Jackson-databind and Default Typing Vulnerabilities

Today, GitHub sent out security notices to owners of projects using old jackson-databind versions (older than 2.8.11.1 and 2.9.5). These notices pertain to this issue. I have talked about its relevance before on IRC, but since it is getting more attention now, I will describe it here again.

The Problem

The “bug” comes from using the so-called default typing. This feature allows a user to deserialize subclasses (or even Object) without specifying the full possible type hierarchy. Consider this model:

interface I {
}
@Data
class A implements I {
    private int i;
}
@Data
class B implements I {
    private boolean b;
}

Now, if we wish to serialize the interface I, you usually need to specify some sort of type info. This is typically done through an annotation on the interface:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
@JsonSubTypes({@JsonSubTypes.Type(value = A.class, name = "A"), @JsonSubTypes.Type(value = B.class, name = "B")})
interface I {
}

If we now serialize an object of type I, we get a result like this:

new ObjectMapper().writerFor(I.class).writeValueAsString(new A())
{"@type":"A","i":0}

Deserializing this JSON works as expected.

Default typing

This annotation-based registration is fine for small use cases, but can get cumbersome if the types are in different modules, or there are just a lot of them, or it would be bad style to reference them from the parent class. Normal Java serialization does not have this problem (it just carries the actual dynamic class name with it), so this could be a barrier for adoption of Jackson for previous Java serialization users.

The problem here is that we want people to migrate from Java serialization. There are lots of reasons, most of them compelling.

Enter default typing. With default typing, we don’t need the type info annotations at all:

new ObjectMapper().enableDefaultTyping().writerFor(I.class).writeValueAsString(new A())
["net.hawo.tv.tvui17.A",{"i":0}]

(Ignore the fact that this is now suddenly an array – this is one of the ways jackson may include type info)
The idea is simple: Include the full class name in the json, and you can simply get the proper class at runtime! Sounds good, right?

The Vulnerability

Well, turns out this is not that good of an idea. Java serialization does a very similar thing (though it still requires the named class to be serializable, which jackson doesn’t) and this has lead to what feels like a third of all serious security vulnerabilities in Java applications, ever. The problem lies with the fact that Java classpaths are often massive, and allowing any class on that classpath to be deserialized at will can be disastrous since it exposes a huge attack surface. If you can get any class on the classpath to execute code when deserialized with jackson, you have successfully achieved remote code execution.
This is exactly what happened with Jackson. Some classes that were common on user classpaths could be deserialized to execute arbitrary code. The fix for this issue is basically a blacklist of a few of these classes that could be exploited. Blacklists are not a solution, though, and since this first list, the list has been amended several times. The maintainers are playing whack-a-mole here, and in my opinion it is a waste of everyone’s time to be adding all exploitable classes to this list.

The Solution

Our experience with this same issue in the Java serialization world tells us not to deserialize untrusted data. Luckily, Jackson is much more secure than Java serialization – if you don’t use default typing. The only acceptable solution to this issue in the long run is: do not use default typing to deserialize untrusted data. Default typing is rarely necessary or even a good idea.
Unfortunately, online resources saying this are sparse. Default typing is an “easy” solution, and many people simply do not have the security awareness to see the issue with it – they will stumble over stackoverflow answers such as this one and simply enable default typing to easily serialize Object fields. The documentation of Jackson also doesn’t highlight this as much as it should.
Two alternatives to default typing exist in Jackson:

  • Normal, annotation-based typing as shown above. This allows you either to use the full name as with default typing, or even specify your own name for greater compatibility (you can later change the class name without affecting the serialized representation). This is the “standard” solution, and the appropriate one for most use cases.
  • Should you not know the possible subtypes of a class you wish to deserialize in advance, you can use the rich registerSubtypes API to dynamically add the types you desire. These types could be detected through an existing module system you are already using, or using something like SPI.

All in all, I am a bit dissatisfied with the attention this issue has gotten. The issue is a security vulnerability by design, and anyone using default typing should have been aware of it. Luckily, default typing is not on by default. I do not have statistics but I would be surprised if many people used it or knew of its existence in the first place, and so I find the attention GitHub has given this a bit over the top – the biggest thing these notifications will spread is uncertainty about Jackson, so this article was an attempt at clearing up what it’s actually all about.

Immutability in Java

Recently in ##java, we had a discussion on what constitutes “immutability” and how to implement it in java properly. Immutability is a core concept for functional programming, and as mainstream languages and ecosystems “catch up” with the safer, less error-prone ideas from functional programming, immutability is wanted in Java projects as well.

What is immutability?

Asking programmers for their definition of immutability, you will usually receive an answer such as this:

An object is immutable if its state never changes.

In this article we will look at different interpretations of that statement and see how to implement it in java.

Unmodifiable vs. Immutable

An unmodifiable object is an object that cannot be mutated. This does not mean it cannot change through other means:

List<String> inner = new ArrayList<>();
inner.add("foo");
List<String> unmodifiable = Collections.unmodifiableList(inner);
System.out.println(unmodifiable); // [foo]
inner.add("bar");
System.out.println(unmodifiable); // [foo, bar]

Unmodifiable objects are generally not considered “truly” immutable. An immutable version of the code above (using Guava‘s ImmutableList) would be:

List<String> inner = new ArrayList<>();
inner.add("foo");
List<String> immutable = ImmutableList.copyOf(inner);
System.out.println(immutable); // [foo]
inner.add("bar");
System.out.println(immutable); // [foo]

Polymorphism

Designing an immutable class to be extended is tricky business. The base class must either relax its immutability or make basically all of its methods final which makes polymorphism basically useless. Relaxing immutability guarantees will often lead to only an unmodifiable class:

public class Vec2 {
    private final int x;
    private final int y;
    public Vec2(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() { return x; }
    public int getY() { return y; }
    @Override public int hashCode() { return getX() * 31 + getY(); }
    @Override public boolean equals(Object o) { ... }
}

This class appears immutable at first glance, but is it really?

public class MutableVec2 extends Vec2 {
    // redundant fields because super fields are private and final
    private int x;
    private int y;
    public MutableVec2(int x, int y) {
        super(x, y);
        this.x = x;
        this.y = y;
    }
    @Override public int getX() { return x; }
    @Override public int getY() { return y; }
    public void setX(int x) { this.x = x; }
    public void setY(int y) { this.y = y; }
}

We’ve suddenly introduced mutability, and not just that, we also override the two getters, so users of the original “immutable” Vec2 class cannot rely on instances of Vec2 never changing state when using them. This makes Vec2 effectively unmodifiable only, not immutable. Immutable classes should be final.
Limited polymorphism is possible when necessary, for example through a package-private constructor so immutability can be strictly controlled. This is error-prone and should be reserved for exceptional cases.

Defensive copies

Immutability itself does not require immutability on class members (in most definitions). However, in many cases where immutability is used, making sure that members also stay unmodified is desired. This can be done through “defensive copies”:

public class ImmutableClass {
    private final List<String> list;
    public ImmutableClass(List<String> list) {
        this.list = Collections.unmodifiableList(new ArrayList<>(list));
        // when using guava:
        //this.list = ImmutableList.copyOf(list);
    }
    // getter, equals, hashCode
}

Internal state

Internal mutable state is allowed in immutable classes. One use case of this is caching. In OpenJDKs java.lang.String class, which is immutable, the hashCode is cached using this method:

public class String {
    ...
    private int hash; // defaults to 0. not explicitly initialized to avoid race condition (see section below)
    @Override
    public int hashCode() {
        int h = hash;
        if (h == 0) {
            hash = h = computeHashCode();
        }
        return h;
    }
}

(This isn’t the actual implementation but it’s the same principle. Want to see this as implemented in an actual Java runtime library? Here it is.)
To users of this class, the internal state is never visible – they do not see a change in behavior. Because of this, the class still counts as immutable.

Concurrency

Java concurrency is a fickle beast, and immutability is an important tool to avoid the shared mutable state that makes concurrency so difficult. However, immutability has to be implemented carefully to work reliably in concurrent applications.
On top of the basic “state never changes” requirement we must introduce two more requirements: (from Java Concurrency In Practice. Read it, it’s a great book.)

  • All fields must be final.
  • The this reference must never escape the constructor. This is called “proper construction”.

To explore why these requirements are necessary, we will take an example straight from the Java Language Specification:

final class MaybeImmutable {
    private int x;
    public MaybeImmutable(int x) {
        this.x = x;
    }
    public int getX() { return x; }
    // the two threads
    static MaybeImmutable sharedField; // this field isn't volatile
    static void thread1() {
        sharedField = new MaybeImmutable(5);
    }
    static void thread2() {
        MaybeImmutable tmp = sharedField;
        if (tmp != null) {
            System.out.println(tmp.getX()); // could be 5... but could also be 0!
        }
    }
}

At first glance the MaybeImmutable class appears immutable – it does not provide any API to modify its state. The problem with this class is that the field x is not final. Let’s take a look at what both threads do:

// thread1
tmp = allocate MaybeImmutable
tmp.x = 5
write sharedField = tmp
// thread2
read tmp = sharedField
check tmp != null
print tmp.x

In this case, the constructor call is “just” setting the field. If you’re already familiar with the java memory model, you will notice that there is no happens-before ordering between those two threads! This makes the following execution order legal:

// thread1
tmp = allocate MaybeImmutable
write sharedField = tmp
tmp.x = 5
// thread2
// same as above

In that case, thread2 could legally print out “0”.
(Reordering is only one reason why this could happen – caches are another, for example.)
This fact makes our MaybeImmutable class not immutable: From thread2, the state of the same exact instance of MaybeImmutable can change over time. How can we fix this? By making the field x final. Looking at the JLS, this guarantees that when we can see the MyImmutable instance in the sharedField, we can also see the correctly initialized x field. thread2 will always print out 5, as we’d expect.
This visibility guarantee is only given if our class does not leak its this reference before construction is complete, which brings us to our second requirement (“proper construction”):

final class NotImmutable {
    public static NotImmutable instance;
    private final int x;
    public NotImmutable(int x) {
        this.x = x;
        instance = this;
    }
    public int getX() { return x; }
    static void thread1() {
        new NotImmutable(5);
    }
    static void thread2() {
        NotImmutable tmp = instance;
        if (tmp != null) {
            System.out.println(tmp.getX()); // could still be 0!
        }
    }
}

Even though the x field is final in this case, thread2 can see an instance of NotImmutable before its constructor completes, meaning it can still see 0 as the value of x.
Interestingly, setting the x field to volatile in doesn’t help in either example (NotImmutable or MaybeImmutable). The guarantees of final fields are actually different than those of volatile fields here. This is covered in Aleksey ShipilĂ«v’s article “Close Encounters of The Java Memory Model Kind.” This article, together with “Java Memory Model Pragmatics,” is a great resource to learn more about the Java memory model.

Builders

Immutable classes, especially those with many fields, can be difficult to construct – the Java language lacks named parameters. This can be made somewhat easier through the builder pattern, which the following example demonstrates (though only with a single field):

public class ImmutableClass {
    private final String url;
    private ImmutableClass(String url) {
        this.url = url;
    }
    public String getX() {
        return url;
    }
    public static class Builder {
        private String url;
        public Builder url(String url) {
            this.url = url;
            return this;
        }
        public ImmutableClass build() {
            return new ImmutableClass(url);
        }
    }
}
ImmutableClass myInstance = new ImmutableClass.Builder().url("https://yawk.at/").build();

This pattern provides an easy way of constructing very large immutable classes – builders are infinitely more readable than constructor calls such as new ImmutableClass("GET", 6, true, true, false, 5, null). Most IDEs also include utilities to quickly generate such builders.

Lombok

The wonderful project lombok provides a very easy way of writing immutable classes in a few lines of code via the @Value annotation:

@Value
public class ImmutableClass {
    private final String url;
}

This makes the fields private final (if they aren’t already), generates getters and a constructor, and makes the class final. As an added bonus it also generates an equals and hashCode for you. On top of that, you can use the @Builder annotation to also let it generate a builder for you, and the experimental @Wither annotation to generate copy mutators.
To see what kind of code Lombok can generate you can either view the examples on their website or use my javap pastebin to see decompiled lombok-generated code (make sure to select “Procyon” on the bottom-right).

Serialization

Immutable classes do not follow the traditional Java bean layout: They do not have a zero-argument constructor and they do not have setters. This makes serialization using reflection difficult. There are solutions, though – an immutable class constructor can be annotated with serialization-framework-specific annotations to give the framework information on which parameter corresponds to which object property.
We will look at the jackson-databind annotations first:

public final class Vec2 {
    private final int x;
    private final int y;
    @JsonCreator
    public Vec2(@JsonProperty("x") int x, @JsonProperty("y") int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() { return x; }
    public int getY() { return y; }
}

A more framework-agnostic solution exists using the java.beans.ConstructorProperties annotation (not present in android) which is supported by Jackson since version 2.7.0:

public final class Vec2 {
    private final int x;
    private final int y;
    @ConstructorProperties({ "x", "y" })
    public Vec2(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() { return x; }
    public int getY() { return y; }
}

This annotation is also generated on Lombok constructors by default making serializing Lombok-generated immutable classes hassle-free with jackson 2.7+.
Gson takes another approach to deserialization of objects without a zero-argument constructor: sun.misc.Unsafe provides a method to allocate an instance of a class without calling its constructor. Gson then proceeds to reflectively set fields of the immutable object. However, Unsafe is unsupported API and the OpenJDK project is attempting to phase out the class and may remove it entirely in a future JDK version, so this solution should be avoided.
Immutability in Java is not as simple in Java as it might appear at first but with the right tools and patterns, it can be very beneficial to your code base, making your application easier to read, debug and extend and making parallelism much simpler. Be careful of accidentally adding mutability and make use of tools like Lombok to reduce boilerplate associated with immutable classes in Java.