CVE-2025-61686: React Router Path Traversal Vulnerability

CVE-2025-61686 Overview

CVE-2025-61686 is a critical path traversal vulnerability affecting React Router's session file storage functionality. The vulnerability exists in @react-router/node versions 7.0.0 through 7.9.3, @remix-run/deno prior to version 2.17.2, and @remix-run/node prior to version 2.17.2. When applications use createFileSessionStorage() with an unsigned cookie, attackers can manipulate session identifiers to cause the application to read or write files outside the designated session directory.

Critical Impact

Attackers can potentially access or modify files outside the session storage directory, leading to unauthorized data access or integrity violations depending on server permissions.

Affected Products

• @react-router/node versions 7.0.0 through 7.9.3
• @remix-run/deno versions prior to 2.17.2
• @remix-run/node versions prior to 2.17.2

Discovery Timeline

• 2026-01-10 - CVE-2025-61686 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2025-61686

Vulnerability Analysis

This path traversal vulnerability (CWE-22) affects the file-based session storage mechanism in React Router and Remix frameworks. The core issue stems from insufficient validation of session identifiers when using the createFileSessionStorage() function with unsigned cookies. Without cryptographic signing, session cookies can be tampered with by attackers to include directory traversal sequences (such as ../) that escape the intended session storage directory.

The impact depends on several factors: the web server process's file system permissions, whether target files conform to the expected session file format, and how the application handles session data. Read operations would only succeed if accessed files match the session format, and even then, the data populates server-side session storage rather than being directly returned to the attacker. However, if application logic exposes session information, data leakage becomes possible. Write operations present a more direct risk, potentially allowing arbitrary file modification within accessible paths.

Root Cause

The vulnerability originates from improper input validation in the session file path construction logic. When createFileSessionStorage() processes session identifiers from unsigned cookies, it fails to sanitize path components before constructing the full file path. This allows malicious session identifiers containing path traversal sequences to escape the designated session directory boundary.

Attack Vector

The attack leverages network-based access with no authentication required. An attacker crafts a malicious session cookie containing directory traversal sequences (e.g., ../../etc/passwd or similar paths). When the server processes this cookie through the createFileSessionStorage() function, it attempts to read or write to the attacker-specified path rather than the legitimate session file location.

The attack's success depends on:

• The application using unsigned session cookies
• The web server process having sufficient file system permissions
• For read operations: target files matching the expected session file format
• For data exfiltration: application logic that returns session data to clients

Detection Methods for CVE-2025-61686

Indicators of Compromise

• Unusual session cookie values containing path traversal sequences like ../, ..%2F, or encoded variants
• Web server error logs showing file access attempts outside the configured session directory
• Application logs indicating session parsing errors for unexpected file formats
• File system access audit logs showing the web server process accessing non-session files

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block cookies containing path traversal patterns
• Monitor session storage directory for unexpected file creation or access patterns
• Deploy application-level logging to track session identifier validation failures
• Configure file integrity monitoring (FIM) on sensitive directories to detect unauthorized access

Monitoring Recommendations

• Enable verbose logging for session management operations in affected React Router and Remix applications
• Set up alerts for HTTP requests containing encoded directory traversal sequences in cookie headers
• Monitor web server process file access using tools like auditd on Linux systems
• Implement real-time alerting for file system operations outside designated application directories

How to Mitigate CVE-2025-61686

Immediate Actions Required

• Upgrade @react-router/node to version 7.9.4 or later immediately
• Upgrade @remix-run/deno to version 2.17.2 or later
• Upgrade @remix-run/node to version 2.17.2 or later
• Audit application logs for any evidence of exploitation attempts

Patch Information

Security patches have been released by the React Router maintainers. The vulnerability is fully addressed in @react-router/node version 7.9.4, @remix-run/deno version 2.17.2, and @remix-run/node version 2.17.2. For detailed patch information, consult the GitHub Security Advisory.

Workarounds

• Implement signed session cookies to prevent session identifier tampering
• Consider switching to alternative session storage mechanisms such as database-backed sessions
• Apply strict file system permissions to limit web server process access to only necessary directories
• Deploy a reverse proxy or WAF with path traversal detection rules as an interim measure

# Update affected packages using npm
npm update @react-router/node@7.9.4
npm update @remix-run/deno@2.17.2
npm update @remix-run/node@2.17.2

# Verify installed versions
npm list @react-router/node @remix-run/deno @remix-run/node