download_attachment

THE RISK

Low Risk

Why download_attachment needs a policy

This tool retrieves existing attachment data from Fastmail. While attachments may contain sensitive information, the operation itself is read-only with no reversible or irreversible modifications to data. The act of downloading does not alter email state, attachment metadata, or any Fastmail records.

From the tool's definition Tool downloads and retrieves email attachment data without modifying, deleting, or creating data. Returns file path/size or download URL—purely retrieval operations with no side effects.

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

POLICY

How to control download_attachment

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

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "download_attachment": {}
  }
}

download_attachment is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.

Create a free account and register Fastmail 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.

CAP THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Data exfiltration → How read access becomes an exfiltration channel when chained with untrusted content.
MCP token cost → What connecting this server costs in context-window tokens on every request.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about download_attachment

What does the download_attachment tool do? +

Download an email attachment. If savePath is provided, saves the file to disk and returns the file path and size. Otherwise returns a download URL. It is categorised as a Read tool in the Fastmail MCP Server MCP Server, which means it retrieves data without modifying state.

How do I enforce a policy on download_attachment? +

Register the Fastmail MCP Server MCP server in PolicyLayer and add a rule for download_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 Fastmail MCP Server. Nothing to install.

What risk level is download_attachment? +

download_attachment is a Read tool with low risk. Read-only tools are generally safe to allow by default.

Can I rate-limit download_attachment? +

Yes. Add a rate_limit block to the download_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 download_attachment completely? +

Set action: deny in the PolicyLayer policy for download_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 download_attachment? +

download_attachment is provided by the Fastmail MCP Server MCP server (madllama25/fastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Fastmail MCP Server tool call.

Start from Fastmail 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 →