open_attachment · Eml MCP: Risk & Policy

WHAT IT DOES

What open_attachment does on Eml

AI agents invoke open_attachment to trigger actions in Eml. 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 open_attachment needs a policy

This tool triggers execution of an external application on the host system to open a file. The specific application launched depends on the file type and system configuration, meaning an attacker could potentially cause arbitrary executables to be launched if a malicious attachment is present. This goes beyond a simple read operation — it invokes system-level execution.

From the tool's definition 'Open an attachment with the system default application'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

open_attachment Rate-limited

open_attachment 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": {
    "open_attachment": {
      "limits": [
        {
          "counter": "open_attachment_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 Eml, so you can run agents against it safely.

FAQ

Questions about open_attachment

What does the open_attachment tool do? +

Open an attachment with the system default application. It is categorised as a Execute tool in the Eml MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on open_attachment? +

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

What risk level is open_attachment? +

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

Can I rate-limit open_attachment? +

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

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

open_attachment is provided by the Eml MCP server (miguelripoll23/eml-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.