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Mutable classes are those which when instantiated, provide a reference such that the contents of the class can be altered anytime. It is important to provide means for creating copies of mutable class instances so as to allow safe passing in and returning of class objects when used within method arguments.

Noncompliant Code Example

In this noncompliant example, MutableClass uses a mutable Date object. If the caller changes the instance of the Date object (like incrementing the month), the class implementation no longer remains consistent with its old state. Both, the constructor as well as the getDate method are susceptible to abuse. This also defies attempts to implement thread safety.

import java.util.Date;

public final class MutableClass {
  private Date d;
	
  public MutableClass(Date d) {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      //check permissions
    }
    this.d = d;
  }
	
  public Date getDate() {
    return this.d;	
  }
}

Compliant Solution

Always provide mechanisms to create copies of the instances of a mutable class. This compliant solution implements the Cloneable interface and overrides the clone method to create a deep copy of both the object and the mutable Date object. Since using clone() independently only produces a shallow copy and still leaves the class mutable, it is advised to also copy all the referenced mutable objects that are passed in or returned from any method.

import java.util.Date;

public final class CloneClass implements Cloneable {
  private Date d;
	
  public CloneClass(Date d) {
    SecurityManager sm = System.getSecurityManager();
      if (sm != null) {
	//check permissions
      }
      this.d = new Date(d.getTime());  //copy-in 
  }

  public Object clone() throws CloneNotSupportedException {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      //check permissions
    }
    final CloneClass cloned = (CloneClass)super.clone();
    cloned.d = (Date)this.d.clone();  //copy mutable Date object
    return cloned;
  }
	
  public Date getDate() {
    return new Date(this.d.getTime()); //copy and return
  }
}

At times, a class is labeled final with no accessible copy methods. Callers can then obtain an instance of the class, create a new instance with the original state and subsequently proceed to use it. Similarly, mutable objects obtained must also be copied when necessary.

Risk Assessment

Creating a mutable class without a clone method may result in the data of the class becoming corrupted.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] [method clone()|http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()]
\[[Security 06|AA. Java References#Security 06]\]
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 2-2 Support copy functionality for a mutable class
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 39: Make defensive copies when needed

SEC34-J. Do not allow the unauthorized construction of sensitive classes      00. Security (SEC)      SEC36-J. Ensure that the bytecode verifier is applied to all involved code upon any modification