javascript_execute

THE RISK

High Risk

Why javascript_execute needs a policy

This tool allows execution of arbitrary JavaScript in a browser context, which can interact with any page content, modify the DOM, exfiltrate data, trigger network requests, or interact with web APIs.

From the tool's definition Tool description explicitly states "Execute custom JavaScript code in the browser context" with capabilities for "DOM manipulation, data extraction, and custom interactions." The verb "Execute" combined with "custom JavaScript code" clearly indicates…

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

POLICY

How to control javascript_execute

PolicyLayer is an MCP gateway — it sits between your AI agents and Firefox MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for javascript_execute:

policy.json

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

javascript_execute 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 Firefox MCP Server — 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

Related tools and policies

More Firefox MCP Server tools

Destructive debug_buffers_clear Clear accumulated debug data buffers to free memory and start fresh monitoring. Destructive browser_close Close the entire browser instance and clean up all sessions and resources. Destructive session_close Close a specific browser session and clean up its resources. Execute browser_launch Launch Firefox browser with multi-session support and comprehensive debugging capabilities Execute debug_monitoring_start Start or restart comprehensive monitoring for console logs, errors, network activity, and Execute element_wait Wait for specific elements to appear or become visible. Essential for handling dynamic con Execute page_navigate Navigate to a specific URL in the browser. Works with any web page or web application. Execute debug_helpers_inject Inject custom debugging utilities and helper functions into the page context for enhanced

All 29 Firefox MCP Server tools →

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Go deeper

High-risk tools in Firefox MCP Server → 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.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about javascript_execute

What does the javascript_execute tool do? +

Execute custom JavaScript code in the browser context. Powerful for advanced DOM manipulation, data extraction, and custom interactions. It is categorised as a Execute tool in the Firefox MCP Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on javascript_execute? +

Register the Firefox MCP Server MCP server in PolicyLayer and add a rule for javascript_execute: 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 Firefox MCP Server. Nothing to install.

What risk level is javascript_execute? +

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

Can I rate-limit javascript_execute? +

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

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

javascript_execute is provided by the Firefox MCP Server MCP server (jediluke/firefox-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Firefox MCP Server tool call.

Start from Firefox MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →