Page History

An object is partially initialized if a constructor has begun building the object but has not finished. As long as the object is not fully initialized, it must be hidden from other classes.

Other classes might access a partially initialized object from concurrently running threads. This rule is a specific instance of TSM01-J. Do not let the this reference escape during object construction but focuses only on single-threaded programs. Multithreaded programs must also comply with TSM03-J. Do not publish partially initialized objects.

Some uses of variables require failure atomicity. This requirement typically arises when a variable constitutes an aggregation of different objects, for example, a composition-and-forwarding-based approach, as described in OBJ02-J. Preserve dependencies in subclasses when changing superclasses. In the absence of failure atomicity, the object can be left in an inconsistent state as a result of partial initialization.

There are three common approaches to dealing with the problem of partially initialized objects:

• Exception in constructor. This approach throws an exception in the object's constructor. Unfortunately, an attacker can maliciously obtain the instance of such an object. For example, an attack that uses the finalizer construct allows an attacker to invoke arbitrary methods within the class, even if the class methods are protected by a security manager.

• Final field. Declaring the variable that is initialized to the object as final prevents the object from being partially initialized. The compiler produces a warning when there is a possibility that the variable's object might not be fully initialized. Declaring the variable final also guarantees initialization safety in multithreaded code. According to The Java Language Specification (JLS), §17.5, "final Field Semantics" [JLS 2015], "An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields." In other words, when a constructor executing in one thread initializes a final field to a known safe value, other threads are unable to see the preinitialized values of the object.

• Initialized flag. This approach allows uninitialized or partially initialized objects to exist in a known failed state; such objects are commonly known as zombie objects. This solution is error prone because any access to such a class must first check whether the object has been correctly initialized.

The following table summarizes these three approaches:

Noncompliant Code Example (Finalizer Attack)

This noncompliant code example, based on an example by Kabutz [Kabutz 2001], defines the constructor of the BankOperations class so that it performs social security number (SSN) verification using the method performSSNVerification(). The implementation of the performSSNVerification() method assumes that an attacker does not know the correct SSN and trivially returns false.

public class BankOperations {
  public BankOperations() {
    if (!performSSNVerification()) {
      throw new SecurityException("Access Denied!");
    }
  }

  private boolean performSSNVerification() {
    return false; // Returns true if data entered is valid, else false
                  // Assume that the attacker always enters an invalid SSN
  }

  public void greet() {
  

For a non-final class, if a constructor throws an exception before fully initializing the object, it becomes possible to maliciously obtain its instance. For example, an attack that uses the finalizer construct allows an attacker to invoke arbitrary methods within the class in spite of authorization measures.

Noncompliant Code Example

The constructor of BankOperations class performs the SSN validation using performSSNVerification(). Assume that an attacker does not know the correct SSN. As a result, this method trivially returns false in this example. A SecurityException is forcefully thrown. The UserApp class appropriately catches this exception and an access denied message is displayed. However, it is still possible for a malicious program to invoke methods of the partially initialized class BankOperations. This is illustrated in the code that follows this example.

public class BankOperations {
  public BankOperations() {
    if (!performSSNVerification()) {
       throw new SecurityException("Invalid SSN!"); 
    }    
  }
  
  private boolean performSSNVerification() {
    return false; // Returns true if data entered is valid, else false. Assume that the attacker just enters invalid SSN.
  }
  
  public void greet() {
    System.out.println("Welcome user! You may now use all the features.");
  }
}

public class UserApp {
  public static void main(String[] args) {
    BankOperations bo;
    try {
      bo = new BankOperations();
    } catch(SecurityException ex) { bo = null; }
   
    Storage.store(bo);
    System.out.println("Proceed with normal logic");
  }
}

public class Storage {
  private static BankOperations bop;

  public static void store(BankOperations bo) {
  // Only store if it is not initialized
    if (bop == null) {  
      if (bo == null) {   
        System.out.println("InvalidWelcome object!");
	System.exit(1);
      }
      bop = bo;
    }
  }
}

To exploit this code, an attacker extends the BankOperations class and overrides the finalize method. The gist of the attack is the capture of a handle of the partially initialized class. When the constructor throws an exception, the garbage collector waits to grab the object reference. However, by overriding the finalizer, a reference is obtained using the this keyword. Consequently, any method on the base class can be invoked maliciously. Note that, even a security manager check can be bypassed this way.

public class Interceptor extends BankOperations {
  private static Interceptor stealInstance = null;
  public static Interceptor get() {
    user! You may now use all the features.");
  }
}

public class Storage {
  private static BankOperations bop;

  public static void store(BankOperations bo) {
  // Store only if it is initialized
    if (bop == null) {
      if (bo == null) {
        System.out.println("Invalid object!");
        System.exit(1);
      }
      bop = bo;
    }
  }
}

public class UserApp {
  public static void main(String[] args) {
    BankOperations bo;
    try {
      bo = new InterceptorBankOperations();
    } catch (ExceptionSecurityException ex) { }bo //= Ignore the exceptionnull; }

    try {Storage.store(bo);
      synchronized(Interceptor.class) {
        while (stealInstance == null) {
          System.gc();
          Interceptor.class.wait(10);
        }
      }
    } catch(InterruptedException ex) { return null; }
    return stealInstance;
  }
  public void finalize() {
    synchronized(Interceptor.class) {
      stealInstance = this;
      Interceptor.class.notify();
    }
    System.out.println("Stolen the instance in finalize of " + this);
  }
}

public class AttackerApp {    // Invoke class and gain access to the restrictive features
  public static void main(String[] args) {
    Interceptor i = Interceptor.get();
    Storage.store(i);      // Can store the stolen object
    i.greet();	           // Now invoke any method of BankOperations class
    UserApp.main(args);    // Invoke the original UserApp
  }
}

This code is an exception to OBJ02-J. Avoid using finalizers

Compliant Solution

The compliant solution declares the partially-initialized class final so that it cannot be extended.

public final class BankOperations {
...

Noncompliant Code Example

The Javabeans pattern uses a no-argument constructor along with a series of parallel setter methods to build an object. This pattern is not thread-safe and can lead to inconsistent object state. Moreover, it permits another thread to access the object even though it may only be partially initialized (not all required fields are initialized).

class UnsafeCurrency {
  // total amount requested (required)
  private int dollars = -1; // initialize to default value
  private int cents = -1; // initialize to default value
  // change requested, denomination (optional)
  private int quarters = 0;
  private int dimes = 0;
  private int nickels = 0;
  private int pennies = 0;
  public UnsafeCurrency() {} // no argument constructor
  /* setter methods */
  public void setDollar(int amount) { dollars = amount;}
  public void setCents(int amount) { cents = amount;}
  public void setQuarters(int quantity) { quarters = quantity;}
  public void setDimes(int quantity) { dimes = quantity;}
  public void setNickels(int quantity) { nickels = quantity;}
  public void setPennies(int quantity) { pennies = quantity;}
}

Compliant Solution

Use the Builder pattern's \[[Gamma 95|AA. Java References#Gamma 95]\] variant suggested by \[[Bloch 08|AA. Java References#Bloch 08]\] to ensure thread safety and atomicity of object creation. The idea is to call the constructor with the _required_ parameters and obtain a _builder_ object. Each _optional_ parameter can be set using setters on the builder. The object construction concludes with the invocation of the {{build()}} method. The class {{Currency}} also becomes immutable as a result. 

class Currency {
  // total amount requested (required)
  private final int dollars;
  private final int cents;
  // change requested, denomination (optional)
  private final int quarters;
  private final int dimes;
  private final int nickels;
  private final int pennies;
  
  /* Static class member */
  private Currency(Builder builder) {
    dollars = builder.dollars;
    cents = builder.cents;
   
    quarters = builder.quarters;
    dimes = builder.dimes;
    nickels = builder.nickels;
    pennies = builder.pennies;
  }
  public static class Builder {
    private final int dollars;
    private final int cents;
    private int quarters = 0;
    private int dimes = 0;
    private int nickels = 0;
    private int pennies = 0;
	 
    public Builder(int dollars, int cents) {
      this.dollars = dollars;
      this.cents = cents;
    }
    public Builder quarters(int quantity) {
      quarters = quantity;  return this; 
    }
    public Builder dimes(int quantity) {
      dimes = quantity; return this;	 
    }
    public Builder nickles(int quantity) {
      nickels = quantity; return this;	 
    }
    public Builder pennies(int quantity) {
      pennies = quantity; return this;	 
    }
    public Currency build() {
      return new Currency(this); 	 
    }
 }
}

Client code:
Currency USD = new Currency.Builder(100,56).quarters(2).dimes(5).pennies(1).build();

If input has to be validated, make sure that the values are copied from the builder class to the containing class's fields prior to checking. The builder class does not violate SCP02-J. Do not expose sensitive private members of the outer class from within a nested class since it maintains a copy of the variables defined in the scope of the containing class. These take precedence and as a result do not break encapsulation.

Exceptions

EX1: When the API cannot be extended (consider a non-final class, for example), it is permissible to use a flag to signal that object construction succeeded. This is shown below.

class BankOperations {
  public volatile boolean initialized = false;
  public BankOperations() {
    if (!performSSNVerification()) {
       throw new SecurityException("Invalid SSN!"); 
    }  
    else
      initialized = true; // object construction succeeded	  
  }
  
  private boolean performSSNVerification() {
    return false;
  }
  
  public void greet() {
    if(initialized == true) {
      System.out.println("Welcome user! You may now use all the features.");
      // ...
    }
    else
      System.out.println("You are not permitted!");
  }
}

EX2: It is permissible to use the telescoping pattern when the overhead of the builder pattern is significant as compared to the number of parameters required to be initialized. This pattern prescribes a constructor to initialize the required parameters and individual constructors for each optional parameter that is added.

Risk Assessment

Allowing a partially initialized object to be accessed can provide an attacker with an opportunity to exploit the object.

Automated Detection

TODO

Related Vulnerabilities

System.out.println("Proceed with normal logic");
  }
}

The constructor throws a SecurityException when SSN verification fails. The UserApp class appropriately catches this exception and displays an "Access Denied" message. However, these precautions fail to prevent a malicious program from invoking methods of the partially initialized class BankOperations, as shown by the following exploit code.

The goal of the attack is to capture a reference to the partially initialized object of the BankOperations class. If a malicious subclass catches the SecurityException thrown by the BankOperations constructor, it is unable to further exploit the vulnerable code because the new object instance has gone out of scope. Instead, an attacker can exploit this code by extending the BankOperations class and overriding the finalize() method. This attack intentionally violates MET12-J. Do not use finalizers.

When the constructor throws an exception, the garbage collector waits to grab the object reference. However, the object cannot be garbage-collected until after the finalizer completes its execution. The attacker's finalizer obtains and stores a reference by using the this keyword. Consequently, the attacker can maliciously invoke any instance method on the base class by using the stolen instance reference. This attack can even bypass a check by a security manager.

public class Interceptor extends BankOperations {
  private static Interceptor stealInstance = null;

  public static Interceptor get() {
    try {
      new Interceptor();
    } catch (Exception ex) {/* Ignore exception */}
    try {
      synchronized (Interceptor.class) {
        while (stealInstance == null) {
          System.gc();
          Interceptor.class.wait(10);
        }
      }
    } catch (InterruptedException ex) { return null; }
    return stealInstance;
  }

  public void finalize() {
    synchronized (Interceptor.class) {
      stealInstance = this;
      Interceptor.class.notify();
    }
    System.out.println("Stole the instance in finalize of " + this);
  }
}

public class AttackerApp { // Invoke class and gain access
                           // to the restrictive features
  public static void main(String[] args) {
    Interceptor i = Interceptor.get(); // Stolen instance

    // Can store the stolen object even though this should have printed
    // "Invalid Object!"
    Storage.store(i);

    // Now invoke any instance method of BankOperations class
    i.greet();

    UserApp.main(args); // Invoke the original UserApp
  }
}

Compliance with ERR00-J. Do not suppress or ignore checked exceptions and ERR03-J. Restore prior object state on method failure can help to ensure that fields are appropriately initialized in catch blocks. A developer who explicitly initializes the variable to null is more likely to document this behavior so that other programmers or clients include the appropriate null reference checks where required. Moreover, this approach guarantees initialization safety in a multithreaded scenario.

Compliant Solution (Final)

This compliant solution declares the partially initialized class final so that it cannot be extended:

public final class BankOperations {
  // ...
}

Compliant Solution (Final finalize())

If the class itself cannot be declared final, it can still thwart the finalizer attack by declaring its own finalize() method and making it final:

public class BankOperations {
  public final void finalize() {
    // Do nothing
  }
}

This solution is allowed under exception MET12-J-EX1, which permits a class to use an empty final finalizer to prevent a finalizer attack.

Compliant Solution (Java SE 6, Public and Private Constructors)

This compliant solution applies to Java SE 6 and later versions in which the JVM will not execute an object's finalizer if the object's constructor throws an exception before the java.lang.Object constructor exits [SCG 2009]. Developers can use this feature to throw an exception in a constructor without risking the escape of a partially initialized object (via the finalizer attack described previously). However, doing so requires a careful coding of the constructor because Java ensures that the java.lang.Object constructor executes on or before the first statement of any constructor. If the first statement in a constructor is a call to either a superclass's constructor or another constructor in the same class, then the java.lang.Object constructor will be executed somewhere in that call. Otherwise, Java will execute the default constructor of the superclass before any of the constructor's code and the java.lang.Object constructor will be executed through that (implicit) invocation.

Consequently, to execute potentially exception-raising checks before the java.lang.Object constructor exits, the programmer must place them in an argument expression of an explicit constructor invocation. For example, the single constructor can be split into three parts: a public constructor whose interface remains unchanged, a private constructor that takes (at least) one argument and performs the actual work of the original constructor, and a method that performs the checks. The public constructor invokes the private constructor on its first line while invoking the method as an argument expression. All code in the expression will be executed before the private constructor, ensuring that any exceptions will be raised before the java.lang.Object constructor is invoked.

This compliant solution demonstrates the design. Note that the performSSNVerification() method is modified to throw an exception rather than returning false if the security check fails.

public class BankOperations {
  public BankOperations() {
    this(performSSNVerification());
  }

  private BankOperations(boolean secure) {
    // secure is always true
    // Constructor without any security checks
  }

  private static boolean performSSNVerification() {
    // Returns true if data entered is valid, else throws a SecurityException
    // Assume that the attacker just enters invalid SSN, so this method always throws the exception
    throw new SecurityException("Invalid SSN!");
  }

  // ...remainder of BankOperations class definition
}

Compliant Solution (Initialized Flag)

Rather than throwing an exception, this compliant solution uses an initialized flag to indicate whether an object was successfully constructed. The flag is initialized to false and set to true when the constructor finishes successfully.

class BankOperations {
  private volatile boolean initialized = false;

  public BankOperations() {
    if (!performSSNVerification()) {
      return;                // object construction failed
    }

    this.initialized = true; // Object construction successful
  }

  private boolean performSSNVerification() {
    return false;
  }

  public void greet() {
    if (!this.initialized) {
      throw new SecurityException("Invalid SSN!");
    }

    System.out.println(
        "Welcome user! You may now use all the features.");
  }
}

The initialized flag prevents any attempt to access the object's methods if the object is not fully constructed. Because each method must check the initialized flag to detect a partially constructed object, this solution imposes a speed penalty on the program. It is also harder to maintain because it is easy for a maintainer to add a method that fails to check the initialized flag.

According to Charlie Lai [Lai 2008]:

If an object is only partially initialized, its internal fields likely contain safe default values such as null. Even in an untrusted environment, such an object is unlikely to be useful to an attacker. If the developer deems the partially initialized object state secure, then the developer doesn't have to pollute the class with the flag. The flag is necessary only when such a state isn't secure or when accessible methods in the class perform sensitive operations without referencing any internal field.

The initialized flag is volatile to ensure that the setting of the flag to true happens-before any reads of the variable. The current code does not allow for multiple threads to read the field before the constructor terminates, but this object could always be subclassed and run in an environment where multiple threads can access the variable.

Noncompliant Code Example (Static Variable)

This noncompliant code example uses a nonfinal static variable. The JLS does not mandate complete initialization and safe publication even though a static initializer has been used. Note that in the event of an exception during initialization, the variable can be incorrectly initialized.

class Trade {
  private static Stock s;
  static {
    try {
      s = new Stock();
    } catch (IOException e) {
      /* Does not initialize s to a safe state */
    }
  }
  // ...
}

Compliant Solution (Final Static Variable)

This compliant solution guarantees safe publication by declaring the Stock field final:

private static final Stock s;

Unlike the previous compliant solution, however, this approach permits a possibly null value but guarantees that a non-null value refers to a completely initialized object.

Risk Assessment

Allowing access to a partially initialized object can provide an attacker with an opportunity to resurrect the object before or during its finalization; as a result, the attacker can bypass security checks.

Automated Detection

Automated detection for this rule is infeasible in the general case. Some instances of nonfinal classes whose constructors can throw exceptions could be straightforward to diagnose.

Related Vulnerabilities

CVE-2008-5353 describes a collection Vulnerability CVE-2008-5339 concerns a series of vulnerabilities in Java. In one of the vulnerabilities, an applet causes an object to be deserialized using ObjectInputStream.readObject(), but the input is controlled by an attacker. The object actually read is a serializable subclass of ClassLoader, and it has a readObject() method that stashes the object instance into a static variable; consequently, the object survives the serialization. As a result, the applet has managed manages to construct a ClassLoader object , by-passing the restrictions against doing so in an applet, and that ClassLoader allows it to construct classes that are not subject to the security restrictions of an applet. The vulnerability is described in depth in SER37-J. Do not deserialize from a privileged context.

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

References

\[[JLS 05|AA. Java References#JLS 05]\] Section 12.6, Finalization of Class Instances
\[[API 06|AA. Java References#API 06]\] [finalize()|http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#finalize()]
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 4-2 Defend against partially initialized instances of non-final classes
\[[Kabutz 01|AA. Java References#Kabutz 01]\] Issue 032 - Exceptional Constructors - Resurrecting the dead
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 7, Avoid finalizers
\[[Darwin 04|AA. Java References#Darwin 04]\] Section 9.5, The Finalize Method
\[[Flanagan 05|AA. Java References#Flanagan 05]\] Section 3.3, Destroying and Finalizing Objects

by passing the restrictions against this in an applet, and the ClassLoader allows it to construct classes that are not subject to the security restrictions of an applet. This vulnerability is described in depth in SER08-J. Minimize privileges before deserializing from a privileged context.

Related Guidelines

Bibliography

...

Image Added Image Added Image AddedOBJ31-J. Do not use public static non-final variables      07. Object Orientation (OBJ)      OBJ33-J. Limit the extensibility of classes and methods