OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely

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 need to create copies of the particular class instance; see guidelines FIO00-J. Defensively copy mutable inputs and mutable internal components and OBJ11-J. Defensively copy private mutable class members before returning their references 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 java.lang.Object's clone() method. Use of the clone() method is secure only for final classes; non-final classes are forbidden to use 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 is insufficient to obviate the need for making defensive copies either of inputs received from, or of outputs that are 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 fails to comply because 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 state of the object may may consequently become inconsistent with 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());  // Copy-in 
  }

  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());  // Copy-in   
  }
 
  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, and by 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 = 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. This ensures that untrusted code cannot declare a subclass that overrides the clone() method so that it supplies 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 and can pass mutable arguments, make defensive copies of the arguments before they are stored in any instance fields. See guideline FIO00-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 guideline OBJ11-J. Defensively copy private mutable class members before returning their references for additional information.

These defensive copies would be unnecessary if untrusted code always invoked object's clone() method on mutable state received from mutable classes and then 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 both 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() throws CloneNotSupportedException {
    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 no accessible copy method may be 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 2007]].

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

Exceptions

EX0: Sensitive classes should not be cloneable, as per guideline OBJ02-J. Sensitive classes must not let themselves be copied.

Risk Assessment

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

Automated Detection

Sound automated detection appears to be infeasible in the general case. Heuristic approaches may be useful.

Related Vulnerabilities

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

Bibliography

[[API 2006]] method clone()
[[Bloch 2008]] Item 39: Make defensive copies when needed and Item 11: Override clone judiciously
[[MITRE 2009]] CWE ID 374 "Mutable Objects Passed by Reference", CWE ID 375 "Passing Mutable Objects to an Untrusted Method"[[Security 2006]]
[[SCG 2007]] Guideline 2-2 Support copy functionality for a mutable class
[[SCG 2009]] Guideline 2-3 Support copy functionality for a mutable class

OBJ07-J. Understand how a superclass can affect a subclass      04. Object Orientation (OBJ)      OBJ11-J. Defensively copy private mutable class members before returning their references