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# Basic Java Deserialization (ObjectInputStream, readObject) 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](http://tutorials.jenkov.com/java-io/objectoutputstream.html) and deserialization (reading) is done with the [ObjectInputStream](http://tutorials.jenkov.com/java-io/objectinputstream.html). 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 b **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:** ```java 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"); } } ``` This example was taken from [https://medium.com/@knownsec404team/java-deserialization-tool-gadgetinspector-first-glimpse-74e99e493649](https://medium.com/@knownsec404team/java-deserialization-tool-gadgetinspector-first-glimpse-74e99e493649) ### Conclusion As you can see in this very basic example, the "vulnerability" here appears because the **readObject** function is **calling other vulnerable functions**.
Support HackTricks and get benefits! Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)! Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family) Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com) **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.** **Share your hacking tricks submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**