vm2 is a sandbox that can run untrusted code with whitelisted Node's built-in modules.
npm install vm2vm2 is a sandbox that can run untrusted code with whitelisted Node's built-in modules.
Before using vm2, you should understand how it works and its limitations.
vm2 attempts to sandbox untrusted JavaScript code within the same Node.js process as your application. It does this through a complex network of Proxies that intercept and mediate every interaction between the sandbox and the host environment.
JavaScript is an extraordinarily dynamic language. Objects can be accessed through prototype chains, constructors can be reached via error objects, symbols provide protocol hooks, and async execution creates timing windows. The sheer number of ways to traverse from one object to another in JavaScript makes building an airtight in-process sandbox extremely difficult.
We are honest about this reality: Despite our best efforts, researchers and security professionals continuously discover new ways to escape the vm2 sandbox. We actively patch these vulnerabilities as they are reported, but the cat-and-mouse nature of in-process sandboxing means that:
1. New bypasses will likely be discovered in the future. Check our security advisories for known vulnerabilities.
2. You must keep vm2 updated to benefit from the latest security fixes. Subscribe to security advisories and update promptly.
3. vm2 should not be your only line of defense. Defense in depth is essential when running untrusted code.
If you require stronger isolation guarantees, consider these alternatives that provide true process or hardware-level isolation:
| Solution | Approach | Performance | Trade-offs |
|----------|----------|-------------|------------|
| isolated-vm | Separate V8 isolates (different V8 heap) | Fast | In maintenance mode; requires manual V8 updates |
| Separate process / Worker | child_process or Worker threads with limited permissions | Medium | Higher IPC overhead; data must be serialized |
| Containers / VMs | Docker, gVisor, Firecracker | Slow | Startup overhead; resource-heavy |
| Managed services | Cloud-based code execution (e.g., AWS Lambda, Cloudflare Workers) | Variable | Network latency; external dependency |
vm2 can be suitable when:
- You need tight integration with host objects and fast synchronous communication
- The untrusted code comes from a relatively trusted source (e.g., internal tools, plugin systems with vetted authors)
- You combine vm2 with other security layers (network isolation, filesystem restrictions, resource limits)
- You accept the risk and actively monitor for security updates
If you're running code from completely untrusted sources (e.g., arbitrary user submissions), we strongly recommend using a solution with stronger isolation guarantees.
- Runs untrusted code securely in a single process with your code side by side
- Full control over the sandbox's console output
- The sandbox has limited access to the process's methods
- It is possible to require modules (built-in and external) from the sandbox
- You can limit access to certain (or all) built-in modules
- You can securely call methods and exchange data and callbacks between sandboxes
- Actively maintained with patches for known escape methods (see Security Disclaimer)
- Transpiler support
- It uses the internal VM module to create a secure context.
- It uses Proxies to prevent escaping from the sandbox.
- It overrides the built-in require to control access to modules.
For an in-depth look at vm2’s internals, see the CONTRIBUTING.md file.
Try it yourself:
``js
import { runInNewContext } from "node:vm";
runInNewContext('this.constructor.constructor("return process")().exit()');
console.log('Never gets executed.');
`
`js
import { VM } from 'vm2';
new VM().run('this.constructor.constructor("return process")().exit()');
// Throws ReferenceError: process is not defined
`
`sh`
npm install vm2
`js
import { VM } from 'vm2';
const vm = new VM();
vm.run(process.exit()); // TypeError: process.exit is not a function`
`js
import { NodeVM } from 'vm2';
const vm = new NodeVM({
require: {
external: true,
root: './',
},
});
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',
);
- VM
- NodeVM
- VMScript
- Error handling
- Debugging a sandboxed code
- Read-only objects
- Protected objects
- Cross-sandbox relationships
- CLI
- 2.x to 3.x changes
- 1.x and 2.x docs
- Contributing
VM is a simple sandbox to synchronously run untrusted code without the require feature. Only JavaScript built-in objects and Node's Buffer are available. Scheduling functions (setInterval, setTimeout and setImmediate) are not available by default.
Options:
- timeout - Script timeout in milliseconds. WARNING: You might want to use this option together with allowAsync=false. Further, operating on returned objects from the sandbox can run arbitrary code and circumvent the timeout. One should test if the returned object is a primitive with typeof and fully discard it (doing logging or creating error messages with such an object might also run arbitrary code again) in the other case.sandbox
- - VM's global object.compiler
- - javascript (default), typescript, coffeescript or custom compiler function. The library expects you to have compiler pre-installed if the value is set to typescript or coffeescript.eval
- - If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc.) will throw an EvalError (default: true).wasm
- - If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError (default: true).allowAsync
- - If set to false any attempt to run code using async will throw a VMError (default: true).
IMPORTANT: Timeout is only effective on synchronous code that you run through run. Timeout does NOT work on any method returned by VM. There are some situations when timeout doesn't work - see #244.
`js
import { VM } from 'vm2';
const vm = new VM({
timeout: 1000,
allowAsync: false,
sandbox: {},
});
vm.run('process.exit()'); // throws ReferenceError: process is not defined
`
You can also retrieve values from VM.
`js`
let number = vm.run('1337'); // returns 1337
TIP: See tests for more usage examples.
Unlike VM, NodeVM allows you to require modules in the same way that you would in the 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), typescript, coffeescript or custom compiler function (which receives the code, and it's file path). The library expects you to have compiler pre-installed if the value is set to typescript or coffeescript.eval
- - If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc.) will throw an EvalError (default: true).wasm
- - If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError (default: true).sourceExtensions
- - Array of file extensions to treat as source code (default: ['js']).require
- - true, an object or a Resolver to enable require method (default: false).require.external
- - Values can be true, an array of allowed external modules, or an object (default: false). All paths matching /node_modules/${any_allowed_external_module}/(?!/node_modules/) are allowed to be required.require.external.modules
- - Array of allowed external modules. Also supports wildcards, so specifying ['@scope/-ver-??], for instance, will allow using all modules having a name of the form @scope/something-ver-aa, @scope/other-ver-11, etc. The wildcard does not match path separators.require.external.transitive
- - Boolean which indicates if transitive dependencies of external modules are allowed (default: false). WARNING: When a module is required transitively, any module is then able to require it normally, even if this was not possible before it was loaded.require.builtin
- - Array of allowed built-in modules, accepts ["\"] for all (default: none). WARNING: "\" can be dangerous as new built-ins can be added.require.root
- - Restricted path(s) where local modules can be required (default: every path).require.mock
- - Collection of mock modules (both external or built-in).require.context
- - host (default) to require modules in the host and proxy them into the sandbox. sandbox to load, compile, and require modules in the sandbox. callback(moduleFilename, ext) to dynamically choose a context per module. The default will be sandbox is nothing is specified. Except for events, built-in modules are always required in the host and proxied into the sandbox.require.import
- - An array of modules to be loaded into NodeVM on start.require.resolve
- - An additional lookup function in case a module wasn't found in one of the traditional node lookup paths.require.customRequire
- - Use instead of the require function to load modules from the host.require.strict
- - false to not force strict mode on modules loaded by require (default: true).require.fs
- - Custom file system implementation.nesting
- - WARNING: Allowing this is a security risk as scripts can create a NodeVM which can require any host module. true to enable VMs nesting (default: false).wrapper
- - commonjs (default) to wrap script into CommonJS wrapper, none to retrieve value returned by the script.argv
- - Array to be passed to process.argv.env
- - Object to be passed to process.env.strict
- - true to loaded modules in strict mode (default: false).
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.
`js
import { NodeVM } from 'vm2';
const vm = new NodeVM({
console: 'inherit',
sandbox: {},
require: {
external: true,
builtin: ['fs', 'path'],
root: './',
mock: {
fs: {
readFileSync: () => '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.
`js`
assert.ok(vm.run('return true') === true);
TIP: See tests for more usage examples.
To load modules by relative path, you must pass the full path of the script you're running as a second argument to vm's run method if the script is a string. The filename is then displayed in any stack traces generated by the script.
`js`
vm.run('require("foobar")', '/data/myvmscript.js');
If the script you are running is a VMScript, the path is given in the VMScript constructor.
`js`
const script = new VMScript('require("foobar")', { filename: '/data/myvmscript.js' });
vm.run(script);
A resolver can be created via makeResolverFromLegacyOptions and be used for multiple NodeVM instances allowing to share compiled module code potentially speeding up load times. The first example of NodeVM can be rewritten using makeResolverFromLegacyOptions as follows.
`js`
const resolver = makeResolverFromLegacyOptions({
external: true,
builtin: ['fs', 'path'],
root: './',
mock: {
fs: {
readFileSync: () => 'Nice try!',
},
},
});
const vm = new NodeVM({
console: 'inherit',
sandbox: {},
require: resolver,
});
You can increase performance by using precompiled scripts. The precompiled VMScript can be run 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.
`js
import { VM, VMScript } from 'vm2';
const vm = new VM();
const script = new VMScript('Math.random()');
console.log(vm.run(script));
console.log(vm.run(script));
`
It works for both VM and NodeVM.
`js
import { NodeVM, VMScript } from '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 the first time it runs. One can compile the code anytime with script.compile(). Once the code is compiled, the method has no effect.
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.
`js
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);
});
`
You can debug or inspect code running in the sandbox as if it was running in a normal process.
- You can use breakpoints (which requires you to specify a script file name)
- You can use debugger keyword.
- You can use step-in to step inside the code running in the sandbox.
/tmp/main.js:
`js
import { VM, VMScript } from 'vm2';
import { readFileSync } from 'node:fs';
const file = ${__dirname}/sandbox.js;
// By providing a file name as second argument you enable breakpoints
const script = new VMScript(readFileSync(file), file);
new VM().run(script);
`
/tmp/sandbox.js
`js
const foo = 'ahoj';
// The debugger keyword works just fine everywhere.
// Even without specifying a file name to the VMScript object.
debugger;
`
To prevent sandboxed scripts from adding, changing, or deleting properties 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 cannot be frozen.
Example without using freeze:
`js
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:
`js
const vm = new VM(); // Objects specified in the sandbox cannot 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 have already been proxied to the VM.
Unlike freeze, this method allows sandboxed scripts to add, change, or delete properties on objects, with one exception - it is not possible to attach functions. Sandboxed scripts are therefore not able to modify methods like toJSON, toString or inspect.
IMPORTANT: It is not possible to protect objects that have already been proxied to the VM.
`js
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);object instanceof Object
assert.ok(vm.run());object
assert.ok(vm.run() instanceof Object);object.__proto__ === Object.prototype
assert.ok(vm.run());object
assert.ok(vm.run().__proto__ === Object.prototype);
assert.ok(vm.run(func) === sandbox.func);func instanceof Function
assert.ok(vm.run());func
assert.ok(vm.run() instanceof Function);func.__proto__ === Function.prototype
assert.ok(vm.run());func
assert.ok(vm.run().__proto__ === Function.prototype);
assert.ok(vm.run(new func() instanceof func));new func()
assert.ok(vm.run() instanceof sandbox.func);new func().__proto__ === func.prototype
assert.ok(vm.run());new func()
assert.ok(vm.run().__proto__ === sandbox.func.prototype);
assert.ok(vm.run(buffer) === sandbox.buffer);buffer instanceof Buffer
assert.ok(vm.run());buffer
assert.ok(vm.run() instanceof Buffer);buffer.__proto__ === Buffer.prototype
assert.ok(vm.run());buffer
assert.ok(vm.run().__proto__ === Buffer.prototype);buffer.slice(0, 1) instanceof Buffer
assert.ok(vm.run());buffer.slice(0, 1)
assert.ok(vm.run() instanceof Buffer);`
Before you can use vm2 in the command line, install it globally with npm install vm2 -g.
`sh`
vm2 ./script.js
- It is not possible to define a class that extends a proxied class. This includes using a proxied class in Object.create`.
- Direct eval does not work.
- Logging sandbox arrays will repeat the array part in the properties.
- Source code transformations can result a different source string for a function.
- There are ways to crash the node process from inside the sandbox.
[npm-image]: https://img.shields.io/npm/v/vm2.svg
[npm-url]: https://www.npmjs.com/package/vm2
[license-image]: https://img.shields.io/npm/l/vm2.svg
[license-url]: https://github.com/patriksimek/vm2/blob/resurrection/LICENSE.md
[downloads-image]: https://img.shields.io/npm/dm/vm2.svg
[downloads-url]: https://www.npmjs.com/package/vm2
[snyk-image]: https://snyk.io/test/github/patriksimek/vm2/badge.svg
[snyk-url]: https://snyk.io/test/github/patriksimek/vm2