How to Secure useSyncExternalStore in React

The useSyncExternalStore hook is a powerful tool in React for subscribing to external data sources, but improper implementation can lead to memory leaks, performance bottlenecks, and security vulnerabilities. This article explains how to secure your implementation of useSyncExternalStore by ensuring proper subscription cleanup, maintaining referential stability, validating external data, and safely handling server-side rendering (SSR).

1. Prevent Memory Leaks with Proper Subscription Cleanup

The subscription function passed to useSyncExternalStore must return a cleanup function. If you fail to clean up the subscription, the callback remains registered in the external store. This leads to memory leaks and unexpected behavior when components mount and unmount.

// Secure implementation with cleanup
const subscribe = (callback) => {
  externalStore.subscribe(callback);
  
  // Always return a cleanup function
  return () => {
    externalStore.unsubscribe(callback);
  };
};

2. Maintain Referential Stability of Callbacks

React uses the identity of the subscribe and getSnapshot functions to determine if it needs to resubscribe or re-evaluate the store. If you pass inline anonymous functions, React will resubscribe on every single render. This causes severe performance degradation and can trigger infinite render loops.

To secure your performance and application stability: * Declare subscribe and getSnapshot outside the component if they do not depend on component props. * If they depend on props or state, wrap them in useCallback or use a memoized selector.

// Secure: Functions declared outside the component
const subscribe = (callback) => {
  window.addEventListener('online', callback);
  return () => window.removeEventListener('online', callback);
};

const getSnapshot = () => navigator.onLine;

function ConnectionStatus() {
  const isOnline = useSyncExternalStore(subscribe, getSnapshot);
  return <div>Status: {isOnline ? 'Online' : 'Offline'}</div>;
}

3. Sanitize and Validate External Store Data

Because the data source is “external” (such as localStorage, a WebSocket, or a global window object), it can be manipulated by malicious actors or browser extensions. Treating this data as trusted without validation can expose your application to Cross-Site Scripting (XSS) or prototype pollution.

Always validate and sanitize the data inside getSnapshot before returning it to React.

import { z } from 'zod';

// Define a schema to validate the external data
const UserThemeSchema = z.union([z.literal('light'), z.literal('dark')]);

const getSnapshot = () => {
  const rawData = localStorage.getItem('theme');
  
  // Validate and provide a fallback if the data is tampered with
  const result = UserThemeSchema.safeParse(rawData);
  return result.success ? result.data : 'light';
};

4. Prevent Hydration Mismatches in Server-Side Rendering (SSR)

If your application uses SSR (like Next.js or Remix), the client-side state of an external store (like window.innerWidth) might differ from the server-rendered HTML. This causes hydration mismatches, which can break the UI or create visual bugs.

To secure the hydration process, always provide the third argument: getServerSnapshot. This function should return a static, safe default value that matches what the server rendered.

const subscribe = (callback) => {
  window.addEventListener('resize', callback);
  return () => window.removeEventListener('resize', callback);
};

// Client snapshot
const getSnapshot = () => window.innerWidth;

// Server snapshot (safe fallback for SSR)
const getServerSnapshot = () => 1024; 

function ResponsiveComponent() {
  const width = useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
  return <div>Viewport Width: {width}px</div>;
}