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

// 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.

// 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

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

# Or using yarn
yarn upgrade react-router@7.5.2