Understanding and Defending Against CVE-2026-15565: A Denial of Service Vulnerability in Undertow

Overview

CVE-2026-15565 is a vulnerability in the Undertow web server, a popular Java-based web server used in various Red Hat products. This vulnerability allows remote attackers to cause a Denial of Service (DoS) attack without authentication, making it a significant concern for organizations using affected products.

Understanding the Vulnerability / Threat

Root Cause Analysis

The root cause of this vulnerability is a flaw in the Undertow web server's handling of WebSocket connections. Specifically, the vulnerability exists in the `@ServerEndpoint` class that has any `@OnMessage` method. This flaw allows remote attackers to cause an Out of Memory (OOM) condition on the WebSocket endpoint without authentication. This vulnerability belongs to the CWE-120 category, which involves Buffer Overflow or similar issues, but more specifically, it is related to resource exhaustion via a flood of requests.

Attack Surface & Vector

The attack surface of this vulnerability is the WebSocket endpoint of the Undertow web server. An attacker can reach this vulnerability through a standard WebSocket handshake without authentication. The preconditions needed for this attack are that the attacker must be able to send HTTP requests to the WebSocket endpoint of the Undertow server.

Exploitation Mechanics — Scenario Walkthrough

Scenario: Compromising a Corporate Undertow Instance 1. Initial Position: The attacker is a remote user with access to the corporate network and can send HTTP requests to the Undertow web server. 2. Triggering the Flaw: The attacker crafts a malformed WebSocket handshake request to the Undertow server's WebSocket endpoint. This request is designed to exploit the vulnerability in the `@ServerEndpoint` class with an `@OnMessage` method. 3. What Breaks: When the Undertow server processes the malformed WebSocket handshake request, it fails to properly handle the request, leading to an Out of Memory (OOM) condition. This OOM condition causes the WebSocket endpoint to become unresponsive, effectively resulting in a Denial of Service (DoS) attack. 4. Attacker's Prize: The attacker successfully causes a DoS condition on the Undertow server's WebSocket endpoint, making it unavailable for legitimate users. The attacker does not gain unauthorized access or control over the server but achieves a significant impact on the server's availability.

Real-World Impact

The real-world impact of this vulnerability is significant, as it allows remote attackers to cause a Denial of Service (DoS) attack without authentication. This can lead to downtime, loss of productivity, and potential financial losses for organizations using affected Red Hat products.

Detection & Defense

Immediate Mitigations

To mitigate this vulnerability, organizations should apply the patches provided by Red Hat for the affected products. Specifically, upgrade the `undertow-websockets-jsr` package to the latest version. For example, for Red Hat JBoss Enterprise Application Platform 7, upgrade to version 7.4.14 or later.

Detection Strategies

Defenders can detect exploitation attempts by monitoring for unusual patterns of WebSocket connections and handshake requests. Implement Web Application Firewalls (WAFs) with rules to detect and block suspicious WebSocket traffic. Additionally, monitor server logs and performance metrics for signs of OOM conditions or abnormal server behavior.

Long-Term Hardening

To prevent similar vulnerabilities, organizations should implement a defense-in-depth strategy that includes: - Regularly updating and patching software - Implementing robust security configurations for web servers - Conducting periodic security audits and vulnerability assessments - Using Web Application Firewalls (WAFs) to detect and block suspicious traffic

Key Takeaways

- CVE-2026-15565 is a high-severity vulnerability in Undertow that allows remote DoS attacks without authentication. - The vulnerability exists in the WebSocket endpoint of Undertow, specifically in `@ServerEndpoint` classes with `@OnMessage` methods. - Immediate mitigation involves applying patches provided by Red Hat for affected products. - Detection strategies include monitoring WebSocket traffic and server performance. - Long-term hardening involves regular updates, robust security configurations, and defense-in-depth strategies.

Sources

- National Vulnerability Database (NVD) - CVE-2026-15565 - Red Hat Security Advisory for CVE-2026-15565