browser_evaluate

THE RISK

High Risk

Why browser_evaluate needs a policy

This tool executes user-supplied JavaScript in the browser context of the active page. While the result is read-only (returns output), the JavaScript execution itself can trigger arbitrary actions including DOM manipulation, network requests via fetch/XMLHttpRequest, local storage access, and cookie theft.

From the tool's definition Tool performs 'Run a JavaScript expression on the current page' — direct code execution capability that can access DOM, make network requests, modify page state, and trigger side effects based on arbitrary input.

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

POLICY

How to control browser_evaluate

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

policy.json

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

browser_evaluate 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 Web Scraper — 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 Web Scraper tools

Destructive clear_cache Clear the response cache. Use when cached data may be stale. Destructive clear_history Clear scraping history. Destructive clear_host_profile Delete one host profile record from the profile store. Destructive clear_session Clear a browser session (cookies, storage). Use for fresh starts. Execute cancel_job Cancel a running async job. Execute browser_navigate Navigate the interactive browser to a URL. Execute browser_solve_challenge Explicitly trigger challenge detection and solving on the current page. Execute browser_wait_for Wait for selector state or a fixed delay on the active page.

All 62 Web Scraper tools →

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Go deeper

High-risk tools in Web Scraper → 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 browser_evaluate

What does the browser_evaluate tool do? +

Run a JavaScript expression on the current page and return the result. It is categorised as a Execute tool in the Web Scraper 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_evaluate? +

Register the Web Scraper MCP server in PolicyLayer and add a rule for browser_evaluate: 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 Web Scraper. Nothing to install.

What risk level is browser_evaluate? +

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

Can I rate-limit browser_evaluate? +

Yes. Add a rate_limit block to the browser_evaluate 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_evaluate completely? +

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

browser_evaluate is provided by the Web Scraper MCP server (imyourboyroy/webscrapertoolkit). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Web Scraper tool call.

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

CHECK YOUR STACK →