React.js RCE: How Exploits Happen & How to Prevent

Why “RCE in React” is a Real Risk

React runs in the browser, but teams often pair it with Node.js (SSR/APIs), third-party packages, and build tools. Attackers abuse these touchpoints to turn a simple bug into Remote Code Execution (RCE) on the server, the CI runner, or even in hybrid shells like Electron. This guide explains the main paths and how to stop them. For more security insights, see our blog: Pentest Testing Blog.

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How RCE Reaches React Projects

1. DOM XSS → Code Execution
   Untrusted HTML + dangerouslySetInnerHTML lets attackers run arbitrary JS, which can pivot to RCE in Electron or backend bridges.

// ❌ Vulnerable: untrusted HTML dropped into the DOM
export default function Post({ html }) {
  return <div dangerouslySetInnerHTML={{ __html: html }} />;
}

// ✅ Safer: sanitize first
import DOMPurify from 'dompurify';
export function SafePost({ html }) {
  const clean = DOMPurify.sanitize(html, { USE_PROFILES: { html: true } });
  return <div dangerouslySetInnerHTML={{ __html: clean }} />;
}

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2. Supply-Chain (npm) → Build/Runtime RCE
   A malicious package script (e.g., postinstall) or compromised dependency can execute system commands during install or build. Lockfile hygiene and provenance checks are critical.

# ✅ Lock your tree
npm ci
npm audit --production
npx npm-why <pkg>

3. SSR/Template Injection → Server RCE
   Rendering untrusted strings in Node can become code execution when unsafe APIs are exposed.

// ❌ Dangerous SSR helper
app.get('/page', (req, res) => {
  const { template } = req.query; // attacker-controlled
  res.send(renderToString(eval(template))); // never eval()
});

// ✅ Safe render (whitelist components)
const templates = { home: <Home/>, about: <About/> };
app.get('/page', (req, res) => {
  const key = String(req.query.template || 'home');
  const view = templates[key] ?? templates.home;
  res.send(renderToString(view));
});

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Sample vulnerability assessment report generated with our free tool, providing insights into possible vulnerabilities.

4. Prototype Pollution → Unexpected Exec Paths
   Polluting objects that later reach templating or command layers can open RCE chains. Always deep-clone and validate merge inputs.

// ✅ Harden merges
import structuredClone from '@ungap/structured-clone';
function safeMerge(a, b) {
  const A = structuredClone(a);
  const B = structuredClone(b);
  return Object.assign({}, A, B); // or a vetted deep-merge that guards __proto__
}

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Prevention Checklist (Copy/Paste)

• Avoid dangerouslySetInnerHTML; if unavoidable, sanitize + CSP (script-src 'self' + nonces).

• Pin dependencies, review diff on updates, enable npm ci, provenance/SLSA if possible.

• Never eval or run dynamic code on the server; whitelist SSR views.

• Validate/encode all inputs; deep-clone before merges; block __proto__ and constructor keys.

• Run SCA/SAST in CI; scan staging with our free tool: Website Security Checker.

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