CVE-2026-126206 Loofah allowed_uri? treats data:text/html as safe text/plain
Attackers can bypass Loofah's URL sanitizer with data URIs containing a solidus entity to enable XSS in Rails apps that render approved URLs.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Harmful if only installed
- No
- Needs a specific call
- Yes
- Fix
- Available
What is CVE-2026-126206?
CVE-2026-126206 is a medium-severity cross-site scripting vulnerability in flavorjones loofah, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Which versions are affected by CVE-2026-126206?
1 open source package ship an affected version, including loofah.
An affected version is only exploitable if your software calls Loofah::HTML5::Scrub.allowed_uri? or Rails::HTML::Sanitizer.allowed_uri?. Many projects that include it are not exposed.
| Package | Affected | Fixed in |
|---|---|---|
loofah RubyGems | From 2.25.2 | 2.25.3 |
How do I fix CVE-2026-126206?
Upgrade loofah to 2.25.3 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-126206 being exploited?
There is no known exploitation of CVE-2026-126206 in the wild and no public exploit code.
Sources
Also known as GHSA-jcff-cj73-4w4f.