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

> Remote attackers can exploit Unicode truncation in Undertow HTTP headers to cause integrity issues or information disclosure via unsanitized input.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Aug 2026
- **Updated:** 14 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-19879, GHSA-2crv-pxj9-66x8, UBUNTU-CVE-2026-19879

## What is CVE-2026-19879?

CVE-2026-19879 is a medium-severity vulnerability in Red Hat Enterprise Linux 10, published on 14 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-681 (Incorrect Conversion between Numeric Types)

## Which versions are affected by CVE-2026-19879?


## How do I fix CVE-2026-19879?

No fix has been published for CVE-2026-19879 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-19879 being exploited?

There is no known exploitation of CVE-2026-19879 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-19879 and fix it if it is affected.

What it is: CVE-2026-19879 is a medium-severity vulnerability in Red Hat Enterprise Linux 10, published on 14 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-19879 in the wild and no public exploit code.

Affected product: Red Hat Enterprise Linux 10. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-19879/ (Markdown: https://insights.sbomify.com/CVE-2026-19879/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-19879
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-19879
- OSV: https://osv.dev/vulnerability/CVE-2026-19879

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-19879/
Summaries are generated and may contain mistakes; the official description above is the record.