OBJ04-J. Do not allow access to partially initialized objects

An attacker can maliciously obtain the instance of an object when a constructor for a non-final class throws an exception before it completes the initialization of the new 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.

Noncompliant Code Example

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

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 always enters an 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");
  }
}

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.

public class Storage {
  private static BankOperations bop;

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

The goal of the attack is to capture a reference to the partially initialized object of the BankOperation 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.

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("Stolen the instance in finalize of " + this);
  }
}

The attacker's code intentionally violates the guideline [MET18-J. Avoid using finalizers] to exploit the vulnerable code.

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 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
  }
}

Compliant Solution (final)

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

public final class BankOperations {
  // ...
}

Compliant Solution (Java SE 6, public and private constructors)

This compliant solution applies to Java SE 6 and later versions, where a finalizer is prevented from being executed when an exception is thrown before the java.lang.Object constructor exits [[SCG 2009]].

In the public constructor, the method call performSSNVerification() is passed as an argument to a private constructor. Also, the performSSNVerification() method actually throws an exception rather than returning false should the security check fail.

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
}

The first statement in any constructor must be a call to either a super constructor, or to another constructor in the same class. If a constructor call was not provided in the public constructor, the default constructor of the superclass executes. Unfortunately, this might allow a finalizer to be added and executed if the superclass constructor exited before the security check.

Compliant Solution (zombie flag)

This compliant solution uses a zombie flag to indicate if an object was successfully constructed. The flag is initiailzed to true and set to false when the constructor completes successfully.

class BankOperations {
  private volatile boolean zombie = true;

  public BankOperations() {
    if (!performSSNVerification()) {
      throw new SecurityException("Invalid SSN!"); 
    }  

    this.zombie = false; // object construction succeeded	  
  }
  
  private boolean performSSNVerification() {
    return false;
  }
  
  public void greet() {
    if (this.zombie) {
      throw new SecurityException("Invalid SSN!"); 
    }

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

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

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; consequently, the attacker can bypass any security checks.

Automated Detection

Automated detection for this guideline appears infeasible in the general case. Some instances of non-final classes whose constructors can throw exceptions may be straightforward to diagnose.

Related Vulnerabilities

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 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 guideline [SER09-J. Do not deserialize from a privileged context].

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

Bibliography

[[API 2006]] finalize()
[[Darwin 2004]] Section 9.5, The Finalize Method
[[Flanagan 2005]] Section 3.3, Destroying and Finalizing Objects
[[JLS 2005]] Section 12.6, Finalization of Class Instances
[[Kabutz 2001]] Issue 032 - Exceptional Constructors - Resurrecting the dead
[[Lai 2008]]
[[SCG 2007]] Guideline 4-2 Defend against partially initialized instances of non-final classes
[[SCG 2009]] Guideline 1-2 Limit the extensibility of classes and methods

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