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