---
id: CVE-2026-19879
title: >-
  Io.undertow/undertow: undertow: http response header integrity issue due to
  character truncation
summary: >-
  A flaw was found in Undertow, an HTTP server, within its HTTP response header
  writing path. The `writeString()` method performs a silent narrowing cast from
  16-bit Unicode characters to 8-bit bytes when writing HTTP response header
  value…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-681
vendor: Red Hat
product: undertow-core
affected:
  - undertow-core (all versions)
  - undertow-parent (all versions)
  - undertow-servlet (all versions)
  - undertow-websockets-jsr (all versions)
  - moditect (all versions)
  - 'pki-core:10.6/resteasy (all versions)'
  - 'pki-deps:10.6/resteasy (all versions)'
  - resteasy (all versions)
  - jboss-eap-7/eap74-els-openjdk11-openshift-rhel8 (all versions)
  - jboss-eap-7/eap74-els-openjdk17-openshift-rhel8 (all versions)
  - jboss-eap-7/eap74-els-openjdk8-openshift-rhel8 (all versions)
  - undertow-core (all versions)
  - undertow-parent (all versions)
  - undertow-servlet (all versions)
  - undertow-websockets-jsr (all versions)
  - undertow-core (all versions)
  - undertow-servlet (all versions)
  - undertow-websockets-jsr (all versions)
  - undertow-core (all versions)
  - undertow-servlet (all versions)
  - undertow-websockets-jsr (all versions)
  - undertow-core (all versions)
  - undertow-servlet (all versions)
  - undertow-websockets-jsr (all versions)
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-08-14T17:41:34.677014Z'
published: '2026-08-14'
updated: '2026-11-05'
sourceUpdated: '2026-11-05T19:26:56.012Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-19879'
references:
  - url: 'https://access.redhat.com/security/cve/CVE-2026-19879'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2516038'
    label: RHBZ#2516038
tags:
  - cve.org
epss: 0.00393
epssPercentile: 0.33609
ingestedAt: '2026-11-05T22:38:31.547Z'
---

## Overview

A flaw was found in Undertow, an HTTP server, within its HTTP response header writing path. The `writeString()` method performs a silent narrowing cast from 16-bit Unicode characters to 8-bit bytes when writing HTTP response header values. A remote attacker can exploit this by supplying specific Unicode characters in user-controlled input that an application places into response headers. This can lead to the truncation of these characters into ASCII control characters or special symbols, potentially resulting in limited integrity impact or information disclosure if the application does not properly sanitize user input.

## Affected

- `undertow-core (all versions)`
- `undertow-parent (all versions)`
- `undertow-servlet (all versions)`
- `undertow-websockets-jsr (all versions)`
- `moditect (all versions)`
- `pki-core:10.6/resteasy (all versions)`
- `pki-deps:10.6/resteasy (all versions)`
- `resteasy (all versions)`
- `jboss-eap-7/eap74-els-openjdk11-openshift-rhel8 (all versions)`
- `jboss-eap-7/eap74-els-openjdk17-openshift-rhel8 (all versions)`
- `jboss-eap-7/eap74-els-openjdk8-openshift-rhel8 (all versions)`
- `undertow-core (all versions)`
- `undertow-parent (all versions)`
- `undertow-servlet (all versions)`
- `undertow-websockets-jsr (all versions)`
- `undertow-core (all versions)`
- `undertow-servlet (all versions)`
- `undertow-websockets-jsr (all versions)`
- `undertow-core (all versions)`
- `undertow-servlet (all versions)`
- `undertow-websockets-jsr (all versions)`
- `undertow-core (all versions)`
- `undertow-servlet (all versions)`
- `undertow-websockets-jsr (all versions)`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

### Workarounds

Applications utilizing Undertow should implement robust input validation and sanitization for all user-controlled data before it is placed into HTTP response headers. It is recommended to restrict header values to printable ASCII characters (0x20–0x7E) at the application level. Where available, use framework-provided header-safe encoding mechanisms to ensure proper handling of special characters.