Undertow HTTP Response Header Unicode Truncation Vulnerability

Executive Summary

CVE-2026-19879 is a flaw in the Undertow HTTP server where the writeString() method incorrectly converts 16-bit Unicode characters to 8-bit bytes when writing HTTP response headers. This can cause user-controlled input containing specific Unicode characters to be truncated into ASCII control characters or special symbols, potentially affecting header integrity or causing information disclosure if input is not properly sanitized.

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Detection Guidance

Detecting this vulnerability requires checking if your Undertow server is running vulnerable versions and monitoring for unusual HTTP response headers. Inspect server logs for header manipulation attempts or truncated Unicode characters. Use network scanners to detect Undertow instances and verify their versions against the affected releases.

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Impact Analysis

This vulnerability may allow remote attackers to manipulate HTTP response headers by inserting crafted Unicode characters. If an application reflects unsanitized user input in headers, it could lead to limited integrity issues or information disclosure. The impact depends on whether the application properly validates and sanitizes user input before including it in headers.

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Mitigation Strategies

Implement strict input validation to restrict header values to printable ASCII characters (0x20–0x7E). Ensure applications sanitize user input before placing it in response headers. Use framework-provided header-safe encoding mechanisms. Monitor for unusual header values or truncation patterns in logs.

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