vm2

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 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
• Is immune to all known methods of attacks
• Transpiler support

How does it work

• 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.

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

Try it yourself:

1const vm = require('vm');
2vm.runInNewContext('this.constructor.constructor("return process")().exit()');
3console.log('Never gets executed.');

1const {VM} = require('vm2');
2new VM().run('this.constructor.constructor("return process")().exit()');
3// Throws ReferenceError: process is not defined

Installation

IMPORTANT: VM2 requires Node.js 6 or newer.

1npm install vm2

Quick Example

1const {VM} = require('vm2');
2const vm = new VM();
3
4vm.run(`process.exit()`); // TypeError: process.exit is not a function

1const {NodeVM} = require('vm2');
2const vm = new NodeVM({
3    require: {
4        external: true,
5        root: './'
6    }
7});
8
9vm.run(`
10    var request = require('request');
11    request('http://www.google.com', function (error, response, body) {
12        console.error(error);
13        if (!error && response.statusCode == 200) {
14            console.log(body); // Show the HTML for the Google homepage.
15        }
16    });
17`, 'vm.js');

Documentation

• 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

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) or coffeescript or custom compiler function. The library expects you to have coffee-script pre-installed if the compiler is set to 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.

1const {VM} = require('vm2');
2
3const vm = new VM({
4    timeout: 1000,
5    allowAsync: false,
6    sandbox: {}
7});
8
9vm.run('process.exit()'); // throws ReferenceError: process is not defined

You can also retrieve values from VM.

1let number = vm.run('1337'); // returns 1337

TIP: See tests for more usage examples.

NodeVM

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) or coffeescript or custom compiler function (which receives the code, and it's file path). The library expects you to have coffee-script pre-installed if the compiler is set to 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.

1const {NodeVM} = require('vm2');
2
3const vm = new NodeVM({
4    console: 'inherit',
5    sandbox: {},
6    require: {
7        external: true,
8        builtin: ['fs', 'path'],
9        root: './',
10        mock: {
11            fs: {
12                readFileSync: () => 'Nice try!'
13            }
14        }
15    }
16});
17
18// Sync
19
20let functionInSandbox = vm.run('module.exports = function(who) { console.log("hello "+ who); }');
21functionInSandbox('world');
22
23// Async
24
25let functionWithCallbackInSandbox = vm.run('module.exports = function(who, callback) { callback("hello "+ who); }');
26functionWithCallbackInSandbox('world', (greeting) => {
27    console.log(greeting);
28});

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

1assert.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 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.

1vm.run('require("foobar")', '/data/myvmscript.js');

If the script you are running is a VMScript, the path is given in the VMScript constructor.

1const script = new VMScript('require("foobar")', {filename: '/data/myvmscript.js'});
2vm.run(script);

Resolver

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.

1const resolver = makeResolverFromLegacyOptions({
2    external: true,
3    builtin: ['fs', 'path'],
4    root: './',
5    mock: {
6        fs: {
7            readFileSync: () => 'Nice try!'
8        }
9    }
10});
11const vm = new NodeVM({
12    console: 'inherit',
13    sandbox: {},
14    require: resolver
15});

VMScript

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.

1const {VM, VMScript} = require('vm2');
2
3const vm = new VM();
4const script = new VMScript('Math.random()');
5console.log(vm.run(script));
6console.log(vm.run(script));

It works for both VM and NodeVM.

1const {NodeVM, VMScript} = require('vm2');
2
3const vm = new NodeVM();
4const script = new VMScript('module.exports = Math.random()');
5console.log(vm.run(script));
6console.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.

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.

1try {
2    var script = new VMScript('Math.random()').compile();
3} catch (err) {
4    console.error('Failed to compile script.', err);
5}
6
7try {
8    vm.run(script);
9} catch (err) {
10    console.error('Failed to execute script.', err);
11}
12
13process.on('uncaughtException', (err) => {
14    console.error('Asynchronous error caught.', err);
15});

Debugging a sandboxed code

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.

Example

/tmp/main.js:

1const {VM, VMScript} = require('.');
2const fs = require('fs');
3const file = `${__dirname}/sandbox.js`;
4
5// By providing a file name as second argument you enable breakpoints
6const script = new VMScript(fs.readFileSync(file), file);
7
8new VM().run(script);

/tmp/sandbox.js

1const foo = 'ahoj';
2
3// The debugger keyword works just fine everywhere.
4// Even without specifying a file name to the VMScript object.
5debugger;

Read-only objects (experimental)

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:

1const util = {
2    add: (a, b) => a + b
3}
4
5const vm = new VM({
6    sandbox: {util}
7});
8
9vm.run('util.add = (a, b) => a - b');
10console.log(util.add(1, 1)); // returns 0

Example with using freeze:

1const vm = new VM(); // Objects specified in the sandbox cannot be frozen.
2vm.freeze(util, 'util'); // Second argument adds object to global.
3
4vm.run('util.add = (a, b) => a - b'); // Fails silently when not in strict mode.
5console.log(util.add(1, 1)); // returns 2

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

Protected objects (experimental)

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.

Cross-sandbox relationships

1const assert = require('assert');
2const {VM} = require('vm2');
3
4const sandbox = {
5    object: new Object(),
6    func: new Function(),
7    buffer: new Buffer([0x01, 0x05])
8}
9
10const vm = new VM({sandbox});
11
12assert.ok(vm.run(`object`) === sandbox.object);
13assert.ok(vm.run(`object instanceof Object`));
14assert.ok(vm.run(`object`) instanceof Object);
15assert.ok(vm.run(`object.__proto__ === Object.prototype`));
16assert.ok(vm.run(`object`).__proto__ === Object.prototype);
17
18assert.ok(vm.run(`func`) === sandbox.func);
19assert.ok(vm.run(`func instanceof Function`));
20assert.ok(vm.run(`func`) instanceof Function);
21assert.ok(vm.run(`func.__proto__ === Function.prototype`));
22assert.ok(vm.run(`func`).__proto__ === Function.prototype);
23
24assert.ok(vm.run(`new func() instanceof func`));
25assert.ok(vm.run(`new func()`) instanceof sandbox.func);
26assert.ok(vm.run(`new func().__proto__ === func.prototype`));
27assert.ok(vm.run(`new func()`).__proto__ === sandbox.func.prototype);
28
29assert.ok(vm.run(`buffer`) === sandbox.buffer);
30assert.ok(vm.run(`buffer instanceof Buffer`));
31assert.ok(vm.run(`buffer`) instanceof Buffer);
32assert.ok(vm.run(`buffer.__proto__ === Buffer.prototype`));
33assert.ok(vm.run(`buffer`).__proto__ === Buffer.prototype);
34assert.ok(vm.run(`buffer.slice(0, 1) instanceof Buffer`));
35assert.ok(vm.run(`buffer.slice(0, 1)`) instanceof Buffer);

CLI

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

1vm2 ./script.js

Known Issues

• 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.

Deployment

1. Update the CHANGELOG.md
2. Update the package.json version number
3. Commit the changes
4. Run npm publish

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