CVE-2026-15565
Received Received - Intake

Out of Memory Denial of Service in Undertow WebSocket Endpoint

Publication date: 2026-08-11

Last updated on: 2026-08-11

Assigner: Red Hat, Inc.

Description
A flaw was found in Undertow. A remote attacker can cause Out of Memory on websockets endpoint without authentication on any @ServerEndpoint class that has any @OnMessage method. This allows an attacker to cause Denial of Service attack without authentication and using only a standard WebSocket handshake.

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Meta Information

Published
2026-08-11
Last Modified
2026-08-11
Generated
2026-08-11
AI Q&A
2026-08-11
EPSS Evaluated
N/A
NVD
CVE-2026-15565
EUVD
EUVD-2026-55988

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
red_hat undertow *

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-120 The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.

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Executive Summary

This vulnerability in Undertow allows a remote attacker to cause an Out of Memory error on a websocket endpoint without authentication. It affects any @ServerEndpoint class with an @OnMessage method by exploiting two defects in message size handling. The first defect ignores configured size limits for text messages, while the second fails to check message size during data arrival, allowing attackers to send large data chunks that consume server resources.

Detection Guidance

Monitor for unusual memory consumption or crashes on systems running Undertow with websocket endpoints. Check for high CPU usage during WebSocket handshakes or message processing. Inspect logs for Out of Memory errors or connection timeouts on @ServerEndpoint classes with @OnMessage methods.

Impact Analysis

This vulnerability can lead to a Denial of Service attack, causing the server to run out of memory and crash. It requires no authentication and uses only a standard WebSocket handshake, making it easy to exploit. Systems using Undertow with websocket endpoints are at risk of resource exhaustion and service disruption.

Compliance Impact

This vulnerability could impact compliance by causing service disruptions or outages, potentially violating availability requirements in GDPR and HIPAA. A DoS attack may lead to unauthorized data access or loss of service, which could result in non-compliance with these regulations' security and availability standards.

Mitigation Strategies

Disable websocket handshakes server-wide by removing websockets configuration from standalone.xml or per-application by setting enable-websockets to false in jboss-web.xml. Apply patches once available from Red Hat or Undertow project. Monitor affected systems for signs of exploitation or resource exhaustion.

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