CVE-2026-40576: Excel MCP Server Path Traversal Flaw

CVE-2026-40576 Overview

A critical path traversal vulnerability has been identified in excel-mcp-server, a Model Context Protocol (MCP) server designed for Excel file manipulation. The vulnerability exists in versions up to and including 0.1.7 and allows unauthenticated attackers to read, write, and overwrite arbitrary files on the host filesystem when the server is running in SSE or Streamable-HTTP transport mode.

Critical Impact

Unauthenticated remote attackers can achieve arbitrary file read, write, and overwrite operations on the host filesystem by exploiting flaws in the get_excel_path() function, potentially leading to complete system compromise.

Affected Products

• excel-mcp-server versions up to and including 0.1.7

Discovery Timeline

• 2026-04-21 - CVE CVE-2026-40576 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-40576

Vulnerability Analysis

This path traversal vulnerability (CWE-22) stems from inadequate input validation in the excel-mcp-server's file path handling mechanism. The server is designed to confine file operations to a directory specified by the EXCEL_FILES_PATH environment variable. However, the function responsible for enforcing this boundary — get_excel_path() — contains two independent implementation flaws that completely bypass these security controls.

The vulnerability is particularly dangerous because it affects all 25 exposed MCP tool handlers, providing multiple attack surfaces. Combined with the server's default configuration of binding to 0.0.0.0 (all network interfaces) and the absence of authentication on network-facing transports, remote exploitation becomes trivial.

Root Cause

The get_excel_path() function fails to enforce directory boundaries due to two critical flaws:

1. Absolute Path Bypass: The function passes absolute paths (e.g., /etc/passwd) through without any validation or restriction checks
2. Relative Path Traversal: When handling relative paths, the function joins them to the base directory without resolving or validating the final result, allowing sequences like ../../ to escape the intended directory

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker with network access to the server can craft malicious filepath arguments to any of the 25 exposed MCP tool handlers. By supplying paths containing directory traversal sequences (e.g., ../../../etc/passwd) or absolute paths, the attacker can bypass the intended file access restrictions.

The vulnerability mechanism works as follows: When a request is received, the get_excel_path() function is called to resolve the file path. Due to the implementation flaws, an attacker can supply either an absolute path that bypasses all checks, or a relative path with traversal sequences that escapes the confined directory after path joining. Since the server binds to all interfaces by default and lacks authentication, any network-accessible attacker can exploit these handlers to read sensitive configuration files, overwrite system files, or plant malicious content on the filesystem.

Detection Methods for CVE-2026-40576

Indicators of Compromise

• File access logs showing requests to paths outside the configured EXCEL_FILES_PATH directory
• Unexpected read or write operations targeting system configuration files (e.g., /etc/passwd, /etc/shadow)
• MCP tool handler requests containing ../ sequences or absolute paths in filepath arguments
• Network connections to the MCP server from unexpected IP addresses or geographic locations

Detection Strategies

• Monitor network traffic for MCP protocol requests containing path traversal patterns (../, ..\\, or absolute paths starting with / or drive letters)
• Implement file integrity monitoring on critical system files and directories
• Deploy network-based intrusion detection rules to identify exploitation attempts targeting MCP server endpoints
• Audit server logs for requests to the 25 exposed MCP tool handlers with suspicious filepath parameters

Monitoring Recommendations

• Enable verbose logging on the excel-mcp-server to capture all filepath arguments passed to tool handlers
• Set up alerts for file operations occurring outside the designated EXCEL_FILES_PATH directory
• Monitor for unusual file system activity patterns, particularly write operations to system directories
• Implement network monitoring to detect connections to the MCP server from untrusted networks

How to Mitigate CVE-2026-40576

Immediate Actions Required

• Upgrade excel-mcp-server to version 0.1.8 or later immediately
• If immediate upgrade is not possible, restrict network access to the MCP server using firewall rules
• Audit systems for signs of compromise, particularly unauthorized file modifications
• Review and restrict the EXCEL_FILES_PATH environment variable to the minimum required scope

Patch Information

The vulnerability has been addressed in excel-mcp-server version 0.1.8. Organizations should update to this version or later to remediate the vulnerability. For detailed patch information, refer to the GitHub Security Advisory.

Workarounds

• Change the bind address from 0.0.0.0 to 127.0.0.1 to prevent remote network access until patching is complete
• Implement network segmentation to isolate the MCP server from untrusted networks
• Deploy a reverse proxy with authentication in front of the MCP server to add an authentication layer
• Run the server in a containerized environment with restricted filesystem access using read-only mounts where possible

# Example: Restrict bind address to localhost only
export MCP_BIND_ADDRESS="127.0.0.1"

# Example: Run with restricted filesystem access using Docker
docker run --read-only \
  -v /path/to/excel/files:/data:rw \
  -e EXCEL_FILES_PATH=/data \
  excel-mcp-server:0.1.8