fw_block_user

THE RISK

High Risk

Why fw_block_user needs a policy

This tool triggers an external operation (blocking a user on a federated platform) that has significant social/access-control consequences. It is not merely writing data — it actively restricts another user's ability to interact, which is a privileged, impactful action. It is rate-limited, suggesting the platform recognizes its potential for misuse.

From the tool's definition Block a single user (by full actor handle @user@server). Inline; rate-limited: 5/hour.

Documented attack patterns abuse exactly the kind of access fw_block_user gives an agent:

POLICY

How to control fw_block_user

PolicyLayer is an MCP gateway — it sits between your AI agents and Crow, and nothing reaches the server without passing your rules. This is the rule we recommend for fw_block_user:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "fw_block_user": {
      "limits": [
        {
          "counter": "fw_block_user_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

fw_block_user stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

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

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

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Go deeper

High-risk tools in Crow → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
Rate Limiting MCP Tool Calls: A Practical Guide →
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FAQ

Questions about fw_block_user

What does the fw_block_user tool do? +

Block a single user (by full actor handle @user@server). Inline; rate-limited: 5/hour. It is categorised as a Execute tool in the Crow MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on fw_block_user? +

Register the Crow MCP server in PolicyLayer and add a rule for fw_block_user: 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 Crow. Nothing to install.

What risk level is fw_block_user? +

fw_block_user is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit fw_block_user? +

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

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

fw_block_user is provided by the Crow MCP server (kh0pper/crow). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Crow tool call.

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

CHECK YOUR STACK →