{"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","slug":"CVE-2026-19879","body":"## Overview\n\nA 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.\n\n## Affected\n\n- `undertow-core (all versions)`\n- `undertow-parent (all versions)`\n- `undertow-servlet (all versions)`\n- `undertow-websockets-jsr (all versions)`\n- `moditect (all versions)`\n- `pki-core:10.6/resteasy (all versions)`\n- `pki-deps:10.6/resteasy (all versions)`\n- `resteasy (all versions)`\n- `jboss-eap-7/eap74-els-openjdk11-openshift-rhel8 (all versions)`\n- `jboss-eap-7/eap74-els-openjdk17-openshift-rhel8 (all versions)`\n- `jboss-eap-7/eap74-els-openjdk8-openshift-rhel8 (all versions)`\n- `undertow-core (all versions)`\n- `undertow-parent (all versions)`\n- `undertow-servlet (all versions)`\n- `undertow-websockets-jsr (all versions)`\n- `undertow-core (all versions)`\n- `undertow-servlet (all versions)`\n- `undertow-websockets-jsr (all versions)`\n- `undertow-core (all versions)`\n- `undertow-servlet (all versions)`\n- `undertow-websockets-jsr (all versions)`\n- `undertow-core (all versions)`\n- `undertow-servlet (all versions)`\n- `undertow-websockets-jsr (all versions)`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.\n\n### Workarounds\n\nApplications 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.","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}