For a non-final class, if a constructor throws an exception before fully initializing the class, 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 despite of all authorization measures.
Non-Compliant Code Example
The constructor of BankOperations class performs the SSN validation using performSSNVerification(). Assume that an attacker does not know the correct SSN, thus this method trivially returns false in this example. A SecurityException is forcefully thrown as a result. The UserApp class appropriately catches this 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 static 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; }
if(bo != null)
bo.greet();
else
System.out.println("Sorry, you were denied access to the features.");
}
}
To exploit this code, an attacker extends the BankOperations class and overrides the finalizer 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() {
try {
new Interceptor();
} catch(Exception ex) { } // ignore the 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);
}
}
public class AttackerApp { //invoke this class and gain access to the restrictive features
public static void main(String[] args) {
Interceptor i = Interceptor.get();
i.greet(); //now invoke any method of BankOperations class
UserApp.main(args); //Invoke the original UserApp
}
}
Compliant Solution
One compliant solution is to use an initialization flag that is explicitly set after the constructor has finished initializing the class. This flag should be tested before code is executed in any method. This (rather) imposing solution is recommended for security sensitive applications. For others, it is reasonable to initialize the default values of class variables to something like null. \Check this
public class BankOperations {
public BankOperations() {
if (!performSSNVerification()) {
throw new SecurityException("Invalid SSN!");
}
else
initialized = true;
}
private boolean performSSNVerification() {
return false;
}
public static void greet() {
if(initialized == true) {
System.out.println("Welcome user! You may now use all the features.");
//other authorized code
}
else
System.out.println("You are not permitted!");
}
}
References
Sun Secure Coding http://java.sun.com/security/seccodeguide.html
The Java Specialists' Newsletter Issue 032 - Exceptional Constructors - Resurrecting the dead Author: Dr. Heinz M. Kabutz
JLS, 12.6 Finalization of Class Instances
Effective Java: Programming Language Guide, Item 6, Avoid finalizers
Java Cookbook, Ian Darwin, 9.5 The Finalize Method
Java in a nutshell, 3.3. Destroying and Finalizing Objects
Java API Documentation http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#finalize()