write-tag-values

THE RISK

Medium Risk

Why write-tag-values needs a policy

This tool modifies operational data in a critical industrial control system (SIEMENS WinCC Unified). While reversible (values can be re-written), misuse could cause significant operational disruption, safety hazards, or product loss. The blast radius is high because SCADA systems control physical infrastructure.

From the tool's definition Tool name is 'write-tag-values' and description states it 'Updates tag values'. In SCADA systems, tag values represent real-world industrial parameters (pressure, temperature, flow rates, setpoints, etc.).

Documented attack patterns abuse exactly the kind of access write-tag-values gives an agent:

POLICY

How to control write-tag-values

PolicyLayer is an MCP gateway — it sits between your AI agents and WinCC Unified MCP XT, and nothing reaches the server without passing your rules. This is the rule we recommend for write-tag-values:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "write-tag-values": {
      "limits": [
        {
          "counter": "write-tag-values_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

write-tag-values 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.

Create a free account and register WinCC Unified MCP XT — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More WinCC Unified MCP XT tools

Execute reset-alarms Resets one or more active alarms based on their identifiers. Execute acknowledge-alarms Acknowledges one or more active alarms based on their identifiers. Read browse-objects Retrieves tags, elements, types, alarms, logging tags, and other named objects based on sp Read get-active-alarms Retrieves active alarms from the specified systems. Read get-logged-alarms Retrieves historical (logged) alarms from the storage system. Read get-logged-tag-values Retrieves historical tag values from the database for the specified tags or logging tags. Read get-tag-values Retrieves tag values from WinCC Unified based on the provided names or filter criteria. Other login-user Authenticates a user in WinCC Unified using a provided username and password.

All 9 WinCC Unified MCP XT tools →

Write tools on other servers

M-Team MCP Server download_torrent YouTube MCP Server generate_video_title Android Forensics ADB MCP Server combine_reports 0xarchive web3_signup

Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about write-tag-values

What does the write-tag-values tool do? +

Updates tag values based on a provided list of TagValueInput entries. It is categorised as a Write tool in the WinCC Unified MCP XT 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-tag-values? +

Register the WinCC Unified MCP XT MCP server in PolicyLayer and add a rule for write-tag-values: 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 WinCC Unified MCP XT. Nothing to install.

What risk level is write-tag-values? +

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

Can I rate-limit write-tag-values? +

Yes. Add a rate_limit block to the write-tag-values 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-tag-values completely? +

Set action: deny in the PolicyLayer policy for write-tag-values. 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-tag-values? +

write-tag-values is provided by the WinCC Unified MCP XT MCP server (mrwan84/wincc-unified-mcp-xt). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every WinCC Unified MCP XT tool call.

Start from WinCC Unified MCP XT, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →