CVE-2023-21830 - Java SE Serialization Vulnerability—How Your Apps Are Exposed

1. What is CVE-2023-21830?

CVE-2023-21830 is a security vulnerability found in Oracle Java SE and Oracle GraalVM Enterprise Edition, specifically in the "Serialization" component. This issue allows an attacker to compromise affected Java platforms without needing credentials and with just network access, using various common protocols.

The attacker can update, insert, or delete data they shouldn’t have access to, potentially leading to further compromise if combined with other bugs.  
CVSS 3.1 Base Score: 5.3 (medium)  
Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)

Impacts data integrity (not confidentiality or denial-of-service)

- Most dangerous in scenarios with untrusted/applet code, NOT backend servers with trusted code

2. Which Java Versions are Affected?

| Product                                       | Affected Versions           |
|-----------------------------------------------|----------------------------|
| Oracle Java SE                                | 8u351, 8u351-perf           |
| Oracle GraalVM Enterprise Edition             | 20.3.8, 21.3.4              |

Note:  
This issue *mainly* affects Java used in client applications—like those using Java Web Start or Java applets—that load code directly from the Internet. It does not generally affect Java backend/servers running only administrator-installed code.

Serialization Explained:

Serialization is a way Java turns objects into bytes (so they can be sent over the network or stored), and then back into objects later (“deserialization”). If attackers can feed maliciously crafted data into this process, they can hijack the normal flow.

The Vulnerability:

In affected Java versions, the deserialization component mishandles untrusted input. If an attacker can get your app to deserialize attacker-supplied data, they may perform unauthorized changes (add, remove, or modify data) within the context of the running application.

Malicious serialized input triggers unexpected behavior

4. Proof of Concept and Exploit Details

Here is a simple code example showing how a vulnerable Java app deserializes input (this is just for illustration—don’t use unsafe deserialization in real applications):

import java.io.*;
import java.net.*;

public class UnsafeDeserializationDemo {
    public static void main(String[] args) throws Exception {
        ServerSocket server = new ServerSocket(12345);
        System.out.println("Waiting for data...");
        Socket socket = server.accept();
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        Object obj = ois.readObject(); // VULNERABLE POINT
        System.out.println("Received object: " + obj);
        ois.close();
        socket.close();
        server.close();
    }
}

How an attacker might exploit this:
By sending a specially crafted serialized object to this server, an attacker can execute operations within the application they shouldn't be able to—such as updating, inserting, or deleting important data, depending on how the object is used afterwards.

Real-World Exploit Steps (Conceptual):

1. Find an application using a vulnerable Java version, accepting serialized objects from the network.
2. Craft a malicious serialized object leveraging known Java deserialization gadgets (like those in ysoserial: https://github.com/frohoff/ysoserial).

Note:

Actual code or public exploits are, as of this writing, not available due to the risk of widespread abuse. Instead, security pros recommend fixing the root vulnerability in your Java instance.

5. Mitigation Steps

- Update your Java SE or GraalVM to a fixed version (*Oracle Java SE version 8u361* or newer; Oracle GraalVM EE updated after Jan 2023).
- Never deserialize untrusted data! If you must, restrict classes that can be loaded, or use tools like ObjectInputFilter.

Review application code for unsafe deserialization.

Oracle Security Advisory for CVE-2023-21830:

https://www.oracle.com/security-alerts/cpujan2023.html#AppendixJAVA

National Vulnerability Database entry:

https://nvd.nist.gov/vuln/detail/CVE-2023-21830

ysoserial (deserialization exploit framework):

https://github.com/frohoff/ysoserial

Oracle release notes:

https://www.oracle.com/java/technologies/javase/8u361-relnotes.html

Summary

CVE-2023-21830 is a medium-severity flaw in Java SE and GraalVM EE's serialization, allowing remote attackers to compromise client-side Java by feeding untrusted serialized data. Servers running *trusted* code are not at risk, but all organizations should patch affected Java versions immediately and make sure their apps don’t deserialize data from untrusted sources.

If you rely on Java applets or Web Start, upgrade and review your defenses now.

*Always keep your Java updated and never trust unvalidated data—especially when it comes from the internet!*

Timeline

Published on: 01/18/2023 00:15:00 UTC
Last modified on: 01/24/2023 19:29:00 UTC