# CVE-2026-126203: Loofah: Mutation XSS in the HTML5 scrubbers through unescaped attribute values

> Attackers can bypass Loofah's HTML5 sanitizers with MathML markup in attributes, producing live HTML in apps relying on it.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:** A user has to click or open something
- **Harmful if only installed:** No
- **Needs a specific call:** Yes (`Loofah.html5_fragment`, `Loofah.html5_document`, `scrub_html5_*`)
- **Fix available:** Yes
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-h6v6-rg66-fqj4

## What is CVE-2026-126203?

CVE-2026-126203 is a medium-severity cross-site scripting vulnerability in flavorjones loofah, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. From 2.21.0 until 2.25.3, Loofah's HTML5 APIs Loofah.html5_fragment, Loofah.html5_document, and the scrub_html5_* methods can emit sanitized output that a browser reparses as different markup when used with Nokogiri before 1.19.5, because those Nokogiri versions do not escape angle brackets in attribute values. MathML can place markup inside an allowed attribute so the reparsed output becomes live HTML. The :strip scrubber is affected in every browser, while :prune and :escape are affected in WebKit browsers. Loofah's HTML4 APIs, Loofah::Helpers.sanitize, :whitewash, rails-html-sanitizer's default configuration, and JRuby are not affected. This issue is fixed in version 2.25.3.

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-2026-126203?

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

An affected version is only exploitable if your software calls Loofah.html5_fragment, Loofah.html5_document or scrub_html5_*. Many projects that include it are not exposed.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `loofah` | RubyGems | From 2.21.0 | 2.25.3 |

## How do I fix CVE-2026-126203?

Upgrade loofah to 2.25.3 or later.

## Is CVE-2026-126203 being exploited?

There is no known exploitation of CVE-2026-126203 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126203 and fix it if it is affected.

What it is: CVE-2026-126203 is a medium-severity cross-site scripting vulnerability in flavorjones loofah, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-126203 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls Loofah.html5_fragment, Loofah.html5_document or scrub_html5_*. Many projects that include it are not exposed.

Affected packages and fixed versions:
- loofah (RubyGems): affected from 2.21.0; fixed in 2.25.3

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.
   If an affected version is present, also search the code for calls to Loofah.html5_fragment, Loofah.html5_document or scrub_html5_* and report whether any exist.
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-2026-126203/ (Markdown: https://insights.sbomify.com/CVE-2026-126203/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126203
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126203
- OSV: https://osv.dev/vulnerability/CVE-2026-126203

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126203/
Summaries are generated and may contain mistakes; the official description above is the record.