files_get_upload_url_external · Slack MCP: Risk & Policy

WHAT IT DOES

What files_get_upload_url_external does on Slack

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

WHY IT NEEDS A POLICY

Why files_get_upload_url_external needs a policy

The tool name indicates it retrieves an upload URL for external files, which is part of a write workflow (uploading files to Slack). Getting an upload URL is a precursor to writing data. The description is empty, lowering confidence. Severity is medium as misuse could enable unauthorized file uploads, but this step alone only generates a URL rather than completing the upload.

From the tool's definition Tool name 'files_get_upload_url_external' suggests obtaining a URL for external file upload, implying a write/upload operation preparation step.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

files_get_upload_url_external Rate-limited

files_get_upload_url_external 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": {
    "files_get_upload_url_external": {
      "limits": [
        {
          "counter": "files_get_upload_url_external_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Slack, so you can run agents against it safely.

FAQ

Questions about files_get_upload_url_external

What does the files_get_upload_url_external tool do? +

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

How do I enforce a policy on files_get_upload_url_external? +

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

What risk level is files_get_upload_url_external? +

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

Can I rate-limit files_get_upload_url_external? +

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

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

files_get_upload_url_external is provided by the Slack MCP server (karbassi/slack-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.