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Low, CVSS 2.3amended

Nokogiri: Mutation XSS through noscript content in Nokogiri's HTML5 serializer

CVE-2026-126063GHSA-4p39-36wj-23h4

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (nokogiri 1.19.5), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems nokogiri

    Affected
    >= 1.12.0, < 1.19.5
    Fixed in
    1.19.5

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
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 · score source
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

Summary Nokogiri's HTML5 serializer did not escape text inside `noscript` elements, or ` ` inside attribute values. When a document parsed with the default options is serialized, a browser with scripting enabled can parse the output into markup that Nokogiri's parser never produced. This is a mutation XSS for applications that sanitize untrusted HTML with `Nokogiri::HTML5`, allow `noscript` elements, and send the serialized output to browsers. Nokogiri 1.19.5 escapes both, as the HTML standard's serialization algorithm requires. Text inside `noscript` is still emitted unescaped for documents parsed with `parse_noscript_content_as_text: true`. Only the C extension (CRuby and TruffleRuby) is affected. JRuby does not implement `Nokogiri::HTML5`.

Severity The Nokogiri maintainers have evaluated this as low severity (CVSS 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`). Exploitation requires an application that allows `noscript` in sanitized HTML. Loofah…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-4p39-36wj-23h4

  2. Revision 1 · created ·

    Published from ghsa record GHSA-4p39-36wj-23h4

Agent brief

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# Patch Next advisory brief: CVE-2026-126063

Advisory: Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer
IDs: CVE-2026-126063, GHSA-4p39-36wj-23h4
Record: https://patchnext.com/advisories/cve-2026-126063 (revision 2, source checked 11 Nov 2026, 07:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nokogiri 1.19.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems nokogiri: affected >= 1.12.0, < 1.19.5; fixed in 1.19.5

## Suggested inspection (read-only)
- RubyGems nokogiri: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nokogiri.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4p39-36wj-23h4 (GitHub-reviewed advisory GHSA-4p39-36wj-23h4 lists affected ranges: RubyGems nokogiri >=1.12.0 <1.19.5)
- Verified: https://osv.dev/vulnerability/GHSA-4p39-36wj-23h4 (OSV record GHSA-4p39-36wj-23h4 lists affected ranges: RubyGems nokogiri >=1.12.0 <1.19.5 (fixed))

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