Test execute a script without registering it (Chrome 135+)
AI agents invoke extension_tool_userscripts_test_execute to trigger actions in WebMCP. 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.
This tool executes arbitrary JavaScript in the browser context without even registering it as a userscript. An AI agent could use it to run malicious code, exfiltrate data, manipulate the DOM, or perform unauthorized actions in the browser. The 'test' prefix does not reduce severity since actual script execution occurs.
From the tool's definition "Test execute a script without registering it" — explicitly runs/executes a script in the browser environment
Documented attack patterns abuse exactly the kind of access extension_tool_userscripts_test_execute gives an agent:
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_test_execute:
{
"version": "1",
"default": "deny",
"tools": {
"extension_tool_userscripts_test_execute": {
"limits": [
{
"counter": "extension_tool_userscripts_test_execute_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}extension_tool_userscripts_test_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.
Free to start. No card required.
Test execute a script without registering it (Chrome 135+). 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.
Register the Web MCP server in PolicyLayer and add a rule for extension_tool_userscripts_test_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 WebMCP. Nothing to install.
extension_tool_userscripts_test_execute is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the extension_tool_userscripts_test_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.
Set action: deny in the PolicyLayer policy for extension_tool_userscripts_test_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.
extension_tool_userscripts_test_execute 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.
Start from WebMCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
13 WebMCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.