vm_start

THE RISK

High Risk

Why vm_start needs a policy

Starting a virtual machine executes an operation that triggers external hypervisor actions and state changes. While not destructive or financial in nature, this falls under Execute because it runs a system command/operation whose effects (VM startup, resource allocation) depend on the provided ID argument.

From the tool's definition Tool name 'vm_start' and description 'Start a virtual machine by its ID' indicates execution of a VM operation. The optional 'memory overcommit' parameter shows this triggers external system operations with effects dependent on arguments.

Documented attack patterns abuse exactly the kind of access vm_start gives an agent:

POLICY

How to control vm_start

PolicyLayer is an MCP gateway — it sits between your AI agents and Truenas, and nothing reaches the server without passing your rules. This is the rule we recommend for vm_start:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "vm_start": {
      "limits": [
        {
          "counter": "vm_start_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

vm_start stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

Create a free account and register Truenas — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

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All 279 Truenas tools →

Execute tools on other servers

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Go deeper

High-risk tools in Truenas → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about vm_start

What does the vm_start tool do? +

Start a virtual machine by its ID. Optionally allow memory overcommit. It is categorised as a Execute tool in the Truenas MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on vm_start? +

Register the Truenas MCP server in PolicyLayer and add a rule for vm_start: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Truenas. Nothing to install.

What risk level is vm_start? +

vm_start is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit vm_start? +

Yes. Add a rate_limit block to the vm_start rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block vm_start completely? +

Set action: deny in the PolicyLayer policy for vm_start. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides vm_start? +

vm_start is provided by the Truenas MCP server (spranab/truenas-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Truenas tool call.

Start from Truenas, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →