GHSA-j98m-w3xp-9f56

Summary

A path traversal vulnerability exists in excel-mcp-server versions up to and including 0.1.7. When running in SSE or Streamable-HTTP transport mode (the documented way to use this server remotely), an unauthenticated attacker on the network can read, write, and overwrite arbitrary files on the host filesystem by supplying crafted filepath arguments to any of the 25 exposed MCP tool handlers.

The server is intended to confine file operations to a directory set by the EXCEL_FILES_PATH environment variable. The function responsible for enforcing this boundary — get_excel_path() — fails to do so due to two independent flaws: it passes absolute paths through without any check, and it joins relative paths without resolving or validating the result. Combined with zero authentication on the default network-facing transport and a default bind address of 0.0.0.0 (all interfaces), this allows trivial remote exploitation.

Details

Vulnerable Code

The root cause is in src/excel_mcp/server.py, lines 75–94:

def get_excel_path(filename: str) -> str:
    """Get full path to Excel file."""

    # FLAW 1 — absolute paths bypass the sandbox entirely
    if os.path.isabs(filename):
        return filename                     # line 86: returned as-is

    # In SSE/HTTP mode, EXCEL_FILES_PATH is set
    if EXCEL_FILES_PATH is None:
        raise ValueError(...)

    # FLAW 2 — relative paths joined without boundary validation
    return os.path.join(EXCEL_FILES_PATH, filename)   # line 94: "../" escapes

Why this is exploitable

Flaw 1 — Absolute path bypass (line 86): If the attacker passes filepath="/etc/shadow" or filepath="/home/user/secrets.xlsx", the function returns it unchanged. The sandbox directory EXCEL_FILES_PATH is never consulted.

Flaw 2 — Relative traversal (line 94): os.path.join("/srv/sandbox", "../../etc/passwd") produces "/srv/sandbox/../../etc/passwd", which resolves to "/etc/passwd". No os.path.realpath() or os.path.commonpath() check is performed, so ../ sequences escape the sandbox.

Contributing factors that increase severity

1. Default bind address is 0.0.0.0 (all interfaces) — server.py line 70:

   host=os.environ.get("FASTMCP_HOST", "0.0.0.0"),
   

   A user who follows the README and runs uvx excel-mcp-server streamable-http without explicitly setting FASTMCP_HOST exposes the server to their entire LAN.

2. Zero authentication — FastMCP's SSE and Streamable-HTTP transports ship with no authentication. The server adds none. Any TCP client that reaches port 8017 can call any tool.

3. All 25 tool handlers are affected — every @mcp.tool() decorated function calls get_excel_path(filepath) as its first action. This is not an isolated endpoint; it is the entire API surface.

4. Arbitrary directory creation — src/excel_mcp/workbook.py line 24 runs path.parent.mkdir(parents=True, exist_ok=True) before saving, meaning the attacker can create directory trees at any writable location.

Proof of Concept

Video demonstration

asciinema recording: https://asciinema.org/a/2HVA3uKvVeFahIXY

I have also attached the full PoC shell script (record-poc.sh) and the Python exploit script (exploit_test.py) to a Google Drive for the maintainer to review and reproduce independently:

Google Drive (PoC scripts): Shared privately via email

Contents:

• record-poc.sh — automated PoC recording script (bash)
• exploit_test.py — Python exploit that tests all 7 primitives against a running server

Setup

# install
pip install excel-mcp-server mcp httpx

# start server with a sandbox directory
mkdir -p /tmp/sandbox
EXCEL_FILES_PATH=/tmp/sandbox FASTMCP_HOST=127.0.0.1 FASTMCP_PORT=8017 \
    excel-mcp-server streamable-http

Exploit script

The following Python script connects to the server with zero credentials and demonstrates all exploitation primitives:

{REMOVING CAUSE FOR SAFETY CONCERNS}

Results

All 7 test cases passed against a live server instance:

CONFIRMED: 7  |  FAILED: 0

[CONFIRMED] AUTH: Connected with ZERO authentication. 25 tools exposed.
[CONFIRMED] P1-WRITE-ABS: file exists=True size=4783B (outside sandbox)
[CONFIRMED] P2-WRITE-TRAVERSAL: escaped sandbox via ../ exists=True
[CONFIRMED] P3-MKDIR: attacker directory tree created=True
[CONFIRMED] P4-READ: exfiltrated SSN=True name=True
[CONFIRMED] P5-OVERWRITE: victim data replaced=True
[CONFIRMED] P6-STAT: server attempted to open /etc/hostname (format error confirms file access)

Filesystem evidence (independently verified after exploit)

Files created outside the sandbox:

$ ls -la /tmp/cve-hunt/outside_sandbox/
-rw-rw-r-- 1 hitarth hitarth 4783 Apr 10 17:36 P1_absolute_write.xlsx
-rw-rw-r-- 1 hitarth hitarth 4783 Apr 10 17:36 P2_traversal_write.xlsx

Attacker-created directory tree:

$ find /tmp/cve-hunt/attacker_dir
/tmp/cve-hunt/attacker_dir
/tmp/cve-hunt/attacker_dir/deep
/tmp/cve-hunt/attacker_dir/deep/nested
/tmp/cve-hunt/attacker_dir/deep/nested/x.xlsx

Victim file after overwrite (original SSN destroyed):

  ('SSN', 'Name')
  ('ATTACKER-CONTROLLED', 'PWNED')     # was: ('123-45-6789', 'Alice Johnson')
  ('987-65-4321', 'Bob Smith')

Impact

An unauthenticated network attacker can:

Who is affected

Anyone running excel-mcp-server in SSE or Streamable-HTTP mode on a reachable network — which is the documented and recommended deployment for remote use. The project README explicitly states:

• "Works both locally and as a remote service"
• "Streamable HTTP Transport (Recommended for remote connections)"

The server has 3,655+ GitHub stars and is published on PyPI with active downloads.

Suggested Fix

Replace get_excel_path() with a version that enforces the sandbox boundary:

import os

def get_excel_path(filename: str) -> str:
    if EXCEL_FILES_PATH is None:
        # stdio mode: local caller is trusted
        if not os.path.isabs(filename):
            raise ValueError("must be absolute path in stdio mode")
        return filename

    # Remote mode (SSE / streamable-http): enforce sandbox
    if os.path.isabs(filename):
        raise ValueError("absolute paths are not permitted in remote mode")
    if "\x00" in filename:
        raise ValueError("NUL byte in filename")

    base = os.path.realpath(EXCEL_FILES_PATH)
    candidate = os.path.realpath(os.path.join(base, filename))

    if not candidate.startswith(base + os.sep) and candidate != base:
        raise ValueError(f"path escapes EXCEL_FILES_PATH: {filename}")

    return candidate

References

• CWE-22: Improper Limitation of a Pathname to a Restricted Directory
• OWASP Path Traversal
• Model Context Protocol Specification
• FastMCP Documentation