Security Isolation (Docker · Sandbox · None)

MCPProxy can confine stdio MCP servers so a malicious or buggy server cannot freely touch the host. There are three isolation modes — docker, sandbox, and none — selected by docker_isolation.mode (global) or isolation.mode (per-server). Most of this page describes Docker mode (the default and most capable); see Isolation Modes for the native Sandbox mode and the scanner behaviour under each mode.

:::note The global config key is still docker_isolation for backward compatibility, but its mode field selects any of the three modes — it is not Docker-only. :::

Overview

Docker isolation automatically wraps stdio-based MCP servers in Docker containers, providing:

• Process Isolation: Each server runs in a separate container
• File System Isolation: Servers cannot access host file system
• Network Isolation: Configurable network modes for security
• Resource Limits: Memory and CPU limits prevent resource exhaustion
• Automatic Runtime Detection: Maps commands to appropriate Docker images

Isolation Modes

MCPProxy resolves an isolation mode for every stdio server. Set it globally with docker_isolation.mode and override per-server with isolation.mode:

{
  "docker_isolation": { "mode": "sandbox" },
  "mcpServers": [
    { "name": "trusted-local", "command": "uvx", "args": ["x"], "isolation": { "mode": "none" } }
  ]
}

Back-compat with the legacy enabled flag

The older boolean docker_isolation.enabled (and per-server isolation.enabled) still works and is mapped to a mode:

• an explicit mode always wins;
• otherwise enabled: true ⇒ docker, enabled: false ⇒ none;
• a missing isolation config ⇒ none.

Per-server precedence: explicit per-server mode → per-server legacy enabled → global mode → global legacy enabled.

Sandbox mode (Landlock)

sandbox mode confines a stdio server without Docker by applying a Linux Landlock LSM ruleset (a writable-path allowlist) plus setrlimit resource caps to the process before it execs, then preserving the raw stdin/stdout JSON-RPC pipes. It needs no user namespaces, so it is unaffected by kernel.apparmor_restrict_unprivileged_userns=1 — which is exactly why it works where bubblewrap/userns-based sandboxes are blocked (e.g. Ubuntu 24.04 with snap-docker).

Scanner behaviour under each mode (MCP-34.4)

The security scanner plugins are Docker-based and, since Spec 077, belong to the opt-in deep-scan layer (they run only when security.deep_scan.enabled: true). Under a non-Docker isolation mode they cannot run at all, so MCPProxy skips them and surfaces the skip informationally rather than failing silently:

Since Spec 077 (FR-008) a skipped or failed deep scanner never downgrades the baseline verdict to degraded — the old security_scan.status: "degraded" behaviour was removed. The always-emitted deep_scan descriptor carries the skip instead: { "enabled": <bool>, "ran": <bool>, "available": <bool>, "scanners_failed": [...], "skipped_scanners": [...] }. The deterministic in-process tpa-descriptions baseline scanner is the sole source of the verdict, so a low/zero risk score from a baseline-only scan is a trustworthy result, and the incomplete Docker coverage is reported as an informational note. To run the full Docker-based scanner fleet, enable deep scan and use mode: docker on a host with a working Docker daemon, or replace snap-docker with a distro Docker package (see the error doc). The skip is also logged at startup:

WARN  Isolation mode runs no Docker for scanner plugins; Docker-based scanners will be skipped …  {"isolation_mode": "sandbox"}

Configuration

Global Docker Isolation

Add to your ~/.mcpproxy/mcp_config.json:

{
  "docker_isolation": {
    "enabled": true,
    "memory_limit": "512m",
    "cpu_limit": "1.0",
    "timeout": "60s",
    "network_mode": "bridge",
    "registry": "docker.io",
    "default_images": {
      "python": "python:3.11",
      "python3": "python:3.11",
      "uvx": "python:3.11",
      "pip": "python:3.11",
      "pipx": "python:3.11",
      "node": "node:20",
      "npm": "node:20",
      "npx": "node:20",
      "yarn": "node:20",
      "go": "golang:1.21-alpine",
      "cargo": "rust:1.75-slim",
      "rustc": "rust:1.75-slim",
      "ruby": "ruby:3.2-alpine",
      "gem": "ruby:3.2-alpine",
      "php": "php:8.2-cli-alpine",
      "composer": "php:8.2-cli-alpine",
      "binary": "alpine:3.18",
      "sh": "alpine:3.18",
      "bash": "alpine:3.18"
    },
    "extra_args": []
  }
}

Configuration Options

Per-Server Configuration

You can override isolation settings per server:

{
  "mcpServers": [
    {
      "name": "custom-python-server",
      "command": "python",
      "args": ["-m", "my_server"],
      "isolation": {
        "enabled": true,
        "image": "my-custom-python:latest",
        "network_mode": "none",
        "working_dir": "/app",
        "extra_args": ["--cap-drop=ALL"]
      },
      "enabled": true
    },
    {
      "name": "no-isolation-server",
      "command": "python",
      "args": ["-m", "trusted_server"],
      "isolation": {
        "enabled": false
      },
      "enabled": true
    }
  ]
}

Runtime Detection

MCPProxy automatically detects the runtime type based on the command:

Python Environments

• python, python3 → python:3.11
• uvx → python:3.11 (includes uv package manager)
• pip, pipx → python:3.11

Node.js Environments

• node → node:20
• npm, npx → node:20
• yarn → node:20

Other Languages

• go → golang:1.21-alpine
• cargo, rustc → rust:1.75-slim
• ruby, gem → ruby:3.2-alpine
• php, composer → php:8.2-cli-alpine

Shell/Binary

• sh, bash → alpine:3.18
• Unknown commands → alpine:3.18

Why Full Images Instead of Slim/Alpine?

MCPProxy uses full Docker images (python:3.11 instead of python:3.11-slim) because:

1. Git Support: Many MCP servers install packages from Git repositories using git+https:// URLs
2. Build Tools: Some packages require compilation during installation
3. System Dependencies: Full images include common libraries needed by MCP servers

This trade-off prioritizes compatibility over image size.

Environment Variables

Environment variables from server configuration are automatically passed to containers:

{
  "mcpServers": [
    {
      "name": "api-server",
      "command": "uvx",
      "args": ["some-package"],
      "env": {
        "API_KEY": "your-secret-key",
        "DEBUG": "true"
      },
      "enabled": true
    }
  ]
}

These become Docker arguments: -e API_KEY=your-secret-key -e DEBUG=true

Docker-in-Docker Prevention

MCPProxy automatically skips isolation for servers that are already Docker commands:

{
  "mcpServers": [
    {
      "name": "existing-docker-server",
      "command": "docker",
      "args": ["run", "-i", "--rm", "mcp/some-server"],
      "enabled": true
    }
  ]
}

This prevents Docker-in-Docker complications.

Debugging

Check Docker Isolation Status

# Run with debug logging
mcpproxy serve --log-level=debug

# Filter for isolation messages
mcpproxy serve --log-level=debug 2>&1 | grep -i "docker isolation"

Monitor Docker Containers

# List MCPProxy containers
docker ps --format "table {{.Names}}\t{{.Image}}\t{{.Status}}"

# View logs from a specific container
docker logs <container-id>

# Watch container resource usage
docker stats

Common Issues

Container startup timeouts:

• Increase timeout in docker_isolation config
• Check if Docker images need to be pulled
• Verify network connectivity for package installations

Environment variables not working:

• Check that variables are defined in server env section
• Use debug logging to see Docker command arguments
• Verify container has access to required environment

Git/package installation failures:

• Ensure using full images (python:3.11 not python:3.11-slim)
• Check container logs for specific error messages
• Verify network access for package repositories

Container Lifecycle

Startup

When a Docker-isolated server starts:

1. MCPProxy detects runtime type (npm, uvx, python, etc.)
2. Selects appropriate Docker image
3. Runs container with stdio transport (docker run -i)
4. Establishes MCP connection via stdin/stdout

Shutdown

When MCPProxy stops, containers are cleaned up with a 30-second timeout:

1. Graceful Stop: docker stop (sends SIGTERM to container)
2. Force Kill: docker kill if container doesn't stop gracefully

Containers are labeled with mcpproxy.managed=true for identification.

Manual Cleanup

If containers remain after MCPProxy stops:

# List MCPProxy-managed containers
docker ps --filter "label=mcpproxy.managed=true"

# Remove all MCPProxy containers
docker rm -f $(docker ps -q --filter "label=mcpproxy.managed=true")

See Shutdown Behavior for detailed subprocess lifecycle documentation.

Honest limitations

sandbox mode is deliberately scoped. Known limitations:

• No uid/gid drop. Dropping to an unprivileged uid/gid requires CAP_SETUID/CAP_SETGID (i.e. running as root). When mcpproxy runs unprivileged, the uid/gid drop is best-effort and typically a no-op — the sandboxed process keeps the launching user's identity. Landlock (filesystem) and setrlimit (resource caps) still apply. Docker mode does drop to a container user. This is an honest trade-off, not a bug.
• Linux-only. Landlock is a Linux 5.13+ feature. On older kernels the launcher degrades best-effort (fewer access-right bits enforced). On macOS/Windows sandbox is a documented no-op and behaves like none.
• Filesystem + resources only. Landlock confines the filesystem write-allowlist; it does not provide network namespacing. For network-sensitive servers, use docker mode with network_mode: none.
• Docker-based scanners do not run under sandbox/none. They are skipped and the skip is surfaced via the informational deep_scan descriptor — it never downgrades the baseline verdict (Spec 077 FR-008). A native scanner runtime is a future enhancement.

Platform support matrix

Security Considerations

Docker isolation provides strong security boundaries but consider:

1. Network Access: Containers can still access the network by default
2. Resource Limits: Set appropriate memory/CPU limits
3. Image Trust: Use trusted base images from official repositories
4. Secrets: Environment variables are visible in container inspect output

For maximum security, consider:

• Using "network_mode": "none" for servers that don't need network access
• Adding --cap-drop=ALL to extra_args to remove Linux capabilities
• Using custom minimal images for specific use cases