matrix_join_room

THE RISK

High Risk

Why matrix_join_room needs a policy

Joining a Matrix room triggers an external operation (federated join across servers) with lasting side effects — the account becomes a member of that room, which is not trivially reversible (leaving is a separate action). This goes beyond a simple write of local data and involves executing a federated protocol action on potentially remote servers, making Execute the most appropriate category.

From the tool's definition Join a room by ID or alias. If the room lives on another server, Dendrite federates the join

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

POLICY

How to control matrix_join_room

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 matrix_join_room:

policy.json

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

matrix_join_room 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.
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FAQ

Questions about matrix_join_room

What does the matrix_join_room tool do? +

Join a room by ID or alias. If the room lives on another server, Dendrite federates the join (may take several seconds). Rate-limited: 30/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 matrix_join_room? +

Register the Crow MCP server in PolicyLayer and add a rule for matrix_join_room: 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 matrix_join_room? +

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

Can I rate-limit matrix_join_room? +

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

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

matrix_join_room 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 →