OBJ03-J. Sensitive classes must not let themselves be copied

Classes containing private, confidential, or otherwise sensitive data are best not copied. If a class is not 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 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 serious 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 OBJ05-J. Do not allow access to partially initialized objects.

Classes that are not sensitive, but that maintain other invariants must be sensitive to the possibility of malicious subclasses accessing or manipulating their data, and possibly invalidating their invariants. See OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely for more information.

Noncompliant Code Example

This noncompliant code example defines class SensitiveClass which contains a character array used to internally 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 {
  private char[] filename;
  private Boolean shared = false;
 
  SensitiveClass(String filename) {
    this.filename = filename.toCharArray();
  }

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

  final String get() {
    if (!shared) {	
      shared = true;
      return String.valueOf(filename);
    } else {
      throw new IllegalStateException("Failed to get instance");
    }
  }
  
  final void printFilename() {
    System.out.println(String.valueOf(filename));
  }
}

When a client requests a String instance by invoking the get() method, the shared flag is set. Operations that can modify the array are subsequently prohibited, to maintain the array's consistency with the returned String object. Consequently, 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 below) that subclasses the non-final 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 its own instance (ms1) and produces a second one (ms2), by cloning the first. It then obtains a new String filename object 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 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 as final.

final class SensitiveClass {
  // ...
}

Compliant Solution (final clone())

Sensitive classes should not implement the Cloneable interface, nor provide a copy constructor. Sensitive classes that extend from a superclass that implements Cloneable (and is consequently cloneable) 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 cloneable by defining a final clone() method that fails.

class SensitiveClass {
  // ...
  public final SensitiveClass clone() throws CloneNotSupportedException {
    throw new 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 how to handle malicious subclasses, see OBJ10-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely.

Risk Assessment

Failure to make sensitive classes non-copyable 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).

Related Vulnerabilities

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

Bibliography

[[Mcgraw 1998]]
[[MITRE 2009]] CWE ID 498 "Information Leak through Class Cloning", CWE ID 491 "Public cloneable() Method Without Final (aka 'Object Hijack')"
[[Wheeler 2003]] 10.6. Java

OBJ02-J. Never conflate immutability of a reference with that of the referenced object      04. Object Orientation (OBJ)      OBJ04-J. Do not use public static non-final variables