shell_spawn · Mcpx MCP: Risk & Policy

WHAT IT DOES

What shell_spawn does on Mcpx

AI agents invoke shell_spawn to trigger actions in Mcpx. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why shell_spawn needs a policy

'shell_spawn' almost certainly creates a new shell or interactive session based on server context. Spawning a shell gives arbitrary command execution capability, making this an Execute-category tool with critical severity. The empty description lowers confidence slightly but the server description and sibling tools make the intent clear.

From the tool's definition Tool name 'shell_spawn' on a server described as enabling 'command execution, interactive sessions' with sibling tools like 'shell_exec' and 'shell_script' strongly implies spawning an interactive shell process.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

shell_spawn Rate-limited

shell_spawn 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.

View as policy code

policy.json

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

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Mcpx, so you can run agents against it safely.

FAQ

Questions about shell_spawn

What does the shell_spawn tool do? +

shell_spawn. It is categorised as a Execute tool in the Mcpx MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on shell_spawn? +

Register the Mcpx MCP server in PolicyLayer and add a rule for shell_spawn: 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 Mcpx. Nothing to install.

What risk level is shell_spawn? +

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

Can I rate-limit shell_spawn? +

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

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

shell_spawn is provided by the Mcpx MCP server (rmednitzer/relay-shell). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.