How to Fix SharedArrayBuffer Error in Browser: Complete Guide for 2026

The SharedArrayBuffer error is one of the most common security-related issues developers encounter when working with multithreaded JavaScript applications. This error occurs when attempting to use SharedArrayBuffer without proper cross-origin isolation, which was implemented as a security measure against Spectre attacks.

This comprehensive guide explains why this error occurs, how to fix it, and provides practical examples for different server environments and deployment platforms.

---

Understanding the SharedArrayBuffer Error

The error typically appears as:

DOMException: SharedArrayBuffer is not defined

Or:

DOMException: Cannot construct SharedArrayBuffer

Why This Error Occurs

• Security Mitigation: Browsers implemented cross-origin isolation to prevent Spectre attacks
• Cross-Origin Isolation Required: SharedArrayBuffer needs specific HTTP headers to function
• Modern Browser Policy: Chrome, Firefox, Safari enforce strict security policies

---

Prerequisites for SharedArrayBuffer

Before using SharedArrayBuffer, your application must have:

1. Cross-Origin Isolation: Proper HTTP headers
2. Secure Context: HTTPS or localhost
3. Same-Origin Policy: Resources must be same-origin or properly configured

---

Required HTTP Headers for Cross-Origin Isolation

Your server must send these headers:

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin

What These Headers Do

• Cross-Origin-Embedder-Policy: require-corp: Prevents loading resources that don’t explicitly allow cross-origin usage
• Cross-Origin-Opener-Policy: same-origin: Ensures your document is in a same-origin group

---

Step 1: Check Browser Support

First, verify if SharedArrayBuffer is available:

// Check if SharedArrayBuffer is supported
if (typeof SharedArrayBuffer === 'undefined') {
  console.error('SharedArrayBuffer is not supported in this environment');
  // Fallback to regular ArrayBuffer or other alternatives
} else {
  console.log('SharedArrayBuffer is available');
}

---

Step 2: Verify Cross-Origin Isolation Status

Check if your page is properly isolated:

// Check cross-origin isolation status
function checkCrossOriginIsolation() {
  if ('crossOriginIsolated' in window) {
    if (window.crossOriginIsolated) {
      console.log('✅ Cross-origin isolated: Ready to use SharedArrayBuffer');
      return true;
    } else {
      console.log('❌ Not cross-origin isolated: Need to configure headers');
      return false;
    }
  } else {
    console.log('⚠️ crossOriginIsolated property not available in this browser');
    return false;
  }
}

// Call the function to check status
checkCrossOriginIsolation();

---

Step 3: Server-Side Configuration (Node.js/Express)

Express.js with CORS Headers

const express = require('express');
const cors = require('cors');
const app = express();

// Middleware to add cross-origin isolation headers
app.use((req, res, next) => {
  res.setHeader('Cross-Origin-Embedder-Policy', 'require-corp');
  res.setHeader('Cross-Origin-Opener-Policy', 'same-origin');
  next();
});

// CORS middleware (if needed)
app.use(cors());

// Your routes
app.use(express.static('public'));

app.listen(3000, () => {
  console.log('Server running on port 3000 with cross-origin isolation headers');
});

Complete Express.js Example

const express = require('express');
const path = require('path');

const app = express();

// Cross-origin isolation headers middleware
app.use((req, res, next) => {
  res.setHeader('Cross-Origin-Embedder-Policy', 'require-corp');
  res.setHeader('Cross-Origin-Opener-Policy', 'same-origin');
  res.setHeader('Cross-Origin-Resource-Policy', 'same-site');
  next();
});

// Serve static files
app.use(express.static(path.join(__dirname, 'public')));

// API endpoint that might use SharedArrayBuffer
app.get('/api/data', (req, res) => {
  res.json({ message: 'Cross-origin isolated endpoint' });
});

app.listen(process.env.PORT || 3000);

---

Step 4: Server-Side Configuration (Apache)

Add these lines to your .htaccess file:

# Cross-origin isolation headers for Apache
Header always set Cross-Origin-Embedder-Policy "require-corp"
Header always set Cross-Origin-Opener-Policy "same-origin"
Header always set Cross-Origin-Resource-Policy "same-site"

Or in your Apache virtual host configuration:

<VirtualHost *:80>
    ServerName yourdomain.com
    DocumentRoot /path/to/your/app

    # Cross-origin isolation headers
    Header always set Cross-Origin-Embedder-Policy "require-corp"
    Header always set Cross-Origin-Opener-Policy "same-origin"
    Header always set Cross-Origin-Resource-Policy "same-site"
</VirtualHost>

---

Step 5: Server-Side Configuration (Nginx)

Add these directives to your Nginx configuration:

server {
    listen 80;
    server_name yourdomain.com;
    root /path/to/your/app;
    index index.html;

    # Cross-origin isolation headers
    add_header Cross-Origin-Embedder-Policy "require-corp" always;
    add_header Cross-Origin-Opener-Policy "same-origin" always;
    add_header Cross-Origin-Resource-Policy "same-site" always;

    location / {
        try_files $uri $uri/ /index.html;
    }
}

---

Step 6: Deployment Platform Configurations

Netlify Configuration

Create a netlify.toml file in your project root:

[[headers]]
  for = "/*"
  [headers.values]
    Cross-Origin-Embedder-Policy = "require-corp"
    Cross-Origin-Opener-Policy = "same-origin"
    Cross-Origin-Resource-Policy = "same-site"

Vercel Configuration

Add to your vercel.json:

{
  "headers": [
    {
      "source": "/(.*)",
      "headers": [
        {
          "key": "Cross-Origin-Embedder-Policy",
          "value": "require-corp"
        },
        {
          "key": "Cross-Origin-Opener-Policy",
          "value": "same-origin"
        },
        {
          "key": "Cross-Origin-Resource-Policy",
          "value": "same-site"
        }
      ]
    }
  ]
}

Cloudflare Workers Example

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
});

async function handleRequest(request) {
  const response = await fetch(request);
  
  const newHeaders = new Headers(response.headers);
  newHeaders.set('Cross-Origin-Embedder-Policy', 'require-corp');
  newHeaders.set('Cross-Origin-Opener-Policy', 'same-origin');
  newHeaders.set('Cross-Origin-Resource-Policy', 'same-site');

  return new Response(response.body, {
    status: response.status,
    statusText: response.statusText,
    headers: newHeaders
  });
}

---

Step 7: Practical SharedArrayBuffer Implementation

Once cross-origin isolation is configured, you can use SharedArrayBuffer:

// Example: Using SharedArrayBuffer with Web Workers
function createSharedBufferExample() {
  if (typeof SharedArrayBuffer !== 'undefined' && window.crossOriginIsolated) {
    // Create a shared buffer (4 bytes)
    const sharedBuffer = new SharedArrayBuffer(4);
    const sharedArray = new Int32Array(sharedBuffer);
    
    // Initialize the buffer
    Atomics.store(sharedArray, 0, 0);
    
    console.log('✅ SharedArrayBuffer created successfully');
    console.log('Initial value:', Atomics.load(sharedArray, 0));
    
    // Example of atomic operations
    Atomics.add(sharedArray, 0, 1);
    console.log('After increment:', Atomics.load(sharedArray, 0));
    
    return { sharedBuffer, sharedArray };
  } else {
    console.warn('SharedArrayBuffer not available, using regular ArrayBuffer as fallback');
    const buffer = new ArrayBuffer(4);
    const array = new Int32Array(buffer);
    return { buffer, array };
  }
}

// Call the function
const buffers = createSharedBufferExample();

---

Step 8: Web Workers with SharedArrayBuffer

Main Thread (main.js)

// main.js
async function setupSharedBufferWithWorker() {
  if (!window.crossOriginIsolated) {
    console.error('Cannot use SharedArrayBuffer: Not cross-origin isolated');
    return;
  }

  // Create shared buffer
  const sharedBuffer = new SharedArrayBuffer(1024);
  const sharedArray = new Int32Array(sharedBuffer);

  // Create worker
  const worker = new Worker('worker.js');

  // Send shared buffer to worker
  worker.postMessage({ buffer: sharedBuffer });

  // Listen for messages from worker
  worker.onmessage = function(e) {
    console.log('Message from worker:', e.data);
  };

  // Periodically check the shared array value
  setInterval(() => {
    console.log('Main thread - Shared value:', Atomics.load(sharedArray, 0));
  }, 1000);
}

Web Worker (worker.js)

// worker.js
let sharedArray;

self.onmessage = function(e) {
  if (e.data.buffer) {
    sharedArray = new Int32Array(e.data.buffer);
    
    // Start incrementing the shared value
    setInterval(() => {
      if (sharedArray) {
        Atomics.add(sharedArray, 0, 1);
        self.postMessage(`Worker updated value to: ${Atomics.load(sharedArray, 0)}`);
      }
    }, 500);
  }
};

---

Step 9: Testing Cross-Origin Isolation

Create a test page to verify your configuration:

<!DOCTYPE html>
<html>
<head>
    <title>SharedArrayBuffer Test</title>
</head>
<body>
    <h1>SharedArrayBuffer Cross-Origin Isolation Test</h1>
    <div id="status"></div>
    <button onclick="testSharedArrayBuffer()">Test SharedArrayBuffer</button>

    <script>
        function updateStatus(message, isError = false) {
            const statusDiv = document.getElementById('status');
            statusDiv.innerHTML = `<p style="${isError ? 'color: red;' : 'color: green;'}">${message}</p>`;
        }

        function checkIsolation() {
            if ('crossOriginIsolated' in window) {
                if (window.crossOriginIsolated) {
                    updateStatus('✅ Cross-origin isolated: Ready to use SharedArrayBuffer');
                    return true;
                } else {
                    updateStatus('❌ Not cross-origin isolated: Configure server headers', true);
                    return false;
                }
            } else {
                updateStatus('⚠️ crossOriginIsolated property not available', true);
                return false;
            }
        }

        function testSharedArrayBuffer() {
            if (typeof SharedArrayBuffer !== 'undefined' && checkIsolation()) {
                try {
                    const sab = new SharedArrayBuffer(8);
                    updateStatus('✅ SharedArrayBuffer created successfully!');
                    
                    // Test atomic operations
                    const intArray = new Int32Array(sab);
                    Atomics.store(intArray, 0, 42);
                    updateStatus('✅ SharedArrayBuffer and Atomics working correctly!');
                } catch (error) {
                    updateStatus(`❌ Error creating SharedArrayBuffer: ${error.message}`, true);
                }
            } else {
                updateStatus('❌ SharedArrayBuffer not supported or not cross-origin isolated', true);
            }
        }

        // Check isolation on page load
        window.addEventListener('load', checkIsolation);
    </script>
</body>
</html>

---

Directory Structure

project-root/
├── public/
│   ├── index.html
│   ├── main.js
│   └── worker.js
├── server.js (for Node.js)
├── .htaccess (for Apache)
├── nginx.conf (for Nginx)
├── netlify.toml (for Netlify)
└── vercel.json (for Vercel)

---

Common Issues and Troubleshooting

Issue 1: Headers Not Being Set

• Solution: Verify server configuration and restart the server
• Check: Use browser dev tools Network tab to verify headers

Issue 2: Local Development

• Solution: Use HTTPS locally or proper headers even on localhost
• Note: localhost is considered secure context

Issue 3: Mixed Content

• Solution: Ensure all resources are served with proper headers
• Check: All iframes, scripts, and assets need same headers

---

Alternative Solutions

If you cannot configure cross-origin isolation:

1. Regular ArrayBuffer

// Fallback to regular ArrayBuffer
const buffer = new ArrayBuffer(1024);
const array = new Int32Array(buffer);

2. Message Passing

// Use postMessage for communication instead of shared memory
worker.postMessage(data);

---

Security Considerations

• Only use when necessary: SharedArrayBuffer has security implications
• Validate data: Always validate data passed through shared buffers
• Minimize scope: Keep shared buffers as small as possible
• Monitor usage: Be aware of potential side-channel attacks

---

Performance Benefits

When properly configured:

• Faster communication: Between main thread and workers
• Reduced copying: No need to clone large data structures
• Real-time updates: Immediate synchronization between threads
• Better UX: Smoother UI performance with background processing

---

Conclusion

The SharedArrayBuffer error is primarily a server configuration issue related to cross-origin isolation policies. By implementing the correct HTTP headers (Cross-Origin-Embedder-Policy and Cross-Origin-Opener-Policy), you can enable SharedArrayBuffer functionality in modern browsers.

Remember to:

• Configure your server with proper headers
• Test cross-origin isolation status
• Implement fallbacks for unsupported environments
• Follow security best practices

With proper configuration, SharedArrayBuffer enables powerful multithreaded JavaScript applications with improved performance and user experience.

---