Twelve rules for developing more secure Java code

Writing security-conscious Java code can help you avoid security surprises

This article introduces 12 rules for writing security-critical Java code; 12 rules that all Java developers should abide by. If you are charged with managing a gaggle of Java developers, or if your business relies on the security of Java, make sure your developers follow these rules.

These rules haven't been sugar-coated for mass consumption. They get fairly technical and require broad knowledge of Java to understand. Though experienced Java developers will understand all the rules that follow, less experienced Java developers may have a bit of homework to do. Nevertheless, following these rules will make your Java code more secure.

We base these rules on the experience of many people who have generously discussed their experience in building secure Java code. We are particularly grateful to Andrew Appel, Dirk Balfanz, Drew Dean, and Dan Wallach, of the Secure Internet Programming Team at Princeton, for helping us understand these issues. Others who have contributed significantly to the behind-the-scenes thinking that went into these rules include David Hopwood, Li Gong, and Jim Roskind.

In creating these 12 rules, we've drawn from much experience in hunting down Java security bugs, and on advice and observations from people who write and review security-critical Java code for a living. Each rule is designed to eliminate an unexpected gotcha that you might face.

Of course, security is an elusive goal. Following these rules certainly won't provide any guarantee that your code is completely secure. It is easy to write insecure code that follows these rules. By following these goals, however, you will minimize or eliminate certain kinds of security attacks that you might not have thought of.

Think of these rules as a first step. If you are writing code that may be linked or run in conjunction with untrusted code, then you should definitely consider following these rules.

Every attempt has been made to keep the rules simple enough that you can treat them as a checklist to be followed in mechanical fashion. That way you can save your brainpower for other security issues.

Rule 1: Don't depend on initialization

Most Java developers think there is no way to allocate an object without running a constructor. But this isn't true: there are several ways to allocate noninitialized objects.

The easy way to protect yourself against this problem is to write your classes so that before any object does anything, it verifies that it has been initialized. You can do this as follows:

  • Make all variables private. If you want to allow outside code to access variables in an object, this should be done via get and set methods. (This keeps outside code from accessing noninitialized variables.) If you're following Rule 3, you'll make the get and set methods final.

  • Add a new private boolean variable, initialized, to each object.

  • Have each constructor set the initialized variable as its last action before returning.

  • Have each nonconstructor method verify that initialized is true before doing anything. (Note that you may have to make exceptions to this rule for methods called by your constructors. If you do this, it's best to make the constructors call only private methods.)

    If your class has a static initializer, you will need to do the same thing at the class level. Specifically, for any class that has a static initializer, follow these steps:

    • Make all static variables private. If you want to allow outside code to access static variables in the class, this should be done via static get and set methods. This keeps outside code from accessing noninitialized static variables. If you're following Rule 3, you'll make the get and set methods final.

    • Add a new private static boolean variable, classInitialized, to the class.

    • Have the static constructor set the initialized variable as its last action before returning.

    • Before doing anything, have each static method and each constructor verify that classInitialized is true. (Note: constructors are required to call a constructor of the superclass, or another constructor of the same class, as their first action. So you will have to do that before you check classInitialized.)

Rule 2: Limit access to your classes, methods, and variables

Every class, method, and variable that is not private provides a potential entry point for an attacker. By default, everything should be private. Make something nonprivate only with good reason, and document that reason.

Rule 3: Make everything final (unless there's a good reason not to)

If a class or method isn't final, an attacker could try to extend it in a dangerous and unforeseen way. By default, everything should be final. Make something nonfinal only if there is a good reason, and document that reason.

You might think you can prevent an attacker from extending your class or its methods by declaring the class nonpublic. But if a class isn't public, it must be accessible from within the same package, and as Rule 4 (below) says, you shouldn't to rely on package scope access restrictions for security.

This advice may seem harsh. After all, the rule is asking you to give up extensibility, which is one of the main benefits of using an object-oriented language like Java. But when you're trying to provide security, extensibility is your enemy: it just provides an attacker with more ways to cause trouble.

Rule 4: Don't depend on package scope

Classes, methods, and variables that aren't explicitly labeled as public, private, or protected are accessible within the same package. Don't rely on this for security. Java classes aren't closed, so an attacker could introduce a new class into your package and use this new class to access the things you thought you were hiding. (A few packages, such as java.lang, are closed by default, and a few Java virtual machines (JVMs) let you close your own packages. But you're better off assuming packages aren't closed.)

Package scope makes a lot of sense from a software-engineering standpoint, since it prevents innocent, accidental access to things you want to hide. But don't depend on it for security.

Maybe we'll get sealed classes in the future.

Rule 5: Don't use inner classes

Some Java language books say inner classes can be accessed only by the outer classes that enclose them. But this isn't true. Java bytecode has no concept of inner classes, so inner classes are translated by the compiler into ordinary classes that happen to be accessible to any code in the same package. And Rule 4 says not to depend on package scope for protection.

But wait, it gets worse. An inner class gets access to the fields of the enclosing outer class, even if the these fields are declared private. And the inner class is translated into a separate class. To let this separate class access the fields of the outer class, the compiler silently changes these fields from private to package scope! It's bad enough that the inner class is exposed; but it's even worse that the compiler is silently overruling your decision to make some fields private. Don't use inner classes if you can help it. (Ironically, the new JDK 1.2 PrivilegedAction API requires you to use an inner class to write privileged code. For more details, see our book Securing Java and the developer.com article referenced below.) That's one reason we don't like the PrivilegedAction API.)

Rule 6: Avoid signing your code

Code that isn't signed will run without any special privileges. And code with no special privileges is much less likely to do damage.

Of course, some of your code might have to acquire and use privileges to perform some dangerous operation. Work hard to minimize the amount of privileged code, and audit the privileged code more carefully than the rest.

Rule 7: If you must sign your code, put it all in one archive file

By following this rule, you will help prevent an attacker from carrying out a mix-and-match attack, in which the attacker constructs a new applet or library that links some of your signed classes together with malicious classes, or links together signed classes that you never meant to be used together. By signing a group of classes together, you make such attacks more difficult. Existing code-signing systems do an inadequate job of preventing mix-and-match attacks, so this rule cannot prevent such attacks completely. But using a single archive can't hurt.

Some code-signing systems let you examine other classes to see who signed them. If you're using a code-signing system that allows this, you can put code into the static constructors of your classes to verify that the "surrounding" classes have been signed by the expected person.

This measure doesn't completely prevent mix-and-match attacks, since an adversary can still mix together classes you signed at different times -- for example, by mixing version 1 of Class A with version 2 of Class B. If you're worried about this kind of inter-version mix-and-match attack, you can put each class's "version stamp" in a public final variable, and then have each class check the version stamps of its surrounding classes.

Rule 8: Make your classes noncloneable

Java's object cloning mechanism can allow an attacker to manufacture new instances of classes you define, without executing any of your constructors. If your class isn't cloneable, the attacker can define a subclass of your class, and make the subclass implement java.lang.Cloneable. This lets an attacker create new instances of your class. The new instances are made by copying the memory images of existing objects; though this is sometimes an acceptable way to make a new object, it often is not.

Rather than worry about this, you're better off making your objects noncloneable. You can do this by defining the following method in each of your classes:

public final void clone() throws java.lang.CloneNotSupportedException { throw new java.lang.CloneNotSupportedException(); } If you want your class to be cloneable, and you've considered the consequences of that choice, then you can still protect yourself. If you're defining a clone method yourself, make it final. If you're relying on a nonfinal clone method in one of your superclasses, then define this method: public final void clone() throws java.lang.CloneNotSupportedException { super.clone(); }

This prevents an attacker from redefining your clone method.

Rule 9: Make your classes nonserializeable

Serialization is dangerous because it allows adversaries to get their hands on the internal state of your objects. An adversary can serialize one of your objects into a byte array that can be read. This allows the adversary to inspect the full internal state of your object, including any fields you marked private, and including the internal state of any objects you reference.

To prevent this, you can make your object impossible to serialize. To achieve this goal, declare the writeObject method:

private final void writeObject(ObjectOutputStream out) throws java.io.IOException { throw new java.io.IOException("Object cannot be serialized"); }

This method is declared final so that a subclass defined by the adversary cannot override it.

Rule 10: Make your classes nondeserializeable

This rule is even more important than the previous one. Even if your class isn't serializeable, it may still be deserializeable. An adversary can create a sequence of bytes that happens to deserialize to an instance of your class. This is dangerous, since you do not have control over what state the deserialized object is in. You can think of deserialization as another kind of public constructor for your object; unfortunately it's a kind of constructor that is difficult for you to control.

You can prevent this kind of attack by making it impossible to deserialize a byte stream into an instance of your class. You can do this by declaring the readObject method:

private final void readObject(ObjectInputStream in) throws java.io.IOException { throw new java.io.IOException("Class cannot be deserialized"); }

As above, this method is declared final to prevent the adversary from overriding it.

Rule 11: Don't compare classes by name

Sometimes you want to compare the classes of two objects to see whether they are the same; or you want to see whether an object has a particular class. When you do this, be aware that there can be multiple classes with the same name in a JVM. It is a mistake to compare classes by name since different classes can have the same name. A better method is to compare class objects for equality directly. For example, given two objects, A and B, if you want to see whether they are the same class, use this code:

if(a.getClass() == b.getClass()){ // objects have the same class }else{ // objects have different classes } You should also be on the lookout for cases of less direct by-name comparisons. Suppose, for example, you want to see whether an object has the class "Foo." Here is the wrong way to do it: if(obj.getClass().getName().equals("Foo")) // Wrong! // objects class is named Foo }else{ // object's class has some other name } Here's a better way to do it: if(obj.getClass() == this.getClassLoader().loadClass("Foo")){ // object's class is equal to the class that this class calls "Foo" }else{ // object's class is not equal to the class that // this class calls "Foo" }

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