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

Affected: sparklemotion nokogiri

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.

GHSA-4p39-36wj-23h4 — Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer

Affected: RubyGems: nokogiri

### Summary

Nokogiri's HTML5 serializer did not escape text inside `noscript` elements, or `<` and `>` 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 and rails-html-sanitizer do not allow it by default, and Sanitize always removes it.

### Mitigation

Upgrade to Nokogiri 1.19.5 or later.

Applications that sanitize HTML and allow `noscript` should parse with `parse_noscript_content_as_text: true` (available since Nokogiri 1.17.0), or remove `noscript` elements. This is the workaround for earlier versions, and it is recommended on every version: some content inside `noscript` is serialized unchanged, as the standard requires.

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

GHSA-4p39-36wj-23h4 — Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer