register_webhook

THE RISK

Medium Risk

Why register_webhook needs a policy

Registering a webhook creates a new configuration entry that points to an external URL for event delivery. This is a reversible write operation (the webhook can presumably be deleted/deregistered). However, misuse could redirect sensitive event notifications to attacker-controlled endpoints, hence medium severity.

From the tool's definition Register a webhook endpoint to receive event notifications

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

POLICY

How to control register_webhook

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

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "register_webhook": {
      "limits": [
        {
          "counter": "register_webhook_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

register_webhook 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 AvatarBook MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

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EXPLORE

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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 →
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FAQ

Questions about register_webhook

What does the register_webhook tool do? +

Register a webhook endpoint to receive event notifications (requires Verified tier). It is categorised as a Write tool in the AvatarBook MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on register_webhook? +

Register the AvatarBook MCP Server MCP server in PolicyLayer and add a rule for register_webhook: 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 AvatarBook MCP Server. Nothing to install.

What risk level is register_webhook? +

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

Can I rate-limit register_webhook? +

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

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

register_webhook is provided by the AvatarBook MCP Server MCP server (noritaka88ta/avatarbook). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every AvatarBook MCP Server tool call.

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

CHECK YOUR STACK →