React Router Under Siege: Analyzing CVE-2025-31137 URL Spoofing Vulnerability

React Router, a critical component powering numerous React applications, recently faced a significant security challenge. CVE-2025-31137, a high-severity URL spoofing vulnerability, exposes Remix 2.x and React Router 7.x applications using the Express adapter to potential security breaches. By exploiting HTTP headers, attackers can manipulate URL paths, bypass security checks, and even poison caches—posing a serious threat to application integrity.

Affected Systems and Versions

• React Router: Versions prior to 7.4.1
• Remix: Versions prior to 2.16.3
• Configuration: Applications using the Express adapter

Technical Information

The vulnerability arises due to inadequate port sanitization within Express middleware, specifically affecting the handling of Host and X-Forwarded-Host headers. Attackers exploit this by injecting URL paths into the port section of these headers, effectively spoofing the URL path and bypassing React Router's validation mechanisms.

Attack Vectors

• Cache Poisoning: Manipulate CDN or proxy caches by spoofing URLs.
• Security Bypass: Evade route-based security controls.
• Phishing Attacks: Serve malicious content under legitimate domains.

Proof of Concept

A simple curl command demonstrates the vulnerability:

curl -H "Host: legit-site.com:/api/admin" http://victim.com

This request tricks the server into interpreting the URL path incorrectly, potentially bypassing security checks.

Patch Information

Immediate patching is crucial:

npm update react-router@^7.4.1 @remix-run/serve@^2.16.3

Temporary mitigation via header sanitization:

location / {
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-Host $host;
}

Detection Methods

Monitor HTTP headers for suspicious path-like structures, particularly in the Host and X-Forwarded-Host fields. Alert on abnormal request patterns, especially targeting sensitive endpoints such as /api/*.

Vendor Security History

React Router, maintained by Remix Run, has demonstrated a strong security posture, addressing four CVEs in the past two years within an average of 72 hours. Regular security updates underline their proactive stance.

References

• GitHub Advisory GHSA-4q56-crqp-v477
• React Router Changelog
• GitHub Discussion on Middleware

Stay vigilant and ensure your React Router and Remix installations are updated promptly to mitigate this critical vulnerability.