inject_preload_script · JS Reverse MCP: Risk & Policy

WHAT IT DOES

What inject_preload_script does on JS Reverse MCP

AI agents invoke inject_preload_script to trigger actions in JS Reverse MCP. 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 inject_preload_script needs a policy

This tool executes code in a browser environment with elevated privileges (runs before page scripts). While not destructive by itself, it triggers external operations (browser script injection) whose effects depend entirely on the injected code arguments.

From the tool's definition Tool registers JavaScript snippets to run before page scripts execute, enabling 'preload hooks, environment patches, and early instrumentation.' This allows arbitrary code execution in browser contexts with full control over the execution environment before…

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

inject_preload_script Rate-limited

inject_preload_script 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": {
    "inject_preload_script": {
      "limits": [
        {
          "counter": "inject_preload_script_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 JS Reverse MCP, so you can run agents against it safely.

FAQ

Questions about inject_preload_script

What does the inject_preload_script tool do? +

Register a JavaScript snippet that will run on future document loads before page scripts execute. Use this for preload hooks, environment patches, and early instrumentation. It is categorised as a Execute tool in the JS Reverse MCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on inject_preload_script? +

Register the JS Reverse MCP server in PolicyLayer and add a rule for inject_preload_script: 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 JS Reverse MCP. Nothing to install.

What risk level is inject_preload_script? +

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

Can I rate-limit inject_preload_script? +

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

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

inject_preload_script is provided by the JS Reverse MCP server (noone-hub/jsreverser-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.