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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 is noncompliant because MutableClass objects lack copy functionality.
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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.
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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.
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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.
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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.
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Mutable classes that define a clone() method must be declared final.
Exceptions
EX0: Sensitive classes should not be cloneable, as per guideline OBJ03-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.
Guideline | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
OBJ10 OBJ08-J | low | likely | medium | P6 | L2 |
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
| Wiki Markup |
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\[[API 2006|AA. Bibliography#API 06]\] [method clone()|http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()] \[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 39: Make defensive copies when needed and Item 11: Override clone judiciously \[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 374|http://cwe.mitre.org/data/definitions/374.html] "Mutable Objects Passed by Reference", [CWE ID 375|http://cwe.mitre.org/data/definitions/375.html] "Passing Mutable Objects to an Untrusted Method"\[[Security 2006|AA. Bibliography#Security 06]\] \[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 2-2 Support copy functionality for a mutable class \[[SCG 2009|AA. Bibliography#SCG 09]\] Guideline 2-3 Support copy functionality for a mutable class |
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OBJ07-J. Understand how a superclass can affect a subclass 04. Object Orientation (OBJ) OBJ11OBJ09-J. Defensively copy private mutable class members before returning their references