GHSA-4P39-36WJ-23H4

Vulnerability from github – Published: 2026-11-10 20:47 – Updated: 2026-11-10 20:47
VLAI
Summary
Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer
Details

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.

Credit

This issue was responsibly reported by @a-zara-n.

Severity
2.3 (Low)
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
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "nokogiri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.12.0"
            },
            {
              "fixed": "1.19.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-126063"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-116",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-11-10T20:47:36Z",
    "nvd_published_at": null,
    "severity": "LOW"
  },
  "details": "### Summary\n\nNokogiri\u0027s HTML5 serializer did not escape text inside `noscript` elements, or `\u003c` and `\u003e` 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\u0027s 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.\n\nNokogiri 1.19.5 escapes both, as the HTML standard\u0027s serialization algorithm requires. Text inside `noscript` is still emitted unescaped for documents parsed with `parse_noscript_content_as_text: true`.\n\nOnly the C extension (CRuby and TruffleRuby) is affected. JRuby does not implement `Nokogiri::HTML5`.\n\n### Severity\n\nThe 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.\n\n### Mitigation\n\nUpgrade to Nokogiri 1.19.5 or later.\n\nApplications 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.\n\n### Credit\n\nThis issue was responsibly reported by @a-zara-n.",
  "id": "GHSA-4p39-36wj-23h4",
  "modified": "2026-11-10T20:47:36Z",
  "published": "2026-11-10T20:47:36Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/sparklemotion/nokogiri/security/advisories/GHSA-4p39-36wj-23h4"
    },
    {
      "type": "WEB",
      "url": "https://github.com/sparklemotion/nokogiri/pull/3752"
    },
    {
      "type": "WEB",
      "url": "https://github.com/sparklemotion/nokogiri/commit/3e84df6ff7ab4d78f446e3a415430bd69a055d32"
    },
    {
      "type": "WEB",
      "url": "https://github.com/sparklemotion/nokogiri/commit/c6c0bffbdcd194694995acafeeed27326b0805f8"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/nokogiri/GHSA-4p39-36wj-23h4.yml"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/sparklemotion/nokogiri"
    },
    {
      "type": "WEB",
      "url": "https://github.com/sparklemotion/nokogiri/releases/tag/v1.19.5"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Nokogiri: Mutation XSS through `noscript` content in Nokogiri\u0027s HTML5 serializer"
}



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