# CVE-2026-104828: HtmlUnit Neko: Mutation XSS (mXSS) in HTMLWriterFilter - TEXTAREA/TITLE entity decode without re-encode

> Attackers can execute arbitrary JavaScript via mutation XSS in HtmlUnit Neko when parsing and serializing HTML containing TEXTAREA or TITLE elements.

- **Severity:** High 7.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/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:** 15 Oct 2026
- **Updated:** 21 Oct 2026
- **Also known as:** GHSA-39xg-prhf-8785

## What is CVE-2026-104828?

CVE-2026-104828 is a high-severity cross-site scripting vulnerability in htmlunit-neko, published on 15 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

HtmlUnit Neko is an HtmlUnit adaptation of NekoHTML. Prior to 5.4.0, HTMLWriterFilter writes decoded TEXTAREA and TITLE character content without entity encoding because both elements are treated as special content. During a parse-and-serialize round trip, entity references inside these escapable raw-text elements are decoded by the parser and then emitted as raw markup by printCharacters(), changing the document's semantics. Applications that use htmlunit-neko in an HTML sanitizer, content filter, or other parse-transform-serialize pipeline can therefore turn attacker-controlled encoded text into executable markup when the output is rendered by a browser. This issue is fixed in version 5.4.0.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-104828?


## How do I fix CVE-2026-104828?

No fix has been published for CVE-2026-104828 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-104828 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-104828 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-104828 and fix it if it is affected.

What it is: CVE-2026-104828 is a high-severity cross-site scripting vulnerability in htmlunit-neko, published on 15 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-104828 exists.

Affected product: htmlunit-neko. 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-2026-104828/ (Markdown: https://insights.sbomify.com/CVE-2026-104828/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-104828
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-104828
- OSV: https://osv.dev/vulnerability/CVE-2026-104828

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-104828/
Summaries are generated and may contain mistakes; the official description above is the record.