AI agents use browser_file_upload to create or update resources in Wmux — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Wmux environment.
This tool performs a write operation: it uploads (transfers) file content to a web form, modifying the state of a web application. It is not Read (no retrieval only), not Execute (doesn't run code or arbitrary commands—only uploads pre-existing files), not Destructive (reversible; files can be re-uploaded or cleared), not Financial.
From the tool's definition Tool name is 'browser_file_upload' and description states it 'Upload files to a file input element', indicating it creates or modifies data on a web page by writing file content to form inputs.
Attacks that exploit this kind of access
Upload files to a file input element. Paths MUST live under ~/.wmux/uploads/ — arbitrary filesystem paths are rejected to prevent exfiltration of credentials or SSH keys via malicious pages. It is categorised as a Write tool in the Wmux MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Wmux MCP server in PolicyLayer and add a rule for browser_file_upload: 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.
browser_file_upload is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the browser_file_upload 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.
Set action: deny in the PolicyLayer policy for browser_file_upload. 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.
browser_file_upload 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.
browser_file_upload is one line of Wmux's registry record.
The record carries the whole server: verified identity, auth posture, risk grade, every tool classified, recommended policy — re-checked continuously.
Teams ship this data inside their own products. See what a licence covers →