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 despite authorization measures.
Noncompliant Code Example
This noncompliant code example, based on an example by Kabutz [[Kabutz 01]], defines the constructor of BankOperations class so that it performs SSN verification using the method performSSNVerification(). Assume that an attacker does not know the correct SSN. As a result, this method trivially returns false in this example.
When the SSN verification fails, a SecurityException is thrown. The UserApp class appropriately catches this exception and an access denied message is displayed. However, this does not prevent a malicious program from invoking methods of the partially initialized class BankOperations. This is illustrated in the code that follows this noncompliant code example 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("Invalid object!");
System.exit(1);
}
bop = bo;
}
}
}
Note that even if a malicious subclass catches the SecurityException that the BankOperations constructor throws, it cannot obtain its instance to cause further harm. 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 base 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 instance method on the base class can be invoked maliciously by using the stolen this instance. 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 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
}
}
The attacker's code violates the guideline OBJ08-J. Avoid using finalizers.
Compliant Solution
This compliant solution declares the partially-initialized class final so that it cannot be extended.
public final class BankOperations {
// ...
}
Compliant Solution
This compliant solution prevents a hostile caller from using a partially initialized instance of the class. In the case of the noncompliant code example, the BankOperations class's superclass's constructor is called implicitly from the BankOperations constructor, just before the check. This exposes the partially initialized object to the finalizer attack. In this compliant solution, the check is carried out before the superclass's constructor executes. This forbids hostile code from obtaining a partially initialized instance.
public class {
public BankOperations() {
this(performSSNVerification());
}
private BankOperations(boolean performSSNVerification) {
// ...
}
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!");
}
public void greet() {
System.out.println("Welcome user! You may now use all the features.");
}
}
Compliant Solution
When the API can be extended (consider a non-final class, for example), it is permissible to use a flag to signal the successful completion of object construction. This is shown below.
class BankOperations {
public volatile boolean initialized = false; // volatile flag
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!");
}
}
}
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]] variant suggested by Bloch [[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. This also has the effect of making the class Currency immutable.
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;
}
// Static class member
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).nickles(1).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 SCP03-J. Do not expose sensitive private members of the outer class from within a nested class because 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: 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 resurrect the object before its finalization and bypass any security checks.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
OBJ04- J |
high |
probable |
medium |
P12 |
L1 |
Automated Detection
TODO
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 SER09-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]] Section 12.6, Finalization of Class Instances
[[API 06]] finalize()
[[SCG 07]] Guideline 4-2 Defend against partially initialized instances of non-final classes
[[Kabutz 01]] Issue 032 - Exceptional Constructors - Resurrecting the dead
[[Bloch 08]] Item 7, Avoid finalizers
[[Darwin 04]] Section 9.5, The Finalize Method
[[Flanagan 05]] Section 3.3, Destroying and Finalizing Objects
OBJ03-J. Do not use public static non-final variables 08. Object Orientation (OBJ) OBJ05-J. Limit the extensibility of non-final classes and methods to only trusted subclasses