OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code

Mutable classes allow code external to the class to alter their instance or class fields. Provide means for creating copies of mutable classes so that disposable instances of such classes can be passed to untrusted code. This functionality is useful when methods in other classes must create copies of the particular class instance (see OBJ06-J. Defensively copy mutable inputs and mutable internal components and OBJ05-J. Do not return references to private mutable class members for additional details).

Mutable classes must provide either a copy constructor or a public static factory method that returns a copy of an instance. Alternatively, final classes may advertise their copy functionality by overriding the clone() method of java.lang.Object. Use of the clone() method is secure only for final classes; nonfinal classes must not take this approach.

Trusted callers can be trusted to use the provided copy functionality to make defensive copies before passing object instances to untrusted code. Untrusted callers cannot be trusted to make such defensive copies. Consequently, providing copy functionality does not obviate the need for making defensive copies of inputs received from untrusted code or outputs returned to untrusted code.

Noncompliant Code Example

In this noncompliant code example, MutableClass uses a mutable field date of type Date. Class Date is also a mutable class. The example is noncompliant because the MutableClass objects lack copy functionality.

public final class MutableClass {
  private Date date;

  public MutableClass(Date d) {
    this.date = d;
  }

  public void setDate(Date d) {
    this.date = d;
  }

  public Date getDate() {
    return date;
  }
}

When a trusted caller passes an instance of MutableClass to untrusted code, and the untrusted code modifies that instance (perhaps by incrementing the month or changing the timezone), the object's state can be made inconsistent with respect to its previous state. Similar problems can arise in the presence of multiple threads, even in the absence of untrusted code.

Compliant Solution (Copy Constructor)

This compliant solution uses a copy constructor that initializes a MutableClass instance when an argument of the same type (or subtype) is passed to it:

public final class MutableClass { // Copy constructor
  private final Date date;

  public MutableClass(MutableClass mc)  {
    this.date = new Date(mc.date.getTime());
  }

  public MutableClass(Date d) {
    this.date = new Date(d.getTime());  // Make defensive copy
  }

  public Date getDate() {
    return (Date) date.clone(); // Copy and return
  }
}

This approach is useful when the instance fields are declared final. Callers request a copy by invoking the copy constructor with an existing MutableClass instance as its argument.

Compliant Solution (Public Static Factory Method)

This compliant solution exports a public static factory method getInstance() that creates and returns a copy of a given MutableClass object instance:

class MutableClass {
  private final Date date;

  private MutableClass(Date d) { // Noninstantiable and nonsubclassable
    this.date = new Date(d.getTime());  // Make defensive copy
  }

  public Date getDate() {
    return (Date) date.clone(); // Copy and return
  }

  public static MutableClass getInstance(MutableClass mc)  {
    return new MutableClass(mc.getDate());
  }
}

This approach is useful when the instance fields are declared final.

Compliant Solution (clone())

This compliant solution provides the needed copy functionality by declaring MutableClass to be final, implementing the Cloneable interface, and providing an Object.clone() method that performs a deep copy of the object:

public final class MutableClass implements Cloneable {
  private Date date;

  public MutableClass(Date d) {
    this.date = new Date(d.getTime());
  }

  public Date getDate() {
    return (Date) date.clone();
  }

  public void setDate(Date d) {
    this.date = (Date) d.clone();
  }

  public Object clone() throws CloneNotSupportedException {
    final MutableClass cloned = (MutableClass) super.clone();
    cloned.date = (Date) date.clone();  // Manually copy mutable Date object
    return cloned;
  }
}

Note that the clone() method must manually clone the Date object. This step is usually unnecessary when the object contains only primitive fields or fields that refer to immutable objects. However, when the fields contain data such as unique identifiers or object creation times, the clone() method must calculate and assign appropriate new values for such fields [Bloch 2008].

Mutable classes that define a clone() method must be declared final to ensure that untrusted code cannot declare a subclass that overrides the clone() method to create a spurious instance. The clone() method should copy all internal mutable state as necessary—in this compliant example, the Date object.

When untrusted code can call accessor methods passing mutable arguments, create defensive copies of the arguments before they are stored in any instance fields (see OBJ06-J. Defensively copy mutable inputs and mutable internal components for additional information). When retrieving internal mutable state, make a defensive copy of that state before returning it to untrusted code (see OBJ05-J. Do not return references to private mutable class members for additional information).

Defensive copies would be unnecessary if untrusted code always invoked an object's clone() method on mutable state received from mutable classes and operated only on the cloned copy. Unfortunately, untrusted code has little incentive to do so, and malicious code has every incentive to misbehave. This compliant solution provides a clone() method to trusted code and also guarantees that the state of the object cannot be compromised when the accessor methods are called directly from untrusted code.

Compliant Solution (clone() with Final Members)

When a mutable class's instance fields are declared final and lack accessible copy methods, provide a clone() method, as shown in this compliant solution:

public final class MutableClass implements Cloneable {
  private final Date date; // final field

  public MutableClass(Date d) {
    this.date = new Date(d.getTime());  // Copy in
  }

  public Date getDate() {
    return (Date) date.clone(); // Copy and return
  }

  public Object clone() {
    Date d = (Date) date.clone();
    MutableClass cloned = new MutableClass(d);
    return cloned;
  }
}

Callers can use the clone() method to obtain an instance of such a mutable class. The clone() method must create a new instance of the final member class and copy the original state to it. The new instance is necessary because there might not be an accessible copy method available in the member class. If the member class evolves in the future, it is critical to include the new state in the manual copy. Finally, the clone() method must create and return a new instance of the enclosing class (MutableClass) using the newly created member instance (d) [SCG 2009].

Mutable classes that define a clone() method must be declared final.

Compliant Solution (Unmodifiable Date Wrapper)

If cloning or copying a mutable object is infeasible or expensive, one alternative is to create an immutable view class. This class overrides mutable methods to throw an exception, protecting the mutable class.

class UnmodifiableDateView extends Date {
  private Date date;

  public UnmodifiableDateView(Date date) {
    this.date = date;
  }

  public void setTime(long date) {
    throw new UnsupportedOperationException();
  }

  // Override all other mutator methods to throw UnsupportedOperationException
}

public final class MutableClass {
  private Date date;

  public MutableClass(Date d) {
    this.date = d;
  }

  public void setDate(Date d) {
    this.date = (Date) d.clone();
  }

  public UnmodifiableDateView getDate() {
    return new UnmodifiableDateView(date);
  }
}

Exceptions

OBJ04-J-EX0: Sensitive classes should not be cloneable, per OBJ07-J. Sensitive classes must not let themselves be copied.

Risk Assessment

Creating a mutable class without providing copy functionality can result in the data of its instance becoming corrupted when the instance is passed to untrusted code.

Automated Detection

Sound automated detection is infeasible in the general case. Heuristic approaches could be useful.

Related Guidelines

Bibliography