add_subscriber · Campaign Monitor MCP: Risk & Policy

WHAT IT DOES

What add_subscriber does on Campaign Monitor MCP

AI agents use add_subscriber to create or update resources in Campaign Monitor MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Campaign Monitor MCP environment.

WHY IT NEEDS A POLICY

Why add_subscriber needs a policy

This tool creates new data (subscriber records) in a reversible manner—subscribers can be removed or modified later. It does not execute arbitrary code, delete data permanently, or move money. The severity is medium because misuse could lead to unauthorized subscriber additions, spam list inflation, or data integrity issues, but the action itself is not inherently destructive or financial.

From the tool's definition Tool name 'add_subscriber' and description 'Add a subscriber to a list' indicate creation of new subscriber records in a mailing list database.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

add_subscriber Rate-limited

add_subscriber stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "add_subscriber": {
      "limits": [
        {
          "counter": "add_subscriber_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about add_subscriber

What does the add_subscriber tool do? +

Add a subscriber to a list. It is categorised as a Write tool in the Campaign Monitor MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on add_subscriber? +

Register the Campaign Monitor MCP server in PolicyLayer and add a rule for add_subscriber: 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 Campaign Monitor MCP. Nothing to install.

What risk level is add_subscriber? +

add_subscriber is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit add_subscriber? +

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

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

add_subscriber is provided by the Campaign Monitor MCP server (pauliowest/cmon-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.