start_custom_app

THE RISK

High Risk

Why start_custom_app needs a policy

Starting a custom app executes external operations (Docker container initialization) whose side effects depend on the app's configuration and code. This is not a simple read operation. While reversible via stop_custom_app, the act of starting an app with unknown workload, network exposure, or resource consumption presents significant risk if an AI misuses it.

From the tool's definition Tool starts a stopped Custom App, which triggers external Docker Compose operations whose effects depend on app configuration.

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

POLICY

How to control start_custom_app

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

policy.json

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

start_custom_app 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 Scale MCP Server — 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

Related tools and policies

More TrueNAS Scale MCP Server tools

Destructive delete_custom_app Delete a Custom App and optionally its data volumes Destructive cleanup_stale_containers Clean up stale containers and fix conflicts Execute bulk_app_operation Start/stop multiple apps at once Execute deploy_custom_app Deploy a new Custom App from Docker Compose configuration Execute stop_custom_app Stop a running Custom App Write update_custom_app Update an existing Custom App with new Docker Compose configuration Read diagnose_docker_issues Diagnose Docker engine issues and network conflicts Read get_app_logs Retrieve logs from a Custom App

All 13 TrueNAS Scale MCP Server tools →

Execute tools on other servers

Android Forensics ADB MCP Server adb_shell_command EMBA-MCP run_emba_scan Frida Game Hacking MCP remove_breakpoint Procmon request_elevation

Go deeper

High-risk tools in TrueNAS Scale MCP Server → 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 start_custom_app

What does the start_custom_app tool do? +

Start a stopped Custom App. It is categorised as a Execute tool in the TrueNAS Scale MCP Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on start_custom_app? +

Register the TrueNAS Scale MCP Server MCP server in PolicyLayer and add a rule for start_custom_app: 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 Scale MCP Server. Nothing to install.

What risk level is start_custom_app? +

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

Can I rate-limit start_custom_app? +

Yes. Add a rate_limit block to the start_custom_app 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 start_custom_app completely? +

Set action: deny in the PolicyLayer policy for start_custom_app. 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 start_custom_app? +

start_custom_app is provided by the TrueNAS Scale MCP Server MCP server (svnstfns/truenas-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every TrueNAS Scale MCP Server tool call.

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

CHECK YOUR STACK →