browser_download · Wmux MCP: Risk & Policy

WHAT IT DOES

What browser_download does on Wmux

AI agents invoke browser_download to trigger actions in Wmux. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why browser_download needs a policy

This tool triggers a browser click action that initiates a file download to the local filesystem. It combines browser automation (Execute) with writing a file to disk. The blast radius is high because an AI agent could be directed to download arbitrary files (including malware or large data) to the host system, and the download target is controlled by the ref argument which could be manipulated.

From the tool's definition 'Click an element (identified by ref) and capture the resulting download. Returns the downloaded file path.'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

browser_download Rate-limited

browser_download 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.

View as policy code

policy.json

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

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about browser_download

What does the browser_download tool do? +

Click an element (identified by ref) and capture the resulting download. Returns the downloaded file path. It is categorised as a Execute tool in the Wmux MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on browser_download? +

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

What risk level is browser_download? +

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

Can I rate-limit browser_download? +

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

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

browser_download is provided by the Wmux MCP server (openwong2kim/wmux). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.