CVE-2025-43865: React Router Data Spoofing Vulnerability

CVE-2025-43865 Overview

A high-severity insufficient verification of data authenticity vulnerability has been identified in React Router, a popular routing library for React applications. In versions on the 7.0 branch prior to version 7.5.2, attackers can modify pre-rendered data by adding a specially crafted header to the request. This vulnerability allows complete spoofing of pre-rendered content, enabling modification of all values within the data object passed to the HTML.

Critical Impact
Attackers can manipulate pre-rendered application data through header injection, potentially leading to data integrity violations and application state manipulation without authentication requirements.

Affected Products

React Router versions 7.0 through 7.5.1
Applications using React Router's pre-rendering feature
Server-side rendered React applications utilizing React Router 7.x

Discovery Timeline

2025-04-25 - CVE CVE-2025-43865 published to NVD
2025-04-29 - Last updated in NVD database

Technical Details for CVE-2025-43865

Vulnerability Analysis

This vulnerability is classified under CWE-345 (Insufficient Verification of Data Authenticity). The core issue stems from improper validation of request headers during the pre-rendering process in React Router. When processing requests, the server-side runtime fails to adequately verify the authenticity of certain headers, allowing attackers to inject malicious values that modify the pre-rendered data object.

The vulnerability affects the server-side rendering pipeline where build-time headers were being processed without proper contextual validation. This means that during normal runtime operations, headers intended only for build-time processing could still be read and processed, enabling data manipulation.

Root Cause

The root cause lies in the routes.ts file within the server-runtime package, specifically in how headers were processed without distinguishing between build-time and runtime contexts. The application failed to restrict access to build-time-only headers during normal request processing, creating an avenue for header-based data manipulation.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker can craft HTTP requests with specific headers that the React Router server-side runtime interprets as legitimate build-time directives. By manipulating these headers, the attacker can alter the pre-rendered data object that gets embedded into the HTML response, potentially affecting:

Application state initialization
User-visible content
Configuration data passed to the client
Security-sensitive application parameters

typescript

// Security patch in packages/react-router/lib/server-runtime/dev.ts - Adjust approach for prerendering/SPA mode via headers (#13453)
  // @ts-expect-error
  return globalThis[globalDevServerHooksKey];
}

// Guarded access to build-time-only headers
export function getBuildTimeHeader(request: Request, headerName: string) {
  if (typeof process !== "undefined") {
    try {
      if (process.env?.IS_RR_BUILD_REQUEST === "yes") {
        return request.headers.get(headerName);
      }
    } catch (e) {}
  }
  return null;
}

Source: GitHub Commit Reference

The patch introduces a getBuildTimeHeader function that guards access to build-time-only headers by checking for the IS_RR_BUILD_REQUEST environment variable, ensuring these headers are only processed during legitimate build operations.

typescript

// Security patch in packages/react-router-dev/vite/plugin.ts - Adjust approach for prerendering/SPA mode via headers (#13453)
            );
          }

+          // Set an environment variable we can look for in the handler to
+          // enable some build-time-only logic
+          process.env.IS_RR_BUILD_REQUEST = "yes";

          if (isPrerenderingEnabled(ctx.reactRouterConfig)) {
            // If we have prerender routes, that takes precedence over SPA mode
            // which is ssr:false and only the root route being rendered

Source: GitHub Commit Reference

Detection Methods for CVE-2025-43865

Indicators of Compromise

Unusual HTTP request headers targeting React Router pre-rendering functionality
Modified or unexpected content in pre-rendered HTML responses
Anomalous server-side rendering behavior or data inconsistencies
Request logs showing attempts to manipulate build-time headers during runtime

Detection Strategies

Monitor HTTP request headers for unexpected or malformed values targeting server-side rendering
Implement integrity checking for pre-rendered content to detect unauthorized modifications
Review application logs for discrepancies between expected and actual pre-rendered data
Deploy web application firewalls (WAF) with rules to detect header manipulation attempts

Monitoring Recommendations

Enable detailed logging for all server-side rendering operations
Set up alerts for requests containing unusual header patterns
Implement content integrity verification for pre-rendered pages
Monitor for changes in application behavior that could indicate data spoofing

How to Mitigate CVE-2025-43865

Immediate Actions Required

Upgrade React Router to version 7.5.2 or later immediately
Audit existing applications for signs of exploitation
Review server logs for suspicious header manipulation attempts
Implement input validation on server-side rendering pipelines

Patch Information

The vulnerability has been patched in React Router version 7.5.2. The fix introduces environment variable checks to ensure that build-time-only headers are only processed during legitimate build operations, not during runtime requests. The patch is available via the GitHub Commit Reference and detailed in the GitHub Security Advisory.

Workarounds

Implement a reverse proxy or WAF rule to strip or validate suspicious headers before they reach the application
Disable pre-rendering functionality temporarily if immediate upgrade is not possible
Add custom middleware to validate and sanitize incoming request headers
Consider using content integrity checks on pre-rendered data objects

bash

# Upgrade React Router to patched version
npm update react-router@7.5.2

# Or using yarn
yarn upgrade react-router@7.5.2