CVE-2025-31137: 
JavaScript vulnerability analysis and mitigation

A critical vulnerability (CVE-2025-31137) has been identified in React Router, a multi-strategy router for React bridging the gap from React 18 to React 19. The vulnerability affects all Remix 2 and React Router 7 consumers using the Express adapter. Discovered and disclosed on April 1, 2025, this security flaw allows attackers to spoof URLs in incoming requests through manipulation of the Host or X-Forwarded-Host headers. The issue has been addressed with patches released in Remix 2.16.3 and React Router 7.4.1 (GitHub Advisory).

The vulnerability stems from improper handling of URL port sections in the React Router's Express adapter. Specifically, attackers can manipulate URLs by inserting a URL pathname in the port section of a URL that is part of a Host or X-Forwarded-Host header sent to a Remix/React Router request handler. The vulnerability has been assigned a CVSS v3.0 base score of 7.5 (High) with a vector string of CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, indicating network accessibility with low attack complexity and no required privileges or user interaction (NVD).

The vulnerability can lead to several serious security implications, including Cache Poisoning Denial of Service (CPDoS), Web Application Firewall (WAF) bypass, and potential escalation of Reflected XSS to Stored XSS when caching systems are in place. The library's widespread usage, with reported 13.2 million weekly downloads, amplifies the potential impact of this vulnerability (Security Online).

The vulnerability has been patched in Remix 2.16.3 and React Router 7.4.1. Users of affected versions are strongly advised to update to these patched versions immediately to mitigate the security risk (GitHub Advisory).

The vulnerability was discovered and reported by security researchers Rachid Allam and Yasser Allam, highlighting the collaborative nature of security research in the React ecosystem (Security Online).