Basic Java Deserialization (ObjectInputStream, readObject) - HackTricks

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In this POST it's going to be explained an example using `java.io.Serializable`.

# Serializable

The Java `Serializable` interface (`java.io.Serializable` is a marker interface your classes must implement if they are to be serialized and deserialized. Java object serialization (writing) is done with the ObjectOutputStream and deserialization (reading) is done with the ObjectInputStream.

Lets see an example with a class Person which is serializable. This class overwrites the readObject function, so when any object of this class is deserialized this function is going to be executed.
In the example, the readObject function of the class Person calls the function `eat()` of his pet and the function `eat()` of a Dog (for some reason) calls a calc.exe. We are going to see how to serialize and deserialize a Person object to execute this calculator:

The following example is from https://medium.com/@knownsec404team/java-deserialization-tool-gadgetinspector-first-glimpse-74e99e493649

    import java.io.Serializable;
    import java.io.*;

    public class TestDeserialization {
        interface Animal {
            public void eat();
        }
        //Class must implements Serializable to be serializable
        public static class Cat implements Animal,Serializable {
            @Override
            public void eat() {
                System.out.println("cat eat fish");
            }
        }
        //Class must implements Serializable to be serializable
        public static class Dog implements Animal,Serializable {
            @Override
            public void eat() {
                try {
                    Runtime.getRuntime().exec("calc");
                } catch (IOException e) {
                    e.printStackTrace();
                }
                System.out.println("dog eat bone");
            }
        }
        //Class must implements Serializable to be serializable
        public static class Person implements Serializable {
            private Animal pet;
            public Person(Animal pet){
                this.pet = pet;
            }
            //readObject implementation, will call the readObject from ObjectInputStream  and then call pet.eat()
            private void readObject(java.io.ObjectInputStream stream)
                    throws IOException, ClassNotFoundException {
                pet = (Animal) stream.readObject();
                pet.eat();
            }
        }
        public static void GeneratePayload(Object instance, String file)
                throws Exception {
            //Serialize the constructed payload and write it to the file
            File f = new File(file);
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(f));
            out.writeObject(instance);
            out.flush();
            out.close();
        }
        public static void payloadTest(String file) throws Exception {
            //Read the written payload and deserialize it
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
            Object obj = in.readObject();
            System.out.println(obj);
            in.close();
        }
        public static void main(String[] args) throws Exception {
            // Example to call Person with a Dog
            Animal animal = new Dog();
            Person person = new Person(animal);
            GeneratePayload(person,"test.ser");
            payloadTest("test.ser");
            // Example to call Person with a Cat
            //Animal animal = new Cat();
            //Person person = new Person(animal);
            //GeneratePayload(person,"test.ser");
            //payloadTest("test.ser");
        }
    }

## Conclusion

As you can see in this very basic example, the "vulnerability" here appears because the readObject function is calling other vulnerable functions.

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