CVE-2026-40576

excel-mcp-server is a Model Context Protocol server for Excel file manipulation. 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. This vulnerability is fixed in 0.1.8.

excel-mcp-server is a Model Context Protocol server for Excel file manipulation. 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. This vulnerability is fixed in 0.1.8.

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

| Field | Value | |-------|-------| | Package | excel-mcp-server (PyPI) | | Repository | https://github.com/haris-musa/excel-mcp-server | | Affected versions | <= 0.1.7 | | Tested version | 0.1.7 — commit de4dc75 | | CWE | CWE-22 — Improper Limitation of a Pathname to a Restricted Directory | | CVSS 3.1 | 8.2 High — AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H | | Transports affected | sse, streamable-http (network-facing) | | Authentication required | None |

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

…

excel-mcp-server is a Model Context Protocol server for Excel file manipulation. 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. This vulnerability is fixed in 0.1.8.

excel-mcp-server is a Model Context Protocol server for Excel file manipulation. 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. This vulnerability is fixed in 0.1.8.

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

| Field | Value | |-------|-------| | Package | excel-mcp-server (PyPI) | | Repository | https://github.com/haris-musa/excel-mcp-server | | Affected versions | <= 0.1.7 | | Tested version | 0.1.7 — commit de4dc75 | | CWE | CWE-22 — Improper Limitation of a Pathname to a Restricted Directory | | CVSS 3.1 | 8.2 High — AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H | | Transports affected | sse, streamable-http (network-facing) | | Authentication required | None |

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

…

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

| Field | Value | |-------|-------| | Package | excel-mcp-server (PyPI) | | Repository | https://github.com/haris-musa/excel-mcp-server | | Affected versions | <= 0.1.7 | | Tested version | 0.1.7 — commit de4dc75 | | CWE | CWE-22 — Improper Limitation of a Pathname to a Restricted Directory | | CVSS 3.1 | 8.2 High — AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H | | Transports affected | sse, streamable-http (network-facing) | | Authentication required | None |

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

…