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vm2

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vm2 is a sandbox that can run untrusted code with whitelisted Node's built-in modules. Securely!

Features

  • Runs untrusted code securely in a single process with your code side by side
  • Full control over sandbox's console output
  • Sandbox has limited access to process's methods
  • Sandbox can require modules (builtin and external)
  • You can limit access to certain (or all) builtin modules
  • You can securely call methods and exchange data and callbacks between sandboxes
  • Is immune to while (true) {} (see docs)
  • Is immune to all known methods of attacks
  • Transpilers support

How does it work

  • It uses internal VM module to create secure context
  • It uses Proxies to prevent escaping the sandbox
  • It overrides builtin require to control access to modules

What is the difference between Node's vm and vm2?

Try it yourself:

const vm = require('vm');
vm.runInNewContext('this.constructor.constructor("return process")().exit()');
console.log('Never gets executed.');
const {VM} = require('vm2');
new VM().run('this.constructor.constructor("return process")().exit()');

Installation

IMPORTANT: Requires Node.js 6 or newer.

npm install vm2

Quick Example

const {VM} = require('vm2');
const vm = new VM();
 
vm.run(`process.exit()`); // TypeError: process.exit is not a function
const {NodeVM} = require('vm2');
const vm = new NodeVM({
    require: {
        external: true
    }
});
 
vm.run(`
    var request = require('request');
    request('http://www.google.com', function (error, response, body) {
        console.error(error);
        if (!error && response.statusCode == 200) {
            console.log(body) // Show the HTML for the Google homepage.
        }
    })
`, 'vm.js');

Documentation

VM

VM is a simple sandbox, without require feature, to synchronously run an untrusted code. Only JavaScript built-in objects + Buffer are available. Scheduling functions (setInterval, setTimeout and setImmediate) are not available by default.

Options:

  • timeout - Script timeout in milliseconds.
  • sandbox - VM's global object.
  • compiler - javascript (default) or coffeescript or custom compiler function. The library expects you to have coffee-script pre-installed if the compiler is set to coffeescript.

IMPORTANT: Timeout is only effective on code you run through run. Timeout is NOT effective on any method returned by VM.

const {VM} = require('vm2');
 
const vm = new VM({
    timeout: 1000,
    sandbox: {}
});
 
vm.run("process.exit()"); // throws ReferenceError: process is not defined

You can also retrieve values from VM.

let number = vm.run("1337"); // returns 1337

TIP: See tests for more usage examples.

NodeVM

Unlike VM, NodeVM lets you require modules same way like in regular Node's context.

Options:

  • console - inherit to enable console, redirect to redirect to events, off to disable console (default: inherit).
  • sandbox - VM's global object.
  • compiler - javascript (default) or coffeescript or custom compiler function (which receives the code, and it's filepath). The library expects you to have coffee-script pre-installed if the compiler is set to coffeescript.
  • require - true or object to enable require method (default: false).
  • require.external - true or an array of allowed external modules (default: false).
  • require.builtin - Array of allowed builtin modules, accepts ["*"] for all (default: none).
  • require.root - Restricted path where local modules can be required (default: every path).
  • require.mock - Collection of mock modules (both external or builtin).
  • require.context - host (default) to require modules in host and proxy them to sandbox. sandbox to load, compile and require modules in sandbox. Builtin modules except events always required in host and proxied to sandbox.
  • require.import - Array of modules to be loaded into NodeVM on start.
  • nesting - true to enable VMs nesting (default: false).
  • wrapper - commonjs (default) to wrap script into CommonJS wrapper, none to retrieve value returned by the script.

IMPORTANT: Timeout is not effective for NodeVM so it is not immune to while (true) {} or similar evil.

REMEMBER: The more modules you allow, the more fragile your sandbox becomes.

const {NodeVM} = require('vm2');
 
const vm = new NodeVM({
    console: 'inherit',
    sandbox: {},
    require: {
        external: true,
        builtin: ['fs', 'path'],
        root: "./",
        mock: {
            fs: {
                readFileSync() { return 'Nice try!'; }
            }
        }
    }
});
 
// Sync
 
let functionInSandbox = vm.run("module.exports = function(who) { console.log('hello '+ who); }");
functionInSandbox('world');
 
// Async
 
let functionWithCallbackInSandbox = vm.run("module.exports = function(who, callback) { callback('hello '+ who); }");
functionWithCallbackInSandbox('world', (greeting) => {
    console.log(greeting);
});

When wrapper is set to none, NodeVM behaves more like VM for synchronous code.

assert.ok(vm.run('return true') === true);

TIP: See tests for more usage examples.

Loading modules by relative path

To load modules by relative path, you must pass full path of the script you're running as a second argument of vm's run method. Filename then also shows up in any stack traces produced from the script.

vm.run("require('foobar')", "/data/myvmscript.js");

VMScript

You can increase performance by using pre-compiled scripts. The pre-compiled VMScript can be run later multiple times. It is important to note that the code is not bound to any VM (context); rather, it is bound before each run, just for that run.

const {VM, VMScript} = require('vm2');
 
const vm = new VM();
const script = new VMScript("Math.random()");
console.log(vm.run(script));
console.log(vm.run(script));

Works for both VM and NodeVM.

const {NodeVM, VMScript} = require('vm2');
 
const vm = new NodeVM();
const script = new VMScript("module.exports = Math.random()");
console.log(vm.run(script));
console.log(vm.run(script));

Code is compiled automatically first time you run it. You can compile the code anytime with script.compile(). Once the code is compiled, the method has no effect.

Error handling

Errors in code compilation and synchronous code execution can be handled by try/catch. Errors in asynchronous code execution can be handled by attaching uncaughtException event handler to Node's process.

try {
    var script = new VMScript("Math.random()").compile();
} catch (err) {
    console.error('Failed to compile script.', err);
}
 
try {
    vm.run(script);
} catch (err) {
    console.error('Failed to execute script.', err);
}
 
process.on('uncaughtException', (err) => {
    console.error('Asynchronous error caught.', err);
})

Read-only objects (experimental)

To prevent sandboxed script to add/change/delete properties to/from the proxied objects, you can use freeze methods to make the object read-only. This is only effective inside VM. Frozen objects are affected deeply. Primitive types can not be frozen.

Example without using freeze:

const util = {
    add: (a, b) => a + b
}
 
const vm = new VM({
    sandbox: {util}
});
 
vm.run('util.add = (a, b) => a - b');
console.log(util.add(1, 1)); // returns 0

Example with using freeze:

const vm = new VM(); // Objects specified in sandbox can not be frozen.
vm.freeze(util, 'util'); // Second argument adds object to global.
 
vm.run('util.add = (a, b) => a - b'); // Fails silently when not in strict mode.
console.log(util.add(1, 1)); // returns 2

IMPORTANT: It is not possible to freeze objects that has already been proxied to the VM.

Protected objects (experimental)

Unlike freeze, this method allows sandboxed script to add/modify/delete properties on object with one exception - it is not possible to attach functions. Sandboxed script is therefore not able to modify methods like toJSON, toString or inspect.

IMPORTANT: It is not possible to protect objects that has already been proxied to the VM.

Cross-sandbox relationships

const assert = require('assert');
const {VM} = require('vm2');
 
const sandbox = {
    object: new Object(),
    func: new Function(),
    buffer: new Buffer([0x01, 0x05])
}
 
const vm = new VM({sandbox});
 
assert.ok(vm.run(`object`) === sandbox.object);
assert.ok(vm.run(`object instanceof Object`));
assert.ok(vm.run(`object`) instanceof Object);
assert.ok(vm.run(`object.__proto__ === Object.prototype`));
assert.ok(vm.run(`object`).__proto__ === Object.prototype);
 
assert.ok(vm.run(`func`) === sandbox.func);
assert.ok(vm.run(`func instanceof Function`));
assert.ok(vm.run(`func`) instanceof Function);
assert.ok(vm.run(`func.__proto__ === Function.prototype`));
assert.ok(vm.run(`func`).__proto__ === Function.prototype);
 
assert.ok(vm.run(`new func() instanceof func`));
assert.ok(vm.run(`new func()`) instanceof sandbox.func);
assert.ok(vm.run(`new func().__proto__ === func.prototype`));
assert.ok(vm.run(`new func()`).__proto__ === sandbox.func.prototype);
 
assert.ok(vm.run(`buffer`) === sandbox.buffer);
assert.ok(vm.run(`buffer instanceof Buffer`));
assert.ok(vm.run(`buffer`) instanceof Buffer);
assert.ok(vm.run(`buffer.__proto__ === Buffer.prototype`));
assert.ok(vm.run(`buffer`).__proto__ === Buffer.prototype);
assert.ok(vm.run(`buffer.slice(0, 1) instanceof Buffer`));
assert.ok(vm.run(`buffer.slice(0, 1)`) instanceof Buffer);

CLI

Before you can use vm2 in command line, install it globally with npm install vm2 -g.

$ vm2 ./script.js

Known Issues

  • It is not possible to define class that extends proxied class.

Sponsors

Development is sponsored by Integromat.

License

Copyright (c) 2014-2017 Patrik Simek

The MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.