# CVE-2026-126203: Loofah HTML5 scrubber markup injection with Nokogiri

Source: https://junglewise.ai/threats/cve-2026-126203-loofah-is-a-general-library-for-manipulating-and-transforming  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-12. Updated: 2026-11-12  
License: CC BY 4.0 with a link back

CVE-2026-126203 · Severity: medium · CVSS 6.1 · Published 2026-11-12

Loofah is a library that sanitizes HTML and XML documents. When used with older versions of Nokogiri, its HTML5 scrubbing functions can output HTML that a browser reinterprets as executable markup, allowing attackers to bypass sanitization. This could enable cross-site scripting (XSS) attacks on applications using affected versions.

Technologies: [Loofah](https://junglewise.ai/threats/technologies/loofah-loofah), [loofah (RubyGems)](https://junglewise.ai/threats/technologies/loofah). Vendors: [Loofah](https://junglewise.ai/threats/vendors/loofah), [RubyGems](https://junglewise.ai/threats/vendors/rubygems).

## Quick answers

**Is CVE-2026-126203 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-126203 being exploited in the wild.

**How severe is CVE-2026-126203?** Medium severity, with a CVSS score of 6.1, published 12 November 2026.

**Which products and versions are affected?** Loofah 2.21.0 to 2.25.2; loofah (RubyGems) >= 2.21.0, < 2.25.3; Sparkling Motion Nokogiri before 1.19.5.

**Is there a fix for CVE-2026-126203?** Yes: fixed in Loofah 2.25.3. Until then: Upgrade Loofah to version 2.25.3, which now requires Nokogiri >= 1.19.5 that properly escapes angle brackets in attribute values.</anigation>
<parameter name="timeline">[
  {
    "kind": "disclosed",
    "at": "2026-11-12"
  }
] Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-126203 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-126203?** Check whether you run Loofah, loofah (RubyGems) or Sparkling Motion Nokogiri at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in Loofah 2.25.3

Upgrade Loofah to version 2.25.3, which now requires Nokogiri >= 1.19.5 that properly escapes angle brackets in attribute values.</anigation>
<parameter name="timeline">[
  {
    "kind": "disclosed",
    "at": "2026-11-12"
  }
]

## Technical details

The vulnerability is a mutation XSS (mXSS) affecting Loofah's HTML5 APIs (html5_fragment, html5_document, and scrub_html5_* methods) when combined with Nokogiri versions before 1.19.5. Nokogiri &lt;1.19.5 fails to escape angle brackets (&lt; and &gt;) in HTML attribute values; MathML elements can exploit this to place markup inside allowed attributes, which browsers then reparse as live HTML. The :strip scrubber is affected in all browsers, while :prune and :escape are vulnerable in WebKit browsers only.

## Affected products

- Loofah Loofah 2.21.0 to 2.25.2
- Sparkling Motion Nokogiri before 1.19.5

## Identifiers

- CVE-2026-126203
- GHSA-h6v6-rg66-fqj4

## References

- [https://github.com/flavorjones/loofah/commit/7ba1e7b77e043fb7dc832ef746fcbf83137135e6](https://github.com/flavorjones/loofah/commit/7ba1e7b77e043fb7dc832ef746fcbf83137135e6)
- [https://github.com/flavorjones/loofah/pull/324](https://github.com/flavorjones/loofah/pull/324)

## Related threats

- [Loofah allowed_uri? XSS bypass via numeric character reference](https://junglewise.ai/threats/cve-2026-126209-loofah-is-a-general-library-for-manipulating-and-transforming)
- [Loofah SVG href sanitization bypass in textPath and mpath](https://junglewise.ai/threats/cve-2026-126208-loofah-is-a-general-library-for-manipulating-and-transforming)
- [Loofah CSS url() sanitization bypass via escaped parentheses](https://junglewise.ai/threats/cve-2026-126207-loofah-css-url-sanitization-bypass-via-escaped-parentheses)
- [Loofah HTML5 Scrub XSS bypass via HTML entity encoding in data URIs](https://junglewise.ai/threats/cve-2026-126206-loofah-allowed-uri-treats-data-text-sol-html-as-safe-text-plain)
- [Loofah regular expression denial of service in SVG local-reference check](https://junglewise.ai/threats/cve-2026-126205-loofah-regular-expression-denial-of-service-in-the-svg-local)