the interface, the skeletal class can provide a non-abstract default implementation that the extending class can optionally override.

public interface User {
  boolean authenticate(String username, char[] password);
  void subscribe(int noOfDays);
  void freeService(); // Introduced after the API is publicly released
}

abstract class SkeletalUser implements User {
  public abstract boolean authenticate(String username, char[] password);
  public abstract void subscribe(int noOfDays);
  public void freeService() { 
    // Added later, provide implementation and re-release class 
  }
}

class Client extends SkeletalUser {
  // Implements authenticate() and subscribe(), not freeService()
}

Although useful, this pattern may be insecure because a provider who is unaware of the extending class's code, may choose an implementation that introduces security weaknesses in the client API.

Compliant Solution (1) (Modularize)

A better design strategy is to anticipate the future evolution of the service. The core functionality should be implemented in the User interface and in this case, only the premium service may be required to extend from it. To avail of the new free service, an existing class may then choose to simply implement the new interface FreeUser or just completely ignore it.

public interface User {
  boolean authenticate(String username, char[] password);
}

public interface PremiumUser extends User {
  void subscribe(int noOfDays);
}

public interface FreeUser {
  void freeService();
}

Compliant Solution (2) (Make New Method Unusable)

Another compliant solution is to throw an exception from within the new, freeService() method defined in the implementing subclass.

class Client implements User {
  public void freeService() {
    throw new AbstractMethodError();
  } 
}

Compliant Solution (3) (Delegate Implementation to Subclasses)

Although allowable, a less flexible compliant solution is to delegate the implementation of the method to subclasses of the client's core interface implementing class.

abstract class Client implements User  {
  public abstract void freeService(); // Delegate implementation of new method to subclasses
  // Other concrete implementations
}

Risk Assessment

Failing to publish stable, flaw-free interfaces can break the contracts of the implementing classes, pollute the client API and possibly introduce security weaknesses in the implementing classes.

Related Vulnerabilities

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

Bibliography

[java:[Bloch 2008]] Item 18: "Prefer interfaces to abstract classes"

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