CVE-2026-5680: Undertow-core: undertow: denial of service via websocket permessage-deflate processing
A flaw was found in Undertow. A remote attacker could exploit this vulnerability by sending specially crafted WebSocket messages with permessage-deflate negotiated. This could lead to excessive memory consumption due to the PerMessageDeflateFunction.largerBuffer() method using exponential doubling, resulting in a Denial of Service (DoS) for the affected application.
Other sources
Summary: Undertow’s PerMessageDeflateFunction.largerBuffer() uses exponential doubling
Requirements To Exploit: Any application using Undertow’s WebSocket with permessage-deflate is
affected. This includes:
WildFly application server (uses Undertow as its web layer)
Red Hat JBoss Enterprise Application Platform (JBoss EAP)
Any standalone Undertow WebSocket application using
PerMessageDeflateHandshake
The vulnerability requires only a standard WebSocket connection with
permessage-deflate negotiated, no authentication, no special configuration.
Component Affected: io.undertow:undertow-core
Version Affected: Undertow 2.3.18.Final
Patch Available: no
Version Fixed: N/A
Cvss: Score: 7.5 HIGH Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Impact: IMPORTANT
Steps To Reproduce: I have attached a self-contained Maven project (undertow-websocket-poc.tar.gz)
containing 6 tests:
Baselines:
Baseline 1a: Valid Origin -> upgrade accepted, message delivered
Baseline 1b: Invalid Origin -> HTTP 403 rejected
Mitigation: Add a maxDecompressedBufferSize parameter to PerMessageDeflateHandshake
(e.g. default 10 MB) that limits the largerBuffer() growth
Add a maximum doubling count or absolute buffer cap in largerBuffer()
Add a maxDecompressionRatio check (reject if wire:decompressed > 100x)
Add a maxFragmentsPerMessage limit in WebSocketChannel
Add Ping rate limiting in WebSocketChannel before generating Pong
Expose these limits in the PerMessageDeflateHandshake constructor
— Red Hat
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed to this denial-of-service condition?
Affected Undertow applications are exposed when they accept WebSocket connections that negotiate the permessage-deflate extension. The issue can cause excessive memory consumption in the affected application.
What does an attacker need to exploit the issue?
An attacker can exploit the flaw remotely without privileges or user interaction by sending specially crafted WebSocket messages after permessage-deflate has been negotiated.
What is the impact of successful exploitation?
Successful exploitation can consume excessive memory and cause a denial of service for the affected application. The provided information does not indicate confidentiality or integrity impact.