update_custom_field · Campaign Monitor MCP: Risk & Policy

WHAT IT DOES

What update_custom_field does on Campaign Monitor MCP

AI agents use update_custom_field 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 update_custom_field needs a policy

This tool modifies subscriber list metadata (custom fields) reversibly. It is a Write operation as it updates data without deleting it. Severity is medium because unauthorized modification of subscriber custom fields could corrupt or misrepresent subscriber data in campaigns, affecting data integrity and potentially campaign targeting, but it does not delete data (would be Destructive), execute arbitrary code…

From the tool's definition Tool name is 'update_custom_field' and description states it 'Update an existing custom field on a subscriber list', indicating modification of existing data.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

update_custom_field Rate-limited

update_custom_field 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": {
    "update_custom_field": {
      "limits": [
        {
          "counter": "update_custom_field_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 update_custom_field

What does the update_custom_field tool do? +

Update an existing custom field on a subscriber 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 update_custom_field? +

Register the Campaign Monitor MCP server in PolicyLayer and add a rule for update_custom_field: 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 update_custom_field? +

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

Can I rate-limit update_custom_field? +

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

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

update_custom_field 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.