Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.2, Loofah's HTML5 sanitizer applies its local-reference restriction only to the xlink:href attribute on SVG use and feImage elements, while browsers also accept the plain href attribute. A crafted sanitized SVG can therefore reference an arbitrary same-origin external document; use may render external SVG content containing scripts or other dangerous content, and feImage may load external images for tracking. Applications that sanitize user-supplied SVG with Loofah's default allowlist are affected. This issue is fixed in version 2.25.2.

Affected: RubyGems: loofah

## Summary

Loofah's HTML5 sanitizer restricted only the `xlink:href` attribute on certain SVG elements to local, same-document references. Browsers also accept a plain `href` attribute as an alternative to the deprecated `xlink:href` per the SVG 2 spec, but Loofah did not apply the same restriction to it, allowing those elements to reference arbitrary external documents.

## Impact

SVG `<use>` can load and render external SVG content by reference. If the referenced external SVG is same-origin and contains scripts or other dangerous content, it could execute in the context of the sanitized document. `<feImage>` can load external images, which can be used for tracking. Modern browsers restrict cross-origin `<use>` fetches, which limits but does not eliminate the risk.

Applications that sanitize user-supplied SVG (directly, or as part of HTML) with Loofah's default allowlist are affected.

## Mitigation

Upgrade to Loofah >= 2.25.2.

## Credit

Found by the maintainer, Mike Dalessio, during a security audit.

Affected: Ubuntu:16.04:LTS: ruby-loofah, Ubuntu:18.04:LTS: ruby-loofah, Ubuntu:20.04:LTS: ruby-loofah, Ubuntu:22.04:LTS: ruby-loofah, Ubuntu:24.04:LTS: ruby-loofah, Ubuntu:26.04:LTS: ruby-loofah

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.2, Loofah's HTML5 sanitizer applies its local-reference restriction only to the xlink:href attribute on SVG use and feImage elements, while browsers also accept the plain href attribute. A crafted sanitized SVG can therefore reference an arbitrary same-origin external document; use may render external SVG content containing scripts or other dangerous content, and feImage may load external images for tracking. Applications that sanitize user-supplied SVG with Loofah's default allowlist are affected. This issue is fixed in version 2.25.2.