CVE-2026-19879: Io.undertow/undertow: undertow: http response header integrity issue due to character truncation

Vulnerability summary

CVE-2026-19879 is a medium (CVSS 5.3) vulnerability in undertow. EPSS estimates a 0.39% chance of exploitation in the next 30 days, higher than 33.4% of scored vulnerabilities.

Timeline

1. 14-08-2026
   PublishedVulnerability first disclosed.
2. 14-08-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-681Incorrect Conversion between Numeric TypesWhen converting from one data type to another, such as long to integer, data can be omitted or translated in a way that produces unexpected values. If the resulting values are used in a sensitive context, then dangerous behaviors may occur.