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 is also a mutable class. The example is noncompliant because the
MutableClass objects lack copy functionality.
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:
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:
This approach is useful when the instance fields are declared final.
Compliant Solution (
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:
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
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:
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.
OBJ04-J-EX0: Sensitive classes should not be cloneable, per OBJ07-J. Sensitive classes must not let themselves be copied.
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.
Sound automated detection is infeasible in the general case. Heuristic approaches could be useful.
May expose internal representation by returning reference to mutable object
May expose internal representation by incorporating reference to mutable object
|Make your 'clone()' method "final" for security|
Enforce returning a defensive copy in 'clone()' methods
Do not pass user-given mutable objects directly to certain types
Do not store user-given mutable objects directly into variables
Provide mutable classes with copy functionality
Guideline 6-4 / MUTABLE-4: Support copy functionality for a mutable class
Item 39, "Make Defensive Copies When Needed"