mcp-deno-sandbox

Deno Sandbox MCP Server

An MCP server that allows you to run TypeScript, JavaScript, and Python code in a sandbox on your local machine using the Deno® sandbox. This server provides a controlled environment for executing code with explicit permission controls (i.e. which websites it can visit, which files it can read).

Note: This project is not affiliated with Deno Land LLC in any way. I'm just a fan of the Deno® runtime. "Deno" is a registered trademark of Deno Land LLC.

How and why

LLMs are great at writing code and it helps if the LLM can run the code itself to test it. The problem is that they cannot be trusted not to do damage, especially if a malicious human can trick the LLM using prompt injection. For example you innocently ask a LLM to summarise an email and someone wrote you an email telling your LLM to run code to delete all your files and send all your bitcoin to them.

A sandbox enforces limitations on what the code written by your LLM can do. For example we might say it can only change files in a specific folder, or contact specific trusted websites.

Each operating system has different ways of creating sandboxes. Some are more secure, some are easier to setup. This project strikes a balance between the two.

Our sandbox uses Deno which in turn uses the same technology Chrome uses to stop malicious websites damaging your computer. This lets you run Typescript and Javascript and relies on very little of my code.

Some hard working people have also made Pyodide which lets you run Python inside a web browser. We use that to let you run Python inside the same Deno environment.

You control permissions by passing arguments which are given directly to the Deno runtime. You can configure which:

• files can be read (e.g. your codebase)
• which files can we written
• deny access to specific files (e.g. ssh keys)
• which websites or IP addresses can be accessed

Non Features

I would like to keep this codebase simpler to read than alternatives so people can audit it themselves. Simplicity is an important security feature; the more people who understand it, the more who can spot a bug.

I have also chosen to sacrifice some performance by not reusing pyodide environments. This creates a small overhead but it makes the implementation easier to reason about.

Inspiration

• Simon Willison's Pyodide sandboxing experiments
• Pydantics's MCP Run Python

Usage with Claude Desktop

This MCP should work with a range of MCP clients.

You either need Node.js or Deno installed. You don't need both.

To use this MCP server with Claude Desktop, add it to your claude_desktop_config.json in:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json

If you have Deno installed

{
  "mcpServers": {
    "denoSandbox": {
      "command": "deno",
      "args": [
        "run",
        "npm:mcp-deno-sandbox",
        "--allow-net=icanhazip.com,example.com",
      ]
    }
  }
}

If you have Node.js installed then Deno will be installed automatically

{
  "mcpServers": {
    "denoSandbox": {
      "command": "npx",
      "args": [
        "mcp-deno-sandbox",
        "--allow-net=icanhazip.com,example.com"
      ]
    }
  }
}

You can help your LLM by encouraging it to use these tools. There is also a resource which defines the permissions available in the sandbox. You can suggest that your LLM checks what permissions it has if it keeps getting permission denied errors.

Permission Examples

You need to set the permissions at runtime; if you change them you need to restart the server.

The permissions are the same as the Deno® permissions and are just passed through.

Examples

1. Network Access

   • Permissive: --allow-net
     • Allows all network access
   • Restricted: --allow-net=api.github.com,example.com
     • Allows network access only to specific domains

2. File System

   • Permissive: --allow-read --allow-write
     • Full file system access
     • Tradeoff: Rogue LLMs can access your dotfiles and anything else you can (including the file it can edit to give itself more permissions)
   • Restricted: --allow-read=/tmp --allow-write=/tmp
     • Limited to specific directories
     • Good for processing isolated files

For a complete list of permissions and detailed documentation, see Deno® Security.

Security Considerations

This server runs code using the permissions specified when starting the server. These permissions are passed through to the Deno® runtime.

Be careful about what permissions you enable. The sandbox is completely undermined by:

• giving blanket FFI or execution permissions
• allowing write access to the file which manages the server permissions (e.g. claude_desktop_config.json)

You should also think carefully about read permissions to sensitive dotfiles you have (e.g. with credentials for AWS, NPM, OpenAI); especially if you have granted network access.

Remember malicious people can use prompt injection to trick your prefered language model into running bad things on your computer. Maybe they can hide some invisible text in a PDF which you cannot read or in the middle of a long document you ask it to summarise.

Deno® has some additional suggestions if you would like even more isolation for untrusted code.

Known Issues

Python

• you cannot use open(PATH, 'w') to write files for some reason. It works for reading files, I don't know what writing doesn't work. I included a hint how to get around this in the tool description. You can also point out to your LLM that it can run import js; js.fs.writeFileSync(PATH, CONTENT). This runs in the same Deno sanbox but is a little bit more annoying.
• If you give -A, -R, or -W permissions, it is hard to tell which files should be mounted into the pyodide environment. For now, I just guess your home directory and /tmp. You can also specify other directories explicitly and these will be mounted.
• I am not sure if this mounting will work correctly on Windows.

Development

# Clone the repository
git clone https://github.com/bewt85/mcp-deno-sandbox.git
cd mcp-deno-sandbox

# Install dependencies
npm install

Check the code formatting and types:

npm run checks

Fix some issues automatically:

npm run fix

Test with the MCP Inspector:

npx @modelcontextprotocol/inspector ./node_modules/.bin/ts-node src/index.ts

Try these examples in the inspector:

1. Basic arithmetic (works without permissions):

   console.log(1 + 2);

2. Network access (requires --allow-net):

   fetch('https://icanhazip.com').then(response => response.text()).then(ip => console.log(`Your IP is: ${ip.trim()}`));

3. File system access (requires --allow-read):

   const text = Deno.readTextFileSync('/path/to/file.txt');
   console.log(text);

Contributing

I don't have a lot of spare time so I will not be able to engage with most feature requests / contributions. It is probably better to fork the repository if you would like to add something. Apologies.

Releases

When you want to do a release:

• update the version in package.json to X.Y.Z
• merge your changes
• make a release in GitHub vX.Y.Z
• wait for it to be automatically deployed to NPM

License

MIT