Recent discoveries have unveiled significant security flaws in React Router, a widely adopted routing library for React applications. These vulnerabilities could allow attackers to manipulate content, poison caches, and alter pre-rendered data, posing substantial risks to affected applications.
Overview of the Vulnerabilities
The identified issues impact applications operating in Framework mode with server-side rendering (SSR) and utilizing loaders. Notably, these vulnerabilities can be exploited remotely without requiring user interaction or special privileges. React Router versions from 7.0 up to 7.5.1 are affected. The development team has addressed these flaws in version 7.5.2; however, numerous applications remain vulnerable until they are updated.
Detailed Examination of the Vulnerabilities
1. Cache Poisoning via Forced SPA Mode (CVE-2025-43864)
This vulnerability, assigned a CVSS score of 7.5, enables attackers to force SSR applications to switch to single-page application (SPA) mode by injecting a malicious header. By adding the `X-React-Router-SPA-Mode` header to a request targeting a page that uses a loader, an error is triggered, corrupting the page content. If the application employs a caching mechanism, this corrupted response can be stored and served to subsequent users, effectively poisoning the cache and leading to a denial of service condition.
Risk Factors:
– Affected Products: React Router (npm package) versions 7.2.0 through 7.5.1
– Impact: Denial of Service (DoS)
– Exploit Prerequisites:
– Application must use React Router in Framework mode
– Target page must use a loader
– No privileges needed
– No user interaction required
– Attacker can inject `X-React-Router-SPA-Mode` header via network request
– CVSS 3.1 Score: 7.5 (High)
2. Pre-Render Data Spoofing (CVE-2025-43865)
The second vulnerability, with a CVSS score of 8.2, allows attackers to manipulate pre-rendered data by injecting a specially crafted `X-React-Router-Prerender-Data` header. This attack vector enables complete content spoofing by modifying values in the data object passed to HTML before it reaches users. The impact is significant, potentially enabling content manipulation, cache poisoning attacks, and possible stored cross-site scripting (XSS) vulnerabilities, depending on how client-side data handling is implemented.
Risk Factors:
– Affected Products: React Router (npm package) versions 7.0 through 7.5.1
– Impact: Cache poisoning with arbitrary data
– Exploit Prerequisites:
– Application must use React Router in Framework mode
– Target page must use a loader
– Attacker can inject `X-React-Router-Prerender-Data` header with crafted JSON
– No privileges or user interaction required
– CVSS 3.1 Score: 8.2 (High)
Understanding React Router’s Framework Mode
React Router’s Framework mode offers a hybrid approach that combines SPA concepts with server-side rendering. By default, it enables SSR but can be configured to run in SPA mode by setting `ssr: false` in the `react-router.config.ts` file. The vulnerabilities specifically target applications using loaders, which are responsible for data fetching in React Router applications.
Mitigation and Recommendations
To protect applications from these vulnerabilities, developers should take the following steps:
1. Update React Router: Upgrade to version 7.5.2 or later, where these vulnerabilities have been patched.
2. Review Application Configuration: Ensure that the application’s configuration aligns with security best practices, particularly concerning SSR and SPA modes.
3. Implement Security Headers: Utilize security headers to prevent unauthorized header injections. For example, configuring the server to reject requests containing unexpected headers can mitigate such attacks.
4. Regular Security Audits: Conduct periodic security assessments to identify and address potential vulnerabilities promptly.
5. Monitor Application Behavior: Implement monitoring tools to detect unusual activities that may indicate exploitation attempts.
Conclusion
The discovery of these critical vulnerabilities in React Router underscores the importance of maintaining up-to-date dependencies and adhering to security best practices. Developers are urged to promptly update their applications to the latest version of React Router and implement the recommended security measures to safeguard against potential exploits.
