mcp-shell

A native Model Context Protocol server that exposes a single execute_command tool over the Streamable HTTP transport. The command runs directly in a subprocess of this server — there is no separate gateway process and no stdio child.

⚠️ Security first

execute_command is remote command execution by design. This server ships locked down by default with three independent app-layer controls, which are meant to sit on top of (not replace) OS- and network-level hardening — defense in depth:

Layer 1 — Bearer token (required).

• Set MCP_SHELL_TOKEN. The server refuses to start without it.

• Every HTTP request must send Authorization: Bearer <token>; anything else gets 401, checked before any MCP session handling.

Layer 2 — Default-deny command allowlist.

• MCP_SHELL_ALLOWED_COMMANDS is empty by default, which means zero commands run out of the box.

• A command executes only if its program (the first token, matched by name or basename) is explicitly allowlisted.

• Commands containing shell control characters (;, &, |, `, $, (), <>, …) are rejected, so an allowlisted program can't be used to chain into a non-allowlisted one.

Layer 3 — Origin validation (DNS-rebinding protection).

• Per the MCP specification, the Origin header is validated on every request. A request carrying a disallowed Origin gets 403, checked before authentication.

• Accepted origins default to loopback (localhost / 127.0.0.1 / [::1]); override with MCP_SHELL_ALLOWED_ORIGINS (comma-separated).

• Requests with no Origin header (non-browser clients — the normal case for this server) pass through; the opaque Origin: null is rejected.

The executed command's environment is also scrubbed of this server's own secret and control variables (MCP_SHELL_TOKEN, the allowlist, PORT, HOST, …), so an allowlisted command such as env can't read the bearer token. PATH, HOME, and unrelated variables pass through unchanged.

Operational hardening (your responsibility, and just as important).

• Run the server as a dedicated least-privileged user, never root.

• Keep the default bind of 127.0.0.1 and put access control in front of it (an authenticated reverse proxy / tunnel). Do not expose it on a public interface.

• Treat the token as a secret; rotate it; scope the allowlist as narrowly as the use case allows.

Related MCP server: MCP Shell Server

The tool

Output is capped at 100 KB per stream (truncated with a marker); a command that exceeds its timeout is killed with a [PROCESS KILLED] note.

Quickstart

npm install

# Token is mandatory; allowlist is default-deny — enable only what you need.
MCP_SHELL_TOKEN="$(openssl rand -hex 32)" \
MCP_SHELL_ALLOWED_COMMANDS="echo,ls,git" \
  npm start

Without a token the server exits immediately:

npm start
# [mcp-shell] FATAL: MCP_SHELL_TOKEN is not set. Refusing to start ...

Unauthenticated requests are refused:

curl -s -o /dev/null -w '%{http_code}\n' -X POST localhost:3000/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}'
# 401

MCP clients must send the bearer token on every request. Clients that support the Streamable HTTP transport with custom headers connect with something like (exact key names vary by client):

{
  "mcpServers": {
    "shell": {
      "type": "streamable-http",
      "url": "http://127.0.0.1:3000/mcp",
      "headers": { "Authorization": "Bearer <MCP_SHELL_TOKEN>" }
    }
  }
}

Configuration (environment)

Endpoints

• POST/GET/DELETE ${MCP_ENDPOINT} — MCP Streamable HTTP transport (per-session).

• GET ${ROUTE_PREFIX}/healthz — liveness.

• GET ${ROUTE_PREFIX}/ready — readiness: spawns a throwaway shell and confirms a trivial command round-trips.

All endpoints require the bearer token, and a request with a disallowed Origin is refused with 403 before auth (CORS preflight OPTIONS excepted).

Testing

npm test   # node --test

Unit tests cover the helpers, the default-deny allowlist (including the shell-metacharacter bypass guard), Origin validation, and child-environment scrubbing. Integration tests boot the real server and, over the MCP Streamable HTTP client, prove: an allowlisted command runs, a non-allowlisted command is rejected without executing, a request with no / invalid token gets 401, and a request with a disallowed Origin gets 403.

License

Apache-2.0