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

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.

Exploitable remotely over the network, without an account and without any user action; the impact on data or service is limited.

Attack vector

Network

Attack complexity

Low

Privileges required

None

Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

Low

Availability

None

Affected products and versions