CVE-2026-19879

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 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.

Affected Software

VendorProductVersion RangeStatus

Weaknesses

  • CWE-681: Incorrect Conversion between Numeric Types

Workarounds

Applications 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.

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: yes
    • Technical Impact: partial

References