excel-mcp-server has a Path Traversal issue

## Summary A path traversal vulnerability exists in [`excel-mcp-server`](https://github.com/haris-musa/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`](https://pypi.org/project/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`](https://github.com/haris-musa/excel-mcp-server/commit/de4dc75f6ba629ccd2e097f5963235846be622e6) | | **CWE** | [CWE-22](https://cwe.mitre.org/data/definitions/22.html) — 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: ```python 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: ```python 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 [](https://asciinema.org/a/2HVA3uKvVeFahIXY) **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 ```bash # 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 a…