execute_javascript · Browser MCP Bridge MCP: Risk & Policy

WHAT IT DOES

What execute_javascript does on Browser MCP Bridge

AI agents invoke execute_javascript to trigger actions in Browser MCP Bridge. 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 execute_javascript needs a policy

This tool allows arbitrary JavaScript execution within a live browser page context, which can trigger side effects including DOM modifications, API calls, form submissions, redirects, and interaction with page state. While it does not directly delete data or move money, it is the mechanism to accomplish such actions and constitutes an Execute category risk.

From the tool's definition Tool name explicitly states "execute_javascript" with description "Execute JavaScript code in the browser page context".

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

execute_javascript Rate-limited

execute_javascript 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": {
    "execute_javascript": {
      "limits": [
        {
          "counter": "execute_javascript_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 Browser MCP Bridge, so you can run agents against it safely.

FAQ

Questions about execute_javascript

What does the execute_javascript tool do? +

Execute JavaScript code in the browser page context. It is categorised as a Execute tool in the Browser MCP Bridge MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on execute_javascript? +

Register the Browser MCP Bridge MCP server in PolicyLayer and add a rule for execute_javascript: 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 Browser MCP Bridge. Nothing to install.

What risk level is execute_javascript? +

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

Can I rate-limit execute_javascript? +

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

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

execute_javascript is provided by the Browser MCP Bridge MCP server (robhicks/browser-mcp-bridge). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.