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.
flavorjones Reporter
Summary
Loofah's HTML5 sanitizer restricted only the
xlink:hrefattribute on certain SVG elements to local, same-document references. Browsers also accept a plainhrefattribute as an alternative to the deprecatedxlink:hrefper 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.