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Java's object cloning mechanism allows an attacker to manufacture new instances of a class by copying the memory images of existing objects rather than by executing the class's constructor. Often, this is an unacceptable way of 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.

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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.

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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) {
    super(filename);
  }

  @Override public MaliciousSubclass clone() {  // Well-behaved clone() method
    MaliciousSubclass s = null;
    try {
      s = (MaliciousSubclass)super.clone();
    } catch(Exception e) {
      System.out.println("not cloneable");
    }
    return s;
  }

  public static void main(String[] args) {
    MaliciousSubclass ms1 = new MaliciousSubclass("file.txt");
    MaliciousSubclass ms2 = ms1.clone(); // Creates a copy
    String s = ms1.get();  // Returns filename
    System.out.println(s); // Filename is "file.txt"
    ms2.replace();         // Replaces all characters with 'x'
    // Both ms1.get() and ms2.get() will subsequently return filename = 'xxxxxxxx'
    ms1.printFilename();   // Filename becomes 'xxxxxxxx'
    ms2.printFilename();   // Filename becomes 'xxxxxxxx'
  }
}

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 {
  // ...
}

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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.

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Bibliography

Related Guidelines

Bibliography

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