CVE
Loofah mutation XSS in HTML5 scrubbers via unescaped attributes
- Exploited
- No known exploitation
- Fix
- Fixed in 2.25.3
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 2.25.3.
- 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
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.27% · 19th percentile
- 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? →
- 23/100 · 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
- Exploitation: none
- Automatable: no
- Technical impact: partial
Summary
Loofah, a library for manipulating HTML and XML, has a mutation cross-site scripting flaw in its HTML5 scrubbers that can turn sanitized output into live HTML. An attacker can exploit it remotely without logging in by placing MathML markup in an allowed attribute, but a user must interact with the affected content for the attack to work.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. From 2.21.0 until 2.25.3, Loofah's HTML5 APIs Loofah.html5_fragment, Loofah.html5_document, and the scrub_html5_* methods can emit sanitized output that a browser reparses as different markup when used with Nokogiri before 1.19.5, because those Nokogiri versions do not escape angle brackets in attribute values. MathML can place markup inside an allowed attribute so the reparsed output becomes live HTML. The :strip scrubber is affected in every browser, while :prune and :escape are affected in WebKit browsers. Loofah's HTML4 APIs, Loofah::Helpers.sanitize, :whitewash, rails-html-sanitizer's default configuration, and JRuby are not affected. This issue is fixed in version 2.25.3.
cve.now Index
- Exploitation0/40No signal
- Exposure7/25Reachable over the network without a login · Unknown: No advisory context yet
- Reach10/20Affects RubyGems packages
- Severity6.1/10CVSS 6.1
- No patch0/5No signal
What would change the score
- Would score about 78 if added to CISA KEV
- Would score about 73 if CISA's SSVC rates exploitation active
- Would score about 38 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 2.25.3.
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-2026-126203 (Loofah: Mutation XSS in the HTML5 scrubbers through unescaped attribute values), 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-2026-126203 (entry last updated 2026-11-12):
Affected:
- flavorjones loofah: from 2.21.0, fixed in 2.25.3
- RubyGems package loofah: from 2.21.0, fixed in 2.25.3
Fixed versions: 2.25.3.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126203 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126203 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 the stack.check operation of the cve.now MCP server (execute) 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/flavorjones/loofah/security/advisories/GHSA-h6v6-rg66-fqj4
- https://github.com/flavorjones/loofah/pull/324
- https://github.com/sparklemotion/nokogiri/pull/3741
- https://github.com/flavorjones/loofah/commit/7ba1e7b77e043fb7dc832ef746fcbf83137135e6
- https://github.com/sparklemotion/nokogiri/commit/635abc8d65c75d594ddf8957fbfbfd822c2c0dbd
- https://github.com/flavorjones/loofah/releases/tag/v2.25.3
- 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-2026-126203
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126203"}.Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- flavorjones loofah
From 2.21.0 · fixed in 2.25.3
- loofahRubyGems
From 2.21.0 · fixed in 2.25.3
Evidence 9
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 6
- Repository
- Repository
- Repository
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126203
Is CVE-2026-126203 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-2026-126203?
loofah: from 2.21.0 before 2.25.3. loofah (RubyGems): from 2.21.0 before 2.25.3.Source: cve.now feed data
How do I fix CVE-2026-126203?
Upgrade to a fixed version: 2.25.3.Source: cve.now feed data