require-post-message-target-origin.md

require-post-message-target-origin

📝 Enforce using the targetOrigin argument with window.postMessage().

🚫 Disabled by default.

💡 This rule is manually fixable by editor suggestions.

Specifying the targetOrigin argument with window.postMessage() is a security best practice. It ensures that sensitive messages are only delivered to the intended recipient, preventing accidentally sending data to a window with an unexpected origin due to navigation or redirects.

This rule cannot distinguish between window.postMessage() and other calls like Worker#postMessage(), MessagePort#postMessage(), Client#postMessage(), and BroadcastChannel#postMessage(), which don't have a targetOrigin parameter.

Examples

// ❌ - Missing targetOrigin, message could be intercepted
window.postMessage(sensitiveData);

// ✅ - Explicit target origin prevents security issues
window.postMessage(sensitiveData, 'https://trusted-domain.com');

// ❌
window.postMessage({token: authToken});

// ✅ - Always specify the target origin
window.postMessage({token: authToken}, 'https://api.example.com');

// ✅ - '*' is acceptable when sending non-sensitive public data
window.postMessage({publicData: 'hello'}, '*');

// ❌ - Sending to iframe without specifying origin
const iframe = document.querySelector('iframe');
iframe.contentWindow.postMessage(data);

// ✅ - Specify the exact origin you expect
iframe.contentWindow.postMessage(data, 'https://expected-iframe-origin.com');