CVE-2026-126209 Loofah allowed_uri? treats javascript: alert(1) as safe
Attackers can bypass Loofah's URI sanitizer with encoded colons and scheme letters to enable javascript: links in Rails apps rendering approved values.
- 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-2026-126209?
CVE-2026-126209 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. From 2.25.0 until 2.25.3, Loofah::HTML5::Scrub.allowed_uri? strips whitespace and control characters before decoding a semicolon-less numeric character reference, changing where the reference ends and hiding a javascript: or vbscript: scheme that a browser reconstructs. The bypass applies to encoded colons and encoded scheme letters when a stripped character terminates the numeric reference. An application that renders the approved value in href or another browser-interpreted URI attribute can allow cross-site scripting, and Rails::HTML::Sanitizer.allowed_uri? re-exports the affected helper. Loofah's default sanitize() path is not affected because Nokogiri decodes entities before URI evaluation. This issue is fixed in version 2.25.3.
Which versions are affected by CVE-2026-126209?
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 2.25.0 | 2.25.3 |
How do I fix CVE-2026-126209?
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-2026-126209 being exploited?
There is no known exploitation of CVE-2026-126209 in the wild and no public exploit code.
Sources
Also known as GHSA-p9wj-w8xq-vjx6.