CVE-2026-65898

Versions of handlebars prior to 3.0.8 or 4.5.3 are vulnerable to prototype pollution. It is possible to add or modify properties to the Object prototype through a malicious template. This may allow attackers to crash the application or execute Arbitrary Code in specific conditions.

Recommendation

Upgrade to version 3.0.8, 4.5.3 or later.

Versions of handlebars prior to 3.0.8 or 4.5.3 are vulnerable to Arbitrary Code Execution. The package's lookup helper fails to properly validate templates, allowing attackers to submit templates that execute arbitrary JavaScript in the system. It is due to an incomplete fix for a previous issue. This vulnerability can be used to run arbitrary code in a server processing Handlebars templates or on a victim's browser (effectively serving as Cross-Site Scripting).

Recommendation

Upgrade to version 3.0.8, 4.5.3 or later.

DOMPurify before 3.4.11 fails to clone the ALLOWED_ATTR allowlist when setConfig() is used with an uponSanitizeAttribute hook, allowing the hook to permanently mutate the shared allowlist. Attackers can register a hook that conditionally allows dangerous attributes like onerror for trusted elements, then submit untrusted content that inherits the polluted allowlist and executes event handlers as stored XSS.

DOMPurify before 3.4.11 fails to clone the ALLOWED_ATTR allowlist when setConfig() is used with an uponSanitizeAttribute hook, allowing the hook to permanently mutate the shared allowlist. Attackers can register a hook that conditionally allows dangerous attributes like onerror for trusted elements, then submit untrusted content that inherits the polluted allowlist and executes event handlers as stored XSS.

DOMPurify before 3.4.11 fails to clone the ALLOWED_ATTR allowlist when setConfig() is used with an uponSanitizeAttribute hook, allowing the hook to permanently mutate the shared allowlist. Attackers can register a hook that conditionally allows dangerous attributes like onerror for trusted elements, then submit untrusted content that inherits the polluted allowlist and executes event handlers as stored XSS.

Versions of handlebars prior to 3.0.8 or 4.5.2 are vulnerable to Arbitrary Code Execution. The package's lookup helper fails to properly validate templates, allowing attackers to submit templates that execute arbitrary JavaScript in the system. It can be used to run arbitrary code in a server processing Handlebars templates or on a victim's browser (effectively serving as Cross-Site Scripting).

The following template can be used to demonstrate the vulnerability:

	{{#with split as |a|}}
		{{pop (push "alert('Vulnerable Handlebars JS');")}}
		{{#with (concat (lookup join (slice 0 1)))}}
			{{#each (slice 2 3)}}
				{{#with (apply 0 a)}}
					{{.}}
				{{/with}}
			{{/each}}
		{{/with}}
	{{/with}}
{{/with}}```


## Recommendation

Upgrade to version 3.0.8, 4.5.2 or later.

Summary

During IN_PLACE sanitization, a hook that removes an element can leave that element's detached descendants executable. A descendant image can retain its attacker-provided onload handler and fire after sanitize() returns, even though the returned root is clean and the image remains disconnected from the document.

Details

In DOMPurify 3.4.12, _sanitizeElements() in src/purify.ts:1862-1904 runs the beforeSanitizeElements or uponSanitizeElement hook and returns immediately when the hook detached the current node. The return does not call _neutralizeSubtree(currentNode).

The detached subtree is not added to DOMPurify.removed, so the post-walk IN_PLACE neutralization cannot reach it. If the browser queued a resource event while the application constructed the detached dirty root, a descendant can therefore retain its handler and execute after sanitization.

The hook only rejects the containing element and does not add or approve the event handler. DOMPurify's ordinary removal path de-arms the same queued event; only the hook-detachment early return skips the existing subtree neutralization.

PoC

Load the published dompurify@3.4.12 dist/purify.js before this script in Chromium:

<div id="result">not fired</div>
<script>
const root = document.createElement('div');
root.innerHTML = `
  <footer>
    <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"
         onload="result.textContent = 'XSS after sanitize'">
  </footer>
  <div>safe</div>`;

DOMPurify.setConfig({
  ALLOWED_TAGS: ['div', '#text', 'footer'],
  IN_PLACE: true
});
DOMPurify.addHook('uponSanitizeElement', node => {
  if (node.tagName === 'FOOTER') node.remove();
});

DOMPurify.sanitize(root);
document.body.append(root);
</script>

sanitize() returns with no handler execution and the returned root contains only the safe div. After the event loop advances, the original image remains disconnected but its retained onload changes the page to XSS after sanitize.

As the claim-matched control, use the same detached input with ALLOWED_TAGS: ['div', '#text'] and no hook. DOMPurify's ordinary removal path removes the original image's handler, the returned root is still <div>safe</div>, and the marker does not fire.

Impact

In an application that uses IN_PLACE with the documented element-removal hook pattern, an attacker who can supply HTML can execute JavaScript in the integrating application's origin after the application sanitizes and renders that content.

The required non-default configuration is IN_PLACE plus a hook that removes a containing element. The hook does not add or approve the event handler, and the dirty root never needs to be connected before sanitization.

Suggested fix

Reuse the existing _neutralizeSubtree(currentNode) helper before returning from both hook-detachment branches in _sanitizeElements(). Add regressions for beforeSanitizeElements and uponSanitizeElement that retain a reference to a descendant resource element and verify that its event handler is removed after the hook detaches its ancestor.

Summary

DOMPurify 3.4.7 shipped a security fix ("permanent hook pollution") that makes a registered uponSanitizeAttribute hook's mutation of data.allowedAttributes non-persistent — so allowing an attribute for one element does not leak into later sanitize() calls. The fix clones ALLOWED_ATTR inside _parseConfig.

That guard is silently bypassed whenever the application uses the persistent-config API DOMPurify.setConfig(). setConfig() sets the module flag SET_CONFIG = true, which causes sanitize() to skip _parseConfig entirely — and the clone-guard lives inside _parseConfig. The hook is then handed the live, shared ALLOWED_ATTR object; any data.allowedAttributes[name] = true it writes mutates that shared object permanently, for the lifetime of the DOMPurify instance, across every subsequent call, and across all elements.

If an application uses setConfig() together with an uponSanitizeAttribute hook that conditionally allows a dangerous attribute (onerror, onclick, onmouseover, srcdoc, formaction, …) for "trusted" elements, then one trusted render permanently allows that attribute on untrusted, attacker-controlled content — yielding stored XSS in viewers' browsers. DOMPurify applies no separate /^on/ event-handler blocklist: attribute stripping is governed entirely by the allowlist, so a polluted allowlist is the only gate, and survival in the output is final.

Affected configuration (preconditions)

The vulnerability is triggered when an application does both:

1. Calls DOMPurify.setConfig(...) once (the recommended pattern for a fixed, persistent policy), and
2. Registers an uponSanitizeAttribute hook that writes data.allowedAttributes[name] = true to conditionally allow an attribute (e.g. only for elements bearing a trust marker).

This hook pattern is demonstrated in DOMPurify's own test suite, and the per-call variant of exactly this leak is what 3.4.7 was released to fix.

Root cause (source: src/purify.ts, v3.4.10)

The 3.4.7 clone-guard — only inside _parseConfig:

// src/purify.ts  _parseConfig()  (lines ~950-968)
// "if a hook is registered AND the set still points at the default constant, clone it.
//  The hook then mutates the clone ... and the next default-cfg call rebinds to the untouched original."
if ( ... && hooks.uponSanitizeAttribute.length > 0) {
  ALLOWED_TAGS = clone(ALLOWED_TAGS);          // line 961
}
if ( ... hooks.uponSanitizeAttribute.length > 0 ... ) {
  ALLOWED_ATTR = clone(ALLOWED_ATTR);          // line 968
}

sanitize() skips _parseConfig on the persistent-config path:

// src/purify.ts  DOMPurify.sanitize()  (line 2369)
if (!SET_CONFIG) {
  _parseConfig(cfg);          // <-- clone-guard lives in here; SKIPPED when SET_CONFIG is true
}

setConfig() sets the flag that disables the guard:

// src/purify.ts  (lines 2596-2598)
DOMPurify.setConfig = function (cfg = {}) {
  _parseConfig(cfg);
  SET_CONFIG = true;          // every later sanitize() now skips _parseConfig
};

The hook is handed the live allowlist binding, and there is no secondary event-handler defense:

// src/purify.ts (line 2088) — hook event exposes the shared object by reference
allowedAttributes: ALLOWED_ATTR,
// (line 2108) hooks.uponSanitizeAttribute executed; a write to data.allowedAttributes mutates ALLOWED_ATTR itself
// _isValidAttribute gates purely on ALLOWED_ATTR[lcName]; DOMPurify uses NO /^on/ blocklist by design.

Net: after setConfig(), the clone-guard never runs, so the hook's allowedAttributes mutation is a permanent write to the instance's shared ALLOWED_ATTR.

Proof of Concept

Environment: npm i dompurify@3.4.10 jsdom (Node; identical mechanism to isomorphic-dompurify, and to a browser instance).

PoC 1 — the leak (trusted render permanently allows onerror on attacker content)

const createDOMPurify =…

Versions of handlebars prior to 4.0.14 are vulnerable to Prototype Pollution. Templates may alter an Objects' prototype, thus allowing an attacker to execute arbitrary code on the server.

Recommendation

For handlebars 4.1.x upgrade to 4.1.2 or later. For handlebars 4.0.x upgrade to 4.0.14 or later.

DOMPurify before 3.4.11 fails to clone the ALLOWED_ATTR allowlist when setConfig() is used with an uponSanitizeAttribute hook, allowing the hook to permanently mutate the shared allowlist. Attackers can register a hook that conditionally allows dangerous attributes like onerror for trusted elements, then submit untrusted content that inherits the polluted allowlist and executes event handlers as stored XSS.

TuxCare rebuilt handlebars to address 1 CVE. Fixed in 1.3.0-tuxcare.1.

TuxCare rebuilt handlebars to address 3 CVEs. Fixed in 1.0.12-tuxcare.2.

TuxCare rebuilt dompurify to address 5 CVEs. Fixed in 2.5.8-tuxcare.4.

TuxCare rebuilt dompurify to address 7 CVEs. Fixed in 3.0.3-tuxcare.2.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 2.4.7-tuxcare.2.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 2.4.3-tuxcare.4.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 2.4.0-tuxcare.2.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 2.3.0-tuxcare.3.

TuxCare rebuilt dompurify to address 8 CVEs. Fixed in 3.2.3-tuxcare.2.

TuxCare rebuilt dompurify to address 10 CVEs. Fixed in 2.3.0-tuxcare.1.

TuxCare rebuilt dompurify to address 16 CVEs. Fixed in 2.4.0-tuxcare.1.

TuxCare rebuilt dompurify to address 11 CVEs. Fixed in 2.4.3-tuxcare.1.

TuxCare rebuilt dompurify to address 14 CVEs. Fixed in 2.4.7-tuxcare.1.

TuxCare rebuilt dompurify to address 10 CVEs. Fixed in 3.2.3-tuxcare.3.

TuxCare rebuilt dompurify to address 34 CVEs. Fixed in 3.0.3-tuxcare.1.

TuxCare rebuilt dompurify to address 7 CVEs. Fixed in 3.1.6-tuxcare.7.

TuxCare rebuilt dompurify to address 2 CVEs. Fixed in 3.4.8-tuxcare.2.

TuxCare rebuilt dompurify to address 10 CVEs. Fixed in 3.2.7-tuxcare.1.

TuxCare rebuilt dompurify to address 11 CVEs. Fixed in 2.3.0-tuxcare.5.

TuxCare rebuilt dompurify to address 9 CVEs. Fixed in 2.4.0-tuxcare.5.

TuxCare rebuilt dompurify to address 12 CVEs. Fixed in 3.0.3-tuxcare.5.

TuxCare rebuilt dompurify to address 9 CVEs. Fixed in 2.4.7-tuxcare.5.

TuxCare rebuilt dompurify to address 12 CVEs. Fixed in 2.4.3-tuxcare.8.

TuxCare rebuilt dompurify to address 8 CVEs. Fixed in 2.5.8-tuxcare.7.

TuxCare rebuilt dompurify to address 9 CVEs. Fixed in 3.4.5-tuxcare.2.

TuxCare rebuilt dompurify to address 2 CVEs. Fixed in 3.4.9-tuxcare.2.

TuxCare rebuilt dompurify to address 12 CVEs. Fixed in 3.1.3-tuxcare.2.

TuxCare rebuilt dompurify to address 5 CVEs. Fixed in 2.3.3-tuxcare.2.

TuxCare rebuilt dompurify to address 9 CVEs. Fixed in 3.1.2-tuxcare.2.

TuxCare rebuilt dompurify to address 11 CVEs. Fixed in 3.1.4-tuxcare.2.

TuxCare rebuilt dompurify to address 9 CVEs. Fixed in 3.0.2-tuxcare.2.

TuxCare rebuilt dompurify to address 6 CVEs. Fixed in 2.0.8-tuxcare.2.

TuxCare rebuilt dompurify to address 10 CVEs. Fixed in 3.0.9-tuxcare.2.

TuxCare rebuilt dompurify to address 11 CVEs. Fixed in 3.0.8-tuxcare.2.

TuxCare rebuilt dompurify to address 5 CVEs. Fixed in 2.3.10-tuxcare.2.

TuxCare rebuilt dompurify to address 10 CVEs. Fixed in 3.0.1-tuxcare.2.

TuxCare rebuilt dompurify to address 13 CVEs. Fixed in 3.1.0-tuxcare.2.

TuxCare rebuilt dompurify to address 12 CVEs. Fixed in 3.0.11-tuxcare.2.

TuxCare rebuilt dompurify to address 5 CVEs. Fixed in 3.4.6-tuxcare.2.

TuxCare rebuilt dompurify to address 4 CVEs. Fixed in 3.4.7-tuxcare.2.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.10-tuxcare.2.