CVE-2025-53109: MCP Filesystem Path Traversal Vulnerability

CVE-2025-53109 Overview

Model Context Protocol (MCP) Servers is a collection of reference implementations for the model context protocol. A symlink attack vulnerability exists in the Filesystem component prior to versions 0.6.4 and 2025.7.01 that could allow unauthorized access to files outside of intended allowed directories. By exploiting symbolic links within permitted directories, an attacker can bypass path restrictions and access sensitive files on the system.

Critical Impact
Attackers can leverage symlinks within allowed directories to escape path restrictions and access arbitrary files on the filesystem, potentially exposing sensitive configuration data, credentials, or other protected resources.

Affected Products

Model Context Protocol Servers - Filesystem component prior to version 0.6.4
Model Context Protocol Servers - Filesystem component prior to version 2025.7.01

Discovery Timeline

2025-07-02 - CVE-2025-53109 published to NVD
2025-07-03 - Last updated in NVD database

Technical Details for CVE-2025-53109

Vulnerability Analysis

This vulnerability is classified as CWE-59 (Improper Link Resolution Before File Access), commonly known as a Symlink Attack. The Filesystem component of Model Context Protocol Servers implements directory-based access controls to restrict file operations to specific allowed directories. However, prior to the patch, the path validation logic failed to properly resolve symbolic links before checking whether the target path falls within permitted directories.

The vulnerability allows an attacker with the ability to create symbolic links within an allowed directory to craft a symlink pointing to files outside the restricted directory tree. When the application subsequently accesses this symlink, it follows the link to the external target without verifying that the resolved path remains within the allowed boundaries.

This bypass can lead to unauthorized read access to sensitive files, including configuration files, credentials, and other protected data that should not be accessible through the MCP Filesystem interface.

Root Cause

The root cause is improper path validation that fails to resolve symbolic links before performing directory containment checks. The original implementation checked only the literal path string without resolving symlinks to their actual target locations, allowing path traversal via symlink indirection.

Attack Vector

The attack requires network access to the MCP server and involves creating a symbolic link within an allowed directory that points to a target outside the permitted directory structure. When the application processes requests for the symlink path, it follows the link and returns content from the unauthorized location.

Attack prerequisites include:

Network access to the vulnerable MCP Filesystem server
Ability to create files or symlinks within an allowed directory
Knowledge of target file paths outside the allowed directories

typescript

+import path from 'path';
+
+/**
+ * Checks if an absolute path is within any of the allowed directories.
+ * 
+ * @param absolutePath - The absolute path to check (will be normalized)
+ * @param allowedDirectories - Array of absolute allowed directory paths (will be normalized)
+ * @returns true if the path is within an allowed directory, false otherwise
+ * @throws Error if given relative paths after normalization
+ */
+export function isPathWithinAllowedDirectories(absolutePath: string, allowedDirectories: string[]): boolean {
+  // Type validation
+  if (typeof absolutePath !== 'string' || !Array.isArray(allowedDirectories)) {
+    return false;
+  }
+
+  // Reject empty inputs
+  if (!absolutePath || allowedDirectories.length === 0) {
+    return false;
+  }
+
+  // Reject null bytes (forbidden in paths)
+  if (absolutePath.includes('\\x00')) {
+    return false;
+  }
+
+  // Normalize the input path
+  let normalizedPath: string;
+  try {
+    normalizedPath = path.resolve(path.normalize(absolutePath));

Source: GitHub Commit Update

Detection Methods for CVE-2025-53109

Indicators of Compromise

Presence of symbolic links within MCP-allowed directories pointing to locations outside permitted paths
Unexpected file access patterns showing reads from sensitive system directories
Log entries indicating file access outside configured allowed directories
Creation of new symlinks in allowed directories by MCP processes

Detection Strategies

Monitor file system operations for symlink creation within MCP-allowed directories
Implement file integrity monitoring to detect unauthorized symlink creation
Review access logs for patterns indicating path traversal or access to sensitive files
Deploy endpoint detection rules to identify symlink-based path traversal attempts

Monitoring Recommendations

Enable verbose logging for MCP Filesystem server operations
Configure alerts for file access outside expected directory boundaries
Implement real-time monitoring of symlink operations in allowed directories
Review application logs for path normalization errors or unexpected path resolutions

How to Mitigate CVE-2025-53109

Immediate Actions Required

Upgrade Model Context Protocol Servers Filesystem component to version 0.6.4 or 2025.7.01 immediately
Audit existing allowed directories for unauthorized symbolic links
Review file access logs to identify potential exploitation attempts
Restrict symlink creation permissions within MCP-allowed directories where possible

Patch Information

The security fix is available in Filesystem versions 0.6.4 and 2025.7.01. The patch implements proper path normalization using path.resolve() and path.normalize() to resolve symbolic links and canonicalize paths before performing directory containment checks. Additional input validation has been added to reject null bytes and validate input types.

For more details, see the GitHub Security Advisory and the security patch commit.

Workarounds

Remove or disable symlink creation permissions for users within MCP-allowed directories
Implement additional access controls at the operating system level to restrict symlink following
Configure MCP Filesystem with minimal required directory access
Monitor and remove any suspicious symlinks found in allowed directories

bash

# Configuration example - remove symlink creation capability
# Find and remove existing symlinks in allowed directories
find /path/to/allowed/directory -type l -exec rm {} \;

# Restrict symlink creation (requires appropriate filesystem support)
# Example: Use filesystem mount options to prevent symlink following
mount -o nosymfollow /path/to/allowed/directory