Get all custom fields for a subscriber list
AI agents call get_custom_fields to retrieve information from Campaign Monitor MCP without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool queries and retrieves custom field metadata associated with a subscriber list. It performs no create, update, delete, or execute operations. The action is purely informational retrieval, which is characteristic of Read category tools. The severity is low because retrieving field definitions poses minimal risk even if accessed by an unauthorized agent.
From the tool's definition Tool name 'get_custom_fields' and description 'Get all custom fields for a subscriber list' indicate a retrieval operation with no modification or side effects.
Attacks that exploit this kind of access
Get all custom fields for a subscriber list. It is categorised as a Read tool in the Campaign Monitor MCP MCP Server, which means it retrieves data without modifying state.
Register the Campaign Monitor MCP server in PolicyLayer and add a rule for get_custom_fields: 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.
get_custom_fields is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the get_custom_fields 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.
Set action: deny in the PolicyLayer policy for get_custom_fields. 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.
get_custom_fields 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.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
Teams ship this data inside their own products. See what a licence covers →