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Java's Object cloning mechanism can allow an attacker to manufacture new instances of classes that have been defined, without executing its constructorClasses containing private, confidential, or otherwise sensitive data are best not copied. If a class is not cloneable, the attacker can define a subclass, and make the subclass implement the java.lang.Cloneable interface. This lets an attacker create new instances of the class. The new instances of the class are made meant to be copied, then failing to define copy mechanisms, such as a copy constructor, is insufficient to prevent copying.

Java's object cloning mechanism allows an attacker to manufacture new instances of a class by copying the memory images of existing objects ; though rather than by executing the class's constructor. Often, this is sometimes an acceptable unacceptable way of making a new object, it often is not.

 Non Compliant Code

Consider the following class definition, the value 1234 is a secret.

creating new objects. An attacker can misuse the clone feature to manufacture multiple instances of a singleton class, create thread-safety issues by subclassing and cloning the subclass, bypass security checks within the constructor, and violate the invariants of critical data.

Classes that have security checks in their constructors must beware of finalization attacks, as explained in OBJ11-J. Be wary of letting constructors throw exceptions.

Classes that are not sensitive but maintain other invariants must be sensitive to the possibility of malicious subclasses accessing or manipulating their data and possibly invalidating their invariants (see OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code for more information).

Noncompliant Code Example

This noncompliant code example defines class SensitiveClass, which contains a character array used to hold a file name, along with a Boolean shared variable, initialized to false. This data is not meant to be copied; consequently, SensitiveClass lacks a copy constructor.

class SensitiveClass

class MyPrivacy {
  private char[] filename;
  private intBoolean keyshared = 1234false;

  SensitiveClass(String filename) {
   //...
    public MyPrivacy this.filename = filename.toCharArray();
  }

  final void replace() {}
    publicif MyPrivacy(int k!shared) {
      for(int  this.key = k;i = 0; i < filename.length; i++) {
    	filename[i]= 'x' ;}
    }
  }

  final String get() {
   protected intif getKey(!shared) {
      shared = true;
      return this.key;
    }
String.valueOf(filename);
    } else {
      throw new IllegalStateException("Failed to get instance");
    }
  }

  final void printFilename() {
    //System.out.println(String.valueOf(filename));
  }
}

The attacker can create a new instance of MyPrivacy class by using a cloneable subclass and by-pass the constructor and leave the field not initialized or even steal data. This can be showed by the following code.

When a client requests a String instance by invoking the get() method, the shared flag is set. To maintain the array's consistency with the returned String object, operations that can modify the array are subsequently prohibited. As a result, the replace() method designed to replace all elements of the array with an x cannot execute normally when the flag is set. Java's cloning feature provides a way to circumvent this constraint even though SensitiveClass does not implement the Cloneable interface.

This class can be exploited by a malicious class, shown in the following noncompliant code example, that subclasses the nonfinal SensitiveClass and provides a public clone() method:

class MaliciousSubclass extends SensitiveClass implements Cloneable {
  protected MaliciousSubclass(String filename

class Test extends MyPrivacy implements Cloneable{
    private int dummy;
    Test(int d) {
        dummy = dsuper(filename);
    }

   @Override public staticMaliciousSubclass void main(String[] argsclone() {
  //      Test t = new Test(0);Well-behaved clone() method
    MaliciousSubclass s   Object obj = null;
	try {
   try {
       s obj = t(MaliciousSubclass)super.clone();
       } }catch(Exception e) {
            System.out.println("not cloneable");
    }
    return s;
  }

  public static void main(String[] args) {
 if (obj != null)
MaliciousSubclass ms1 = new MaliciousSubclass("file.txt");
    MaliciousSubclass ms2 =  System.out.println(((MyPrivacy)obj).getKey());//steal key
    }
}

 Compliant Solution 1

Classes should be made non cloneable to prevent this from occuring. The following method maybe implemented for achieving this.

class MyPrivacy {
    private int key = 1234;ms1.clone(); // Creates a copy
    String s = ms1.get();  // Returns filename
    //System...
    public MyPrivacy() {}out.println(s); // Filename is "file.txt"
    public MyPrivacy(int k) {
ms2.replace();         this.key = k;
    }// Replaces all characters with 'x'
    protected// intBoth getKeyms1.get() {
        return this.key;and ms2.get() will subsequently return filename = 'xxxxxxxx'
    }

ms1.printFilename();    //...
 Filename becomes 'xxxxxxxx'
 public final void clonems2.printFilename() throws java.lang.CloneNotSupportedException{
;   // Filename becomes 'xxxxxxxx'
 throw new java.lang.CloneNotSupportedException();
    }
}

Compliant Solution 2

One can also make a class non subclassable. This can be achieved by finalizing a class.

 final class MyPrivacy {
 // Rest of the definition remains the same
 }

If, it is absolutely required to make the class cloneable, even then protective measures can be taken.

#1. If clone method is being over-riden, make it final

#2. If the class is reliant on a non-final clone method of one of the superclasses, then define the following

 }
}

The malicious class creates an instance ms1 and produces a second instance ms2 by cloning the first. It then obtains a new filename by invoking the get() method on the first instance. At this point, the shared flag is set to true. Because the second instance ms2 does not have its shared flag set to true, it is possible to alter the first instance ms1 using the replace() method. This approach obviates any security efforts and severely violates the class's invariants.

Compliant Solution (Final Class)

The easiest way to prevent malicious subclasses is to declare SensitiveClass to be final.

final class SensitiveClass {
  // ...
}

Compliant Solution (Final clone())

Sensitive classes should neither implement the Cloneable interface nor provide a copy constructor. Sensitive classes that extend from a superclass that implements Cloneable (and are cloneable as a result) must provide a clone() method that throws a CloneNotSupportedException. This exception must be caught and handled by the client code. A sensitive class that does not implement Cloneable must also follow this advice because it inherits the clone() method from Object. The class can prevent subclasses from being made cloneable by defining a final clone() method that always fails.

class SensitiveClass {
  // ...
  public final SensitiveClass clone() 
                              throws 

public final void clone() throws java.lang.CloneNotSupportedException {
    throw new    super.clone();
}
CloneNotSupportedException();
  }
}

This class fails to prevent malicious subclasses but does protect the data in SensitiveClass. Its methods are protected by being declared final. For more information on handling malicious subclasses, see OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code.

Risk Assessment

Failure to make sensitive classes noncopyable can permit violations of class invariants and provide malicious subclasses with the opportunity to exploit the code to create new instances of objects, even in the presence of the default security manager (in the absence of custom security checks).

Automated Detection

Related Guidelines

Bibliography

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