write_registers · Kc Modbus MCP: Risk & Policy

WHAT IT DOES

What write_registers does on Kc Modbus

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

WHY IT NEEDS A POLICY

Why write_registers needs a policy

This tool writes data to Modbus registers in industrial controllers (PLCs). Writing to PLC registers can alter operational parameters, setpoints, or control states, making it a reversible Write action but with potentially serious consequences in a production environment (high severity). It is not Destructive (reversible), not Execute (doesn't run arbitrary code), and not Financial.

From the tool's definition Tool name 'write_registers' combined with server description stating 'write PLC registers' indicates the tool modifies register values in industrial control systems. Empty description limits specificity but the name and server context are conclusive.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

write_registers Rate-limited

write_registers 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": {
    "write_registers": {
      "limits": [
        {
          "counter": "write_registers_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 Kc Modbus, so you can run agents against it safely.

FAQ

Questions about write_registers

What does the write_registers tool do? +

write_registers. It is categorised as a Write tool in the Kc Modbus MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on write_registers? +

Register the Kc Modbus MCP server in PolicyLayer and add a rule for write_registers: 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 Kc Modbus. Nothing to install.

What risk level is write_registers? +

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

Can I rate-limit write_registers? +

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

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

write_registers is provided by the Kc Modbus MCP server (kerberosclaw/kc_modbus_mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.