CVE-2025-43865 is a medium-severity vulnerability affecting the React Router library, specifically versions from 7.0 up to but not including 7.5.2. React Router is a widely used routing library for React applications, responsible for managing navigation and rendering components based on URL paths. The vulnerability arises from insufficient verification of data authenticity (CWE-345) in the handling of pre-rendered data. An attacker can exploit this flaw by injecting a specially crafted HTTP header into a request, which allows them to completely spoof the contents of the pre-rendered data object passed to the HTML. This means that all values within this data object can be manipulated by the attacker. Such manipulation can lead to the injection of malicious content, potentially enabling cross-site scripting (XSS), content spoofing, or other client-side attacks that compromise the integrity and trustworthiness of the rendered web page. The issue does not require authentication, and exploitation can be performed remotely by sending crafted requests to vulnerable applications. The vulnerability was patched in React Router version 7.5.2, and users of affected versions are strongly advised to upgrade. There are no known exploits in the wild at the time of publication, but the presence of this flaw in a popular frontend routing library makes it a significant risk for web applications relying on React Router for client-side routing and data hydration.

For European organizations, the impact of this vulnerability can be substantial, especially for those operating web applications built with React and using the affected versions of React Router. The ability to spoof pre-rendered data compromises the integrity of web content, potentially leading to client-side attacks such as XSS, which can result in session hijacking, user impersonation, or the delivery of malicious payloads to end users. This undermines user trust and can lead to data breaches or regulatory non-compliance, particularly under GDPR where data integrity and protection are critical. Organizations in sectors such as finance, e-commerce, healthcare, and government services are particularly at risk due to the sensitive nature of their data and the high reliance on web applications. Additionally, the vulnerability could be leveraged for phishing or social engineering attacks by manipulating displayed content. The lack of authentication requirement and ease of exploitation increase the risk of widespread abuse. While no active exploits are reported, the popularity of React Router means that many European organizations could be vulnerable if they have not applied the patch, potentially affecting millions of users.

1. Immediate upgrade to React Router version 7.5.2 or later to ensure the vulnerability is patched. 2. Implement strict Content Security Policy (CSP) headers to mitigate the impact of potential XSS attacks resulting from data spoofing. 3. Validate and sanitize all data received from client requests, including headers, on the server side before rendering or passing to the client. 4. Employ runtime application self-protection (RASP) or web application firewalls (WAFs) configured to detect and block anomalous headers or suspicious request patterns targeting React Router endpoints. 5. Conduct thorough code reviews and security testing focusing on client-side data hydration and rendering logic to identify and remediate similar data authenticity issues. 6. Educate development teams on secure coding practices related to client-side routing and data handling in React applications. 7. Monitor application logs for unusual header injection attempts or unexpected data modifications. These steps go beyond generic patching by emphasizing layered defenses and proactive detection to reduce exploitation risk.