CVE-2026-19879
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.
Statement
This Moderate impact defense-in-depth flaw in Undertow's HTTP response header handling occurs when applications place unsanitized user-controlled Unicode input into response headers. While Undertow prevents HTTP response splitting, the silent truncation of characters to ASCII control characters or special symbols could pose a risk in applications that reflect these header values without proper validation.
Mitigation
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.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | N/A | 5.3 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | Low | N/A | Low |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Understanding the Weakness (CWE)
Other,Integrity
Technical Impact: Unexpected State; Quality Degradation
The program could wind up using the wrong number and generate incorrect results. If the number is used to allocate resources or make a security decision, then this could introduce a vulnerability.
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