CVE-2026-84371: ApostropheCMS: Stored XSS via SVG SMIL URI-list scheme-policy bypass
Summary When SVG animation is allowed, attributeName="href" makes values a list of URL destinations. sanitize-html accepts a list that starts with a safe fragment even when values is explicitly scheme-checked, allowing a later javascript: destination to execute when the sanitized link is activated.
Details index.js:371-383 validates each attribute as one flat URL. It does not recognize that attributeName="href" gives the sibling values attribute SMIL URI-list semantics. For values="#safe;javascript:...", the leading fragment passes the flat check and the complete list is retained.
PoC This was reproduced with sanitize-html@2.17.6 and Chromium 150.0.7871.124. The configuration adds SVG animation to the defaults and applies the existing scheme policy to values; it does not allow javascript:. Save this as poc.js:
js const sanitize = require('sanitize-html');
const input = <svg><a><animate attributeName="href" values="#safe;javascript:alert('XSS')" dur=".01s" fill="freeze"></animate><text y="30">Click me</text></a></svg>; const output = sanitize(input, { allowedTags: sanitize.defaults.allowedTags.concat(['svg', 'animate', 'text']), allowedAttributes: { ...sanitize.defaults.allowedAttributes, animate: ['attributename', 'values', 'dur', 'fill'], text: ['y'] }, allowedSchemesAppliedToAttributes: sanitize.defaults.allowedSchemesAppliedToAttributes.concat(['values']) }); console.log(output);
Install and run it, then open poc.html and click Click me:
sh npm install sanitize-html@2.17.6 node poc.js > poc.html
The output retains the javascript: entry, and clicking the sanitized SVG displays XSS. With input changed to <a href="javascript:alert(1)">control</a>, the same configuration removes href.
Impact In an application that accepts attacker-authored SVG animation, the attacker can store this payload without scripts or event handlers. A victim who activates the sanitized link executes JavaScript in the application's origin despite the configured scheme policy.
Suggested fix Reject attributeName values selecting href or xlink:href on SVG animate and set, while retaining safe targets such as fill. Add values, from, and to regression cases.
Other sources
ApostropheCMS is an open-source Node.js content management system, and sanitize-html provides a simple HTML sanitizer with a clear API. From version 1.9.0 until version 2.17.7, packages/sanitize-html/index.js validates an animation value attribute as one flat URL and does not recognize that attributeName selecting href or xlink:href gives the sibling values, from, to, or by attribute SVG SMIL URL semantics. In configurations that allow the animate, animateColor, animateMotion, animateTransform, or set elements, a values list can begin with a safe fragment and contain a later executable destination that survives allowedSchemesAppliedToAttributes checking. When the sanitized SVG is rendered, the browser can copy that later destination into the live link, and a victim who activates the link can execute script in the application's origin. This issue is fixed in version 2.17.7.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/sanitize-htmlto a version that resolves this vulnerability.Fixed in 2.17.7 - Upgrade
Upgrade
sanitize-htmlto a version that resolves this vulnerability.Fixed in 2.17.7
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Deployments using sanitize-html versions 1.9.0 through 2.17.6 are affected when their configuration allows SVG SMIL elements such as animate, animateColor, animateMotion, animateTransform, or set. The issue applies to ApostropheCMS and sanitize-html usage described in the advisory.
What must an attacker and victim do for exploitation to succeed?
An attacker needs the ability to store sanitized SVG content containing a crafted SMIL values list and an animation targeting href or xlink:href. A victim must render that SVG and activate the resulting link for script to execute in the application's origin.
Is the default sanitizer behavior affected?
The issue requires a configuration that allows the specified SVG SMIL elements. The provided information does not establish whether those elements are allowed by default.
What version fixes the vulnerability?
Upgrade sanitize-html to version 2.17.7 or later. Version 2.17.7 is identified as the fix.