Nokogiri: Mutation XSS through `noscript` content in Nokogiri's HTML5 serializer
- Exploited
- No known exploitation
- Fix
- Fixed in 1.19.5
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 1.19.5.
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 19/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure7/25Reachable over the network without a login · Unknown: No advisory context yet
- Reach10/20Affects RubyGems packages
- Severity2.3/10CVSS 2.3
- No patch0/5No signal
What would change the score
- Would score about 74 if added to CISA KEV
- Would score about 69 if CISA's SSVC rates exploitation active
- Would score about 34 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 1.19.5.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- sparklemotion nokogiri
From 1.12.0 · fixed in 1.19.5
- nokogiriRubyGems
From 1.12.0 · fixed in 1.19.5
Evidence 7
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 4
- Repository
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126063
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: cve.now feed data
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: cve.now feed data
How do I fix CVE-2026-126063?
Upgrade to a fixed version: 1.19.5.Source: cve.now feed data