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("Invalid 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() {
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();
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]] variant suggested by [[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.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
OBJ32- J |
medium |
probable |
medium |
P8 |
L2 |
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 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]] 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
OBJ31-J. Do not use public static non-final variables 07. Object Orientation (OBJ) OBJ33-J. Limit the extensibility of classes and methods to only trusted subclasses