React Router is a widely used routing library for React applications. It allows developers to have a seamless and efficient way of navigating between different components within their application, creating rich and engaging user experiences. However, a recently discovered vulnerability has brought light to a potential security threat to a vast number of applications.

The vulnerability, identified by CVE ID CVE-2025-43864, affects React Router versions 7.2. through 7.5.1. It exploits an issue that could force applications built using React Router to switch from server-side rendering (SSR) to single-page application (SPA) mode. To understand this issue, let's dig deeper into the mechanics of the flaw and how it impacts the affected applications.

Details of the Vulnerability

When an application built using React Router is forced to switch from SSR to SPA mode by the attacker, it can trigger an error that corrupts the entire application page. Since this vulnerability is highly detrimental to an application's availability, it poses a significant risk to users and their data.

What's even more alarming is that this vulnerability allows attackers to poison the cache and store the corrupted version of the application. This cache poisoning can significantly impact the availability of the affected application, causing users to experience an error whenever they access the compromised page.

To demonstrate how this vulnerability can be exploited, let's consider the following code snippet

const onRequest = (req, res) => {
  const context = {};
  const routeMarkup = ReactDOMServer.renderToString(
    <StaticRouter location={req.url} context={context}>
      <App />
    </StaticRouter>
  );

  // Check if the application switched to SPA mode
  if (context.url) {
    res.redirect(context.url);
  } else {
    res.send(`
      <!doctype html>
      <html lang="en">
        <head>
          <meta charset="utf-8">
          <title>React Router Vulnerability Demo</title>
        </head>
        <body>
          <div id="root">${routeMarkup}</div>
          <script src="/bundle.js"></script>
        </body>
      </html>
    `);
  }
};

In this example, the onRequest function handles incoming requests and determines whether to render the application in SSR or SPA mode. The attacker can force the application to switch to SPA mode by including a malicious header in their request like so:

GET /myapplication HTTP/1.1
Host: vulnerableapplication.com
my-malicious-header: trigger-spa

This will cause an error in the application, leading to cache poisoning and disrupting the app's availability.

Mitigation and Patch

The issue has been patched in React Router version 7.5.2. It is strongly recommended that developers using React Router update their applications to the latest version to avoid this vulnerability. In addition, it is crucial for developers to be aware of the risks associated with cache poisoning and take preventive measures against such attacks.

You can find the official patch and more information in the React Router repository on GitHub

- React Router Repository

- React Router 7.5.2 Release

In conclusion, the CVE-2025-43864 vulnerability poses a serious risk to applications built using React Router versions 7.2. through 7.5.1. To protect your applications and users, it's essential to update to the latest version of React Router and stay vigilant to potential security risks.

Timeline

Published on: 04/25/2025 01:15:43 UTC
Last modified on: 04/29/2025 13:52:28 UTC