extension_tool_userscripts_register

THE RISK

High Risk

Why extension_tool_userscripts_register needs a policy

Registering a userscript injects arbitrary JavaScript code into the browser environment via Chrome's userScripts API. This constitutes code execution in the browser context, with potentially broad blast radius as injected scripts can access page content, credentials, cookies, and perform further actions on behalf of the user.

From the tool's definition Register a new userscript with Chrome userScripts API

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

POLICY

How to control extension_tool_userscripts_register

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

policy.json

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

extension_tool_userscripts_register 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 WebMCP — 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 →

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EXPLORE

Related tools and policies

More WebMCP tools

Destructive extension_tool_userscripts_unregister Unregister userscripts by ID or filter Execute call_extension_tool Execute a browser extension tool. Use this to call extension tools that provide browser-le Execute call_website_tool Execute a tool from a website MCP server. Use this to call website-specific tools that you Execute extension_tool_userscripts_test_execute Test execute a script without registering it (Chrome 135+) Execute extension_tool_userscripts_configure_world Configure USER_SCRIPT world properties Write extension_tool_userscripts_reset_world Reset USER_SCRIPT world configuration to defaults Write extension_tool_userscripts_update Update existing userscripts Read extension_tool_check_available_apis Check which Chrome Extension APIs are available to the extension

All 13 WebMCP tools →

Execute tools on other servers

Android Forensics ADB MCP Server adb_shell_command EMBA-MCP run_emba_scan Frida Game Hacking MCP remove_breakpoint Procmon request_elevation

Go deeper

High-risk tools in WebMCP → 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 extension_tool_userscripts_register

What does the extension_tool_userscripts_register tool do? +

Register a new userscript with Chrome userScripts API. It is categorised as a Execute tool in the WebMCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on extension_tool_userscripts_register? +

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

What risk level is extension_tool_userscripts_register? +

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

Can I rate-limit extension_tool_userscripts_register? +

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

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

extension_tool_userscripts_register is provided by the Web MCP server (miguelspizza/webmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every WebMCP tool call.

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

CHECK YOUR STACK →