CVE-2026-19879 is a moderate severity vulnerability in Undertow, an HTTP server, involving 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 truncation can convert characters into ASCII control characters or special symbols, potentially causing limited integrity impact or information disclosure if the application does not sanitize input properly. Undertow prevents HTTP response splitting, but this flaw can still pose risks in applications reflecting unsanitized header values. Red Hat advisory recommends application-level input validation and restricting header values to printable ASCII characters. No patch or official fix is currently documented in the advisory.

The vulnerability can lead to limited integrity impact or information disclosure due to the truncation of Unicode characters into ASCII control characters or special symbols in HTTP response headers. This occurs only if applications place unsanitized user-controlled input into response headers. Undertow itself prevents HTTP response splitting, so the impact is limited to integrity and potential information leakage in such scenarios.

No official patch or fix is currently confirmed for this vulnerability. Applications using Undertow should implement robust input validation and sanitization for all user-controlled data before placing it into HTTP response headers. It is recommended to restrict header values to printable ASCII characters (0x20–0x7E) at the application level. Where possible, use framework-provided header-safe encoding mechanisms to ensure proper handling of special characters.