SSH MCP Server v2

OpenSSF Scorecard

SSH MCP Server is a security-first Model Context Protocol server that gives LLM agents controlled SSH access to remote hosts — with command classification, policy-based authorization, human-in-the-loop approval, and full audit logging.

The risk this server exists to manage. Giving an LLM shell access on a remote host puts private data, untrusted input and network egress in one place — Simon Willison's "lethal trifecta". Prompt injection has no general fix, so ssh-mcp assumes any command may be attacker-influenced: it classifies before executing, authorizes against a role × host-group matrix, gates destructive work behind approval, and records the decision either way. That narrows the blast radius; it does not remove the risk. Two things stay yours: never point it at a root account, and never set auto approval on a production profile. SECURITY.md has the full threat model.

Quick Start

1. Install

npm install -g ssh-mcp

2. Configure

Without a config the server still starts, so a client or directory can complete the MCP handshake and read tools/list — but every tool call is refused until you configure it, with a message naming the path below. Nothing runs on a host until this step is done.

Create the config file at the path for your platform:

[defaults]
defaultProfile = "dev"
approvalMode = "ask-destructive"

[[profiles]]
name = "dev"
host = "192.168.1.100"
port = 22
user = "deploy"           # NOT root!
auth = "key"
keyRef = "~/.ssh/id_ed25519"
role = "admin"
approvalPolicy = "auto"    # dev is permissive

chmod 700 ~/.config/ssh-mcp && chmod 600 ~/.config/ssh-mcp/config.toml

The config decides which hosts, roles and policy rules this server honours, so it checks that nobody but you can read it — and treats the two platforms differently, because the question has a much clearer answer on one of them.

Linux and macOS: enforced. The mode check above, on the file and the directory — which is why chmod 700 is in that command, since mkdir -p under the default umask leaves the directory 0755. The server refuses to start otherwise. "Only the owner" is unambiguous here and chmod is a one-line fix.

Windows: split by what the ACL actually allows. There are no mode bits, so the ACL is read instead — and read exposure and write exposure are not treated alike, because Windows is much clearer about one of them than the other.

A config under %APPDATA% inherits access for you, SYSTEM and Administrators and needs nothing done to it. One created elsewhere does not: a file under C:\ inherits read for every local account and modify for every authenticated one. The message names the two icacls commands that fix it either way.

Read exposure is reported rather than refused because that is where Windows is genuinely muddier than POSIX, and refusing over it blocked a config at the documented location (#138). Write exposure is refused because it is not muddy at all: another account being able to rewrite the file that decides which hosts, roles and approval policy this server honours is an authorization bypass, not a disclosure.

Two flags move the whole thing: --strictConfigAcl refuses everything the check objects to, read-only grants included; --allowUncheckedConfigAcl reports everything and refuses nothing. Neither combination leaves you without an exit, which is the lesson of #138.

Exit statuses

A supervisor that treats any non-zero status as a failure needs no change. One that matched on 1 to detect a startup problem should match on 2 as well.

Starting with nothing configured is not an exit-2 condition, as of the release that added introspection without a config: the server starts so it can be described, and refuses each tool call instead. A supervisor that used a non-zero exit to catch an unconfigured deployment should watch for starting unconfigured on stderr, or read configured from GET /health when running the HTTP transport.

3. Set credentials via environment variables

export SSH_MCP_PASSWORD="your-password"        # if using auth=password
# OR use SSH agent (recommended):
export SSH_AUTH_SOCK="$SSH_AUTH_SOCK"           # already set if agent running

4. Connect from your MCP client

Claude Code:

claude mcp add --transport stdio ssh-mcp -- ssh-mcp

Claude Desktop / Cursor / Windsurf:

{
  "mcpServers": {
    "ssh-mcp": {
      "command": "ssh-mcp",
      "env": {
        "SSH_MCP_PASSWORD": "your-password"
      }
    }
  }
}

Never pass passwords as CLI arguments — they're visible via ps aux. Use env vars, config files, SSH agent, or OS keychain.

Tools (11)

Interactive Sessions

Sessions maintain state (CWD, environment variables) between commands:

Agent: open-session(name="deploy", type="interactive")
Agent: run-command(session="deploy", command="cd /opt/myapp")
Agent: run-command(session="deploy", command="git pull")    # runs in /opt/myapp
Agent: run-command(session="deploy", command="npm ci")      # CWD persists
Agent: close-session(name="deploy")

Background Sessions

Long-running processes (logs, builds):

Agent: open-session(name="logs", type="background", command="tail -f /var/log/syslog")
Agent: read-session-output(name="logs", lines=20)   # poll
Agent: close-session(name="logs")

Remote host support

Tested against Linux (Debian/bash, Alpine/busybox ash), Dropbear, and Windows OpenSSH on Windows 11.

Interactive sessions require a POSIX shell (sh, bash, ash, zsh). They work by bracketing each command with printf markers and reading $? and $PWD from a trailer — none of which exist in cmd.exe, the default shell for Windows OpenSSH. Opening one against such a host fails immediately with an explicit error rather than timing out; everything else works normally.

Setting PowerShell as the OpenSSH DefaultShell does not help: the protocol is POSIX-specific, not merely non-cmd.

Configuration

Profile options

[defaults]
defaultProfile = "dev"
sessionMaxPerConnection = 5
sessionIdleTimeoutMs = 600000       # 10min
sessionBackgroundMaxMs = 3600000    # 1hr
commandTimeoutMs = 60000
commandMaxChars = 5000              # 0 = unlimited, the config spelling of --maxChars=none
commandMaxOutputBytes = 1048576     # 1MB
connectionIdleReapMs = 900000       # 15min
commandQuotaPerDay = 0              # 0 = unlimited; circuit breaker for runaway agents
approvalGrantTtlMs = 0              # 0 = always prompt; see "Approval Grants"
approvalMode = "ask-destructive"    # auto | ask-destructive | ask-all | deny

[[profiles]]
name = "prod-web-1"
host = "10.0.1.50"
port = 22
user = "deploy"
auth = "agent"                      # agent | key | password | keychain
keyRef = "~/.ssh/id_ed25519"        # for auth=key
keychainEntry = "ssh-mcp/prod"      # for auth=keychain (requires @napi-rs/keyring)
via = "bastion"                     # ProxyJump — route through bastion profile
group = "prod"                      # Policy tier: prod | staging | dev, or your own (see [policy])
workdir = "/var/www"
trustedHostKey = "SHA256:..."       # Pin host key (optional)
tty = false
role = "operator"                   # viewer | operator | admin
readOnly = false
approvalPolicy = "ask-all"
cert = false                        # SSH CA cert auth — auto-detects keyRef-cert.pub
sessionMaxPerConnection = 3         # per-profile override
sessionIdleTimeoutMs = 300000       # stricter for prod
commandQuotaPerDay = 200            # per-profile override
maxChars = 2000                     # per-profile override; stricter for prod

# Optional. Merged over the built-in role matrix; see "Policy Engine" below.
# roleBindings is keyed by role and then by tier, so the block below changes
# operator on prod and leaves operator's other tiers, and viewer and admin,
# on their defaults.
[policy]
denylist = ["^terraform\\s+destroy"]

[policy.roleBindings.operator]
prod = ["read-only", "safe", "destructive"]

Unknown sections and keys are a startup error, not a warning, so a typo cannot leave you running defaults you thought you had overridden. That extends to role and tier names: every one you write under [policy.roleBindings] has to be reachable by some profile, and every profile's role and tier has to resolve to real bindings. Both directions are checked at startup.

ProxyJump (Bastion)

Reach internal hosts behind a bastion/jump server. The via field specifies a profile name to tunnel through:

[[profiles]]
name = "bastion"
host = "bastion.example.com"
user = "deploy"
auth = "agent"

[[profiles]]
name = "internal-db"
host = "10.0.1.50"                 # private IP — not directly reachable
user = "dbadmin"
auth = "key"
keyRef = "~/.ssh/db_key"
via = "bastion"                     # tunnel through bastion

No agent forwarding — only a TCP tunnel via forwardOut. The bastion stays connected and reusable for multiple internal hosts.

SSH CA Certificates

For enterprise setups with a central SSH Certificate Authority:

[[profiles]]
name = "prod-db"
host = "db.internal"
user = "admin"
auth = "key"
keyRef = "~/.ssh/id_ed25519"
cert = true                         # enable CA cert auth

The certificate file is auto-detected using OpenSSH convention (keyRef + -cert.pub, e.g. ~/.ssh/id_ed25519-cert.pub). You can override the path with SSH_MCP_<NAME>_CERT env var. The cert is concatenated with the private key per ssh2 convention.

Credential Resolution Order

1. SSH agent (SSH_AUTH_SOCK) — no key material in process memory
2. OS keychain (macOS Keychain / Windows Credential Manager / Linux Secret Service) — requires auth = "keychain" and @napi-rs/keyring
3. Environment variables — SSH_MCP_PASSWORD, SSH_MCP_KEY, SSH_MCP_SUDO_PASSWORD, or profile-specific SSH_MCP_<NAME>_PASSWORD
4. Key file — keyRef path or SSH_MCP_KEY env var

Never CLI arguments. v2 removes --password, --sudoPassword, --suPassword entirely.

Policy Engine

Roles

Which column applies comes from the profile's group. Set it explicitly — without it the tier is guessed from the profile name (prod/staging/dev, local, test, sandbox), and an unrecognised name resolves to prod, the strictest tier. A production host named web-01 is therefore treated as production rather than silently getting dev permissions.

Note what this means for sudo: admin has no privileged on prod, so privileged-command is refused there by design — including on a quick-start profile, which has no name to infer from and therefore lands on prod. If the host is not production, say so:

npx ssh-mcp --host=10.0.0.5 --user=deploy --group=dev

[[profiles]]
name = "build-box"
group = "dev"

Configuring the matrix

The table above is the default, not a limit. An optional [policy] section is merged over it at startup, so granting sudo on a host you have honestly labelled prod is a reviewable line in a config file rather than a relabelling:

[policy.roleBindings.admin]
prod = ["read-only", "safe", "destructive", "privileged"]

The merge is at role and tier depth. That block changes admin on prod and nothing else: admin on staging and dev keep their defaults, and viewer and operator are untouched. Roles and tiers the defaults have never heard of are added rather than rejected, which is what makes a custom group resolve to real bindings instead of falling back to the strictest tier:

[[profiles]]
name = "build-box"
role = "admin"
group = "tier-1"

[policy.roleBindings.admin]
"tier-1" = ["read-only", "safe", "destructive"]

Extra deny patterns live in the same section, and are applied on top of the never-allowed list rather than replacing it:

[policy]
denylist = ["^terraform\\s+destroy"]

Because role and tier names are free strings, nothing in the merge itself can tell a new custom role from a misspelling of an existing one. A cross-check at startup does, and these all fail there rather than at the point of use:

• a command class outside read-only | safe | destructive | privileged, so a priviledged typo cannot parse into a grant of nothing and then read as a policy decision when a command is refused;
• any unrecognised section or key anywhere in the config, so a block the parser does not understand is an error rather than a clean startup with none of the behaviour you configured;
• a role or tier under [policy.roleBindings] that no profile uses, so [policy.roleBindings.operater] cannot merge in as a fourth role while the profiles you meant to restrict keep running on defaults;
• a profile whose role has no bindings, or whose tier has none under that role, so a host cannot end up on read-only for a reason nobody wrote down.

The last one covers the tier you did not set as well as the one you did. A profile with no group still resolves to one by name, and that inferred tier has to exist under the profile's role like any other.

A tier with no bindings for a role grants read-only, and never another tier's classes. There is no fallback between tiers: while the matrix was compiled in, falling back to prod meant falling back to a role's strictest cell, but a [policy] block can write that cell now.

An OPA sidecar is not an alternative route to the same grant. OPA is consulted only for commands the local policy already allows, so it can refuse more but never widen. Widening happens here or not at all.

Command Classification

Every command is classified before execution:

• read-only: Allowlisted commands (ls, cat, grep, df, stat, systemctl status, ...)
• safe: Non-destructive mutations (npm install, git pull, ...)
• destructive: mutations that need approval (rm -rf /tmp/build, ...)
• privileged: sudo, su, doas, pkexec

A separate forbidden list is never allowed, whatever the role or approval policy: rm -rf /, mkfs, dd of=/dev/, shutdown, curl|sh, fork bombs, writes to /etc/cron, /etc/systemd or authorized_keys, iptables -F, and recursive chmod 777 / / chown /. Add your own patterns via the policy denylist; an invalid pattern fails at startup rather than degrading silently.

Approval Modes

• auto — no prompts (dev only!)
• ask-destructive — prompt for destructive/privileged (default). Narrower than it sounds: outside the never-allowed list, destructive is one rm -rf /path pattern, find with a write/exec flag, an unresolvable command word, a program handed to an interpreter this server cannot read (python3 -c, perl -e, node -e, a program arriving on a pipe — but not awk, whose program is not read), and sftp-upload/interactive open-session — elevation classifies privileged, which also prompts. Ordinary writes, service control and signals do not. See SECURITY.md before relying on this in production.
• ask-all — prompt for every command
• deny — reject destructive/privileged commands outright (no prompt)

Approval Grants (just-in-time)

approvalGrantTtlMs lets one explicit approval cover repeats of the exact same command on the same profile for a bounded time (e.g. 300000 for five minutes). It exists because approving rm -rf /tmp/build every few seconds during an iterative task trains you to click through prompts — which is worse for safety than a grant you chose deliberately.

A grant is bound to the exact command text, the profile and the command class: approving rm -rf /tmp/build does not cover rm -rf /tmp/build-prod, the same command on another host, or the same command escalated to sudo. Runs covered by a grant appear in the audit log with approver: "jit-grant", so they stay distinguishable from a fresh human answer.

Off by default (0 = always prompt). Auto-approval weakens the gate that makes destructive commands safe, so turning it on should be a decision.

Answering the prompt

Approval goes through the MCP elicitation request, so what you see is your client's dialog. Accepting it approves the command — there is no second field to fill in.

You have 10 minutes to answer. Past that the request expires and the command is refused rather than left pending, and the refusal says so; the prompt may still be open in your client, in which case run the command again once you are ready. If your client does not support elicitation at all, every destructive and privileged command is refused with APPROVAL_UNAVAILABLE naming that cause — approval fails closed by design.

Command Quota

commandQuotaPerDay bounds how many commands a profile may run in a rolling 24-hour window (0 = unlimited). The approval gate stops destructive commands and the HTTP rate limiter caps request rate, but neither bounds total work — a prompt-injected agent looping over allowed commands stays under both. The quota is the circuit breaker for that case.

Counted after policy allows a command and before it runs, so a denied command does not spend budget. The window slides rather than resetting at midnight, which would let an agent spend a full quota just before the reset and another immediately after.

External Policy Engine (OPA)

For organizations that standardize on Open Policy Agent / Rego:

ssh-mcp --opaUrl=http://localhost:8181

When --opaUrl is set, commands the built-in engine allows are additionally evaluated by OPA. OPA can only narrow. A command the built-in engine has already denied returns that denial without OPA being consulted at all, so a sidecar answering allow cannot grant a class the role bindings withhold. To widen, edit [policy].

An outage falls back to the local decision and logs one warning per minute. That is the default because OPA is an additional deny layer and stopping all work would be the worse failure — but an operator who deployed OPA as the authorization gate loses that gate during the outage, and the only signal is a stderr line MCP clients usually discard. --opaFailClosed makes the gate being down mean no; the refusal carries ruleId: "opa-unavailable" so the audit record says the gate was down rather than implying a policy refused the command.

The request shape follows the AuthZEN Access Evaluation contract:

{
  "input": {
    "subject": { "role": "operator", "profile": "prod-web-1" },
    "action": { "tool": "run-command", "commandClass": "destructive" },
    "resource": { "command": "rm -rf /tmp/cache", "binary": "rm", "host": "10.0.1.50" },
    "context": { "readOnly": false }
  }
}

OPA responds with { "result": true/false }. If OPA denies (result: false), the command is blocked even if the built-in engine allows it. If OPA is unreachable, the built-in engine's decision stands by default (fail-open, to avoid locking out access); --opaFailClosed refuses instead. A 200 that carries no boolean result counts as unreachable — that is what OPA answers for an undefined document, so a misnamed package or an unactivated bundle is an outage rather than consent.

Example Rego policy (ssh-mcp.rego):

package ssh.mcp

default allow := false

# Admins pass the OPA gate on dev hosts. The built-in policy still applies on
# top: this widens nothing that the role bindings withhold.
allow if {
  input.subject.role == "admin"
  startswith(input.subject.profile, "dev")
}

# Deny all destructive commands on prod
deny if {
  input.action.commandClass == "destructive"
  startswith(input.subject.profile, "prod")
}

Security

Threat Model

See SECURITY.md for the full threat model, vulnerability reporting policy, and deployment checklist.

Supply chain

Releases carry signed attestations, published through Sigstore and recorded in its public transparency log. They live in two different stores, which is what decides how each is verified:

Provenance comes from npm trusted publishing: the release workflow authenticates with a short-lived OIDC token and no stored credential, so there is no long-lived npm token to leak.

npm audit signatures        # provenance, against an installed tree

npm pack ssh-mcp            # the SBOM attestation is bound to the tarball, so fetch it
gh attestation verify ssh-mcp-*.tgz --repo tufantunc/ssh-mcp --predicate-type https://cyclonedx.org/bom

Both flags on the last command are load-bearing. gh attestation verify defaults to the SLSA predicate, so without --predicate-type it filters the SBOM out and reports nothing found — and the provenance it would look for instead is in npm's store, not the GitHub store --repo queries. Use https://spdx.dev/Document for the SPDX one.

Both SBOMs are also attached to each GitHub release, for reading rather than verifying.

Level 2, not 3. Provenance is signed by the generic GitHub-hosted runner — builder.id is https://github.com/actions/runner/github-hosted — which the build itself can influence; Build L3 requires an isolated builder it cannot. Reaching L3 is not currently compatible with trusted publishing: npm turns on its own provenance whenever that setting is left at its default, and then ignores any externally generated one. So L3 today would mean returning to a long-lived npm token — trading the property described above for a level number.

Safe Defaults

• Non-root user in all examples
• TOFU host key verification (accept on first connect, verify after — within one process; see SECURITY.md)
• RFC 9142 algorithm allow-list (no SHA-1, no CBC, no ssh-rsa)
• exec()-only (no persistent su shells — fixes PTY leak)
• Sudo via stdin (not argv — fixes process list leak)
• Sanitizer strips CR/LF/NUL from all metadata
• 3-layer redaction (field → regex → entropy) on audit logs
• No CLI-arg secrets (use env vars, keychain, or config)

Hardening Checklist

• Create dedicated low-privilege service account on target hosts
• Use command-specific sudoers instead of NOPASSWD: ALL
• Enable ask-all approval for production profiles
• Restrict network egress on target hosts
• Use readOnly = true for monitoring profiles
• Review audit logs regularly
• Run chmod 700 <config dir> && chmod 600 config.toml (Windows: the ACL under %APPDATA% is already restricted)

Transports

stdio (default)

For local MCP clients (Claude Code, Cursor, Windsurf). No network exposure.

ssh-mcp                          # reads config from XDG path
ssh-mcp --config=/path/to.toml   # custom config path

HTTP (optional)

For remote/web clients behind a reverse proxy with TLS:

ssh-mcp --transport=http --httpPort=3000 --bearerToken=secret
ssh-mcp --transport=http --httpPort=3000 --bearerToken=secret --rateLimit=60

Endpoints: POST / (MCP Streamable HTTP), GET /status, GET /health

GET /health answers {"healthy": true, "configured": <bool>}. It stays 200 either way — healthy is liveness — while configured is false when no profile is set, which is the case of a config bind mount that silently did not attach: the server binds the port and refuses every tool call. GET /status carries the profile list itself and stays behind the bearer token.

When rate limit is exceeded, the server returns HTTP 429 with Retry-After header and a JSON-RPC error body so MCP clients can handle it gracefully.

Failed authentication is throttled separately, and on by default. --rateLimit never saw a wrong bearer token, because the auth check answers before the limiter is reached — so guessing ran at network speed. --authFailureLimit gives each client its own small budget, spent only on a 401; a correct token never consumes from it, so a working client never throttles itself. Once an address has spent its budget every request from it waits, including one with the right token — that is deliberate, since answering the guess would otherwise tell the caller which token was right. Clients are told apart by socket address. Behind a reverse proxy that means every client shares one budget, so set --trustProxy when the proxy is yours — the server prints a warning the first time it sees X-Forwarded-For without it. --trustProxy takes the rightmost X-Forwarded-For entry, which is the hop the proxy itself appended; everything to its left came from the client, so reading the leftmost would let a client choose its own budget or spend a victim's. That only holds if a proxy actually appended the entry, so the header is read only when the peer is the proxy — bare --trustProxy means a loopback peer, which is the deployment above; name a proxy elsewhere with --trustedProxies. When the header cannot be read as an address, or the peer is not trusted, the server falls back to the socket address and says so once, so a flag that is not taking effect is not silent. One trusted hop is assumed. A malformed --authFailureLimit is refused at startup rather than silently disabling the check; only 0 turns it off.

Always terminate TLS at a reverse proxy (Caddy/nginx). The server listens on 127.0.0.1 only.

Docker

# Build
docker build -t ssh-mcp .

# Run (config file + env vars for credentials)
docker run -i \
  -v ./config.toml:/home/appuser/.config/ssh-mcp/config.toml:ro \
  -e SSH_MCP_PASSWORD=secret \
  ssh-mcp

Or with docker-compose:

docker-compose --profile app up

The Docker image runs as non-root UID 65532, with a minimal node:22-slim base.

CLI Flags (v2)

Secrets are never passed as CLI arguments.

Migrating from v1

v2 is a breaking release. Passing a removed flag now fails at startup with the replacement, rather than failing later as a confusing auth error.

Tools

Command results now carry status. In v1 a failed command rejected with Error (code N). In v2 a non-zero exit comes back as an error result including the exit code and stderr — so an empty response no longer means "it worked".

Flags

Credentials moved off the command line because CLI arguments are world-readable via /proc/<pid>/cmdline on Linux (CWE-214). Credentials now resolve through an SSH agent → OS keychain → env var → key file cascade.

Example

// v1
{ "command": "npx", "args": ["ssh-mcp", "--host=1.2.3.4", "--user=root", "--password=hunter2"] }

// v2 — credentials via env
{
  "command": "npx",
  "args": ["ssh-mcp", "--host=1.2.3.4", "--user=root"],
  "env": { "SSH_MCP_PASSWORD": "hunter2" }
}

For more than one host, move to a TOML config file (see Configuration) and pass --config <path>; profiles carry per-host roles and approval policy.

Host key verification

v1 did not verify host keys. v2 defaults to trust-on-first-use and records the key in memory, for the life of the process; a later mismatch in that same process fails the connection. Nothing is written to disk and ~/.ssh/known_hosts is not consulted, so a restart accepts afresh — see SECURITY.md. Pin explicitly with trustedHostKey in a profile, which is the only control here that survives a restart — and which no host key mode overrides, so --insecureHostKey is an opt-out only for hosts you have not pinned (test environments only).

Testing

# Start test SSH server
docker-compose --profile test up -d

# Run all tests
npm test

# Run only unit tests
npm test -- test/unit/

# Run with coverage
npm run coverage

MCP Inspector

npm run inspect

Contributing

See CONTRIBUTING.md. Please follow the security checklist in all PRs.

Support

If you find SSH MCP Server helpful, consider starring the repository or sponsoring!

Listed on

Also on the official MCP registry as io.github.tufantunc/ssh-mcp.