# CVE-2026-126063: Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer


- **Severity:** Low 2.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 10 Nov 2026
- **Updated:** 10 Nov 2026
- **Also known as:** DEBIAN-CVE-2026-126063, GHSA-4p39-36wj-23h4, UBUNTU-CVE-2026-126063

## What is CVE-2026-126063?

CVE-2026-126063 is a low-severity vulnerability in sparklemotion nokogiri, published on 10 Nov 2026. 

### Official description

Nokogiri is an open source XML and HTML library for the Ruby programming language. From 1.12.0 until 1.19.5, the C extension on CRuby and TruffleRuby has a Nokogiri::HTML5 serializer that does not escape text inside noscript elements or angle brackets inside attribute values. When an application sanitizes untrusted HTML with the default parsing options, allows noscript, and sends serialized output to a scripting-enabled browser, the browser can parse markup that Nokogiri did not produce, enabling mutation cross-site scripting. Using parse_noscript_content_as_text: true is the recommended workaround, not the vulnerable default. JRuby is not affected because it does not implement Nokogiri::HTML5. This issue is fixed in version 1.19.5.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-116 (Improper Encoding or Escaping of Output)

## Which versions are affected by CVE-2026-126063?

1 open source package ship an affected version, including nokogiri.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nokogiri` | RubyGems | From 1.12.0 | 1.19.5 |

## How do I fix CVE-2026-126063?

Upgrade nokogiri to 1.19.5 or later.

## Is CVE-2026-126063 being exploited?

There is no known exploitation of CVE-2026-126063 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126063 and fix it if it is affected.

What it is: CVE-2026-126063 is a low-severity vulnerability in sparklemotion nokogiri, published on 10 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-126063 in the wild and no public exploit code.

Affected packages and fixed versions:
- nokogiri (RubyGems): affected from 1.12.0; fixed in 1.19.5

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-2026-126063/ (Markdown: https://insights.sbomify.com/CVE-2026-126063/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126063
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126063
- OSV: https://osv.dev/vulnerability/CVE-2026-126063

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126063/
Summaries are generated and may contain mistakes; the official description above is the record.