execute_js · Selenium MCP: Risk & Policy

WHAT IT DOES

What execute_js does on Selenium

AI agents invoke execute_js to trigger actions in Selenium. 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_js needs a policy

The tool directly executes code (JavaScript) in a live browser environment. While the immediate effects are browser-scoped rather than system-wide, this is a powerful Execute category tool because: (1) it runs arbitrary code provided as arguments, (2) effects depend entirely on what code is passed to it, (3) an AI agent could be tricked into executing malicious JavaScript (e.g., stealing session tokens, exfiltrating…

From the tool's definition Tool executes arbitrary JavaScript in the browser context via 'Execute JavaScript in the browser'. This is code execution that can perform any action the browser allows, including modifying DOM, making network requests, stealing data, or triggering unintended…

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

execute_js Rate-limited

execute_js 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_js": {
      "limits": [
        {
          "counter": "execute_js_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 Selenium, so you can run agents against it safely.

FAQ

Questions about execute_js

What does the execute_js tool do? +

Execute JavaScript in the browser and return the result. It is categorised as a Execute tool in the Selenium 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_js? +

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

What risk level is execute_js? +

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

Can I rate-limit execute_js? +

Yes. Add a rate_limit block to the execute_js 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_js completely? +

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

execute_js is provided by the Selenium MCP server (scv-consultants/selenium-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.