execute untrusted code with custom permissions

Jailed — flexible JS sandbox

Jailed is a small JavaScript library for running untrusted code in a
sandbox. The library is written in vanilla-js and has no dependencies.

With Jailed you can:

• Load an untrusted code into a secure sandbox;

• Export a set of external functions into the sandbox.

The untrusted code may then interract with the main application by
directly calling those functions, but the application owner decides
which functions to export, and therefore what will be allowed for the
untrusted code to perform.

The code is executed as a plugin, a special instance running as a
restricted subprocess (in Node.js), or in a web-worker inside a
sandboxed frame (in case of web-browser environment). The iframe is
created locally, so that you don't need to host it on a separate
(sub)domain.

You can use Jailed to:

• Setup a safe environment for executing untrusted code, without a
  need to create a sandboxed worker / subprocess manually;

• Do that in an isomorphic way: the syntax is same both for Node.js
  and web-browser, the code works unchanged;

• Execute a code from a string or from a file;

• Initiate and interrupt the execution anytime;

• Demo safely
  execute user-submitted code;

• Demo embed
  3rd-party code and provide it the precise set of functions to
  harmlessly operate on the part of your application;

• Export the particular set of application functions into the sandbox
  (or in the opposite direction), and let those functions be invoked
  from the other site (without a need for manual messaging) thus
  building any custom API and set of permissions.

For instance:

JS
var path = 'http://path.to/the/plugin.js';

// exported methods, will be available to the plugin  
var api = {  
    alert: alert  
};

var plugin = new jailed.Plugin(path, api);  

plugin.js:

JS
// runs in a sandboxed worker, cannot access the main application,  
// with except for the explicitly exported alert() method

// exported methods are stored in the application.remote object  
application.remote.alert('Hello from the plugin!');  

(exporting the alert() method is not that good idea actually)

Under the hood, an application may only communicate to a plugin
(sandboxed worker / jailed subprocess) through a messaging mechanism,
which is reused by Jailed in order to simulate the exporting of
particular functions. Each exported function is duplicated on the
opposite site with a special wrapper method with the same name. Upon
the wrapper method is called, arguments are serialized, and the
corresponding message is sent, which leads to the actual function
invocation on the other site. If the executed function then issues a
callback, the responce message will be sent back and handled by the
opposite site, which will, in turn, execute the actual callback
previously stored upon the initial wrapper method invocation. A
callback is in fact a short-term exported function and behaves in the
same way, particularly it may invoke a newer callback in reply.

Installation

For the web-browser environment — download and unpack the
distribution, or install it using Bower:

SH
$ bower install jailed  

Load the jailed.js in a preferrable way. That is an UMD module, thus
for instance it may simply be loaded as a plain JavaScript file using
the <script> tag:

HTML
<script src="jailed/jailed.js"></script>  

For Node.js — install Jailed with npm:

SH
$ npm install jailed  

and then in your code:

JS
var jailed = require('jailed');  

Optionally you may load the script from the
distribution:

JS
var jailed = require('path/to/jailed.js');  

After the module is loaded, the two plugin constructors are available:
jailed.Plugin and jailed.DynamicPlugin.

Usage

The messaging mechanism reused beyond the remote method invocation
introduces some natural limitations for the exported functions and
their usage (nevertheless the most common use-cases are still
straightforward):

• Exported function arguments may only be either simple objects (which
  are then serialized and sent within a message), or callbacks (which
  are preserved and replaced with special identifiers before
  sending). Custom object instance may not be used as an argument.

• A callback can not be executed several times, it will be destroyed
  upon the first invocation.

• If several callbacks are provided, only one of them may be called.

• Returned value of an exported function is ignored, result should be
  provided to a callback instead.

In Node.js the
send()
method of a child process object is used for transfering messages,
which serializes an object into a JSON-string. In a web-browser
environment, the messages are transfered via
postMessage()
method, which implements the structured clone
algorithm
for the serialization. That algorithm is more capable than JSON (for
instance, in a web-browser you may send a RegExp object, which is not
possible in Node.js). More details about structured clone algorithm
and its comparsion to
JSON.

A plugin object may be created either from a string containing a
source code to be executed, or with a path to the script. To load a
plugin code from a file, create the plugin using jailed.Plugin
constructor and provide the path:

JS
var path = 'http://path.to/some/plugin.js';

// set of methods to be exported into the plugin  
var api = {  
    alert: alert  
}

var plugin = new jailed.Plugin(path, api);  

plugin.js:

JS
application.remote.alert('Hello from the plugin!');  

Creating a plugin from a string containing a code is very similar,
this is performed using jailed.DynamicPlugin constructor:

JS
var code = "application.remote.alert('Hello from the plugin!');";

var api = {  
    alert: alert  
}

var plugin = new jailed.DynamicPlugin(code, api);  

The second api argument provided to the jailed.Plugin and
jailed.DynamicPlugin constructors is an interface object with a set
of functions to be exported into the plugin. It is also possible to
export functions in the opposite direction — from a plugin to the main
application. It may be used for instance if a plugin provides a method
to perform a calculation. In this case the second argument of a plugin
constructor may be omitted. To export some plugin functions, use
application.setInterface() method in the plugin code:

JS
// create a plugin  
var path = "http://path.to/some/plugin.js";  
var plugin = new jailed.Plugin(path);

// called after the plugin is loaded  
var start = function() {  
    // exported method is available at this point  
    plugin.remote.square(2, reportResult);  
}

var reportResult = function(result) {  
    window.alert("Result is: " + result);  
}

// execute start() upon the plugin is loaded  
plugin.whenConnected(start);  

plugin.js:

JS
// provides the method to square a number  
var api = {  
    square: function(num, cb) {  
        // result reported to the callback  
        cb(num*num);  
    }  
}

// exports the api to the application environment  
application.setInterface(api);  

In this example the whenConnected() plugin method is used at the
application site: that method subscribes the given function to the
plugin connection event, after which the functions exported by the
plugin become accessible at the remote property of a plugin.

The whenConnected() method may be used as many times as needed and
thus subscribe several handlers for a single connection event. For
additional convenience, it is also possible to set a connection
handler even after the plugin has already been connected — in this
case the handler is issued immediately (yet asynchronously).

When a plugin code is executed, a set of functions exported by the
application is already prepared. But if one of those functions is
invoked, it will actually be called on the application site. If in
this case the code of that function will try to use a function
exported by the plugin, it may not be prepared yet. To solve this, the
similar application.whenConnected() method is available on the
plugin site. The method works same as the one of the plugin object:
the subscribed handler function will be executed after the connection
is initialized, and a set of functions exported by each site is
available on the opposite site.

Therefore:

• If you need to load a plugin and supply it with a set of exported
  functions, simply provide those functions into the plugin
  constructor, and then access those at application.remote property
  on the plugin site — the exported functions are already prepared
  when the plugin code is exectued.

• If you need to load a plugin and use the functions it provides
  through exporting, set up a handler using plugin.whenConnected()
  method on the application site. After the event is fired, the
  functions exported by the plugin are available at its remote
  property of the plugin object;.

• If both application and a plugin use the exported functions of each
  other, and the communication is initiated by the plugin, you will
  most likely need to use the application.whenConnected() method on
  the plugin site before initiating the communication, in order to
  make sure that the functions exported by the plugin are already
  available to the application.

To disconnect a plugin, use the disconnect() method: it kills a
worker / subprocess immediately without any chance for its code to
react.

A plugin may also disconnect itself by calling the
application.disconnect() method.

In addition to whenConnected() method, the plugin object also
provides similar whenFailed() and whenDisconnected() methods:

• whenFailed() subscribes a handler function to the connection
  failure event, which happens if there have been some error during
  the plugin initialization, like a network problem or a syntax error
  in the plugin initialization code.

• whenDisconnected() subscribes a function to the disconnect event,
  which happens if a plugin was disconnected by calling the
  disconnect() method, or a plugin has disconnected itself by
  calling application.disconnect(), or if a plugin failed to
  initialize (along with the failure event mentioned above). After the
  event is fired, the plugin is not usable anymore.

Just like as for whenConnected() method, those two methods may also
be used several times or even after the event has actually been fired.

Compatibility

Jailed was tested and should work in Node.js, and in the following
browsers:

• Internet Explorer 10+, Edge
• Firefox 26+
• Opera 12+
• Safari 6+
• Chrome 10+

Security

This is how the sandbox is built:

In a web-browser:

• a sandboxed
  iframe
  is created with its sandbox attribute only set to "allow-scripts"
  (to prevent the content of the frame from accessing anything of the
  main application origin);

• then a web-worker is started inside that frame;

• finally the code is loaded by the worker and executed.

Note: when Jailed library is loaded from the local source (its path
starts with file://), the "allow-same-origin" permission is added
to the sandbox attribute of the iframe. Local installations are
mostly used for testing, and without that permission it would not be
possible to load the plugin code from a local file. This means that
the plugin code has an access to the local filesystem, and to some
origin-shared things like IndexedDB (though the main application page
is still not accessible from the worker). Therefore if you need to
safely execute untrusted code on a local system, reuse the Jailed
library in Node.js.

In Node.js:

Warning: according to recent reports
(#33) this way of
sandboxing is not secure any longer, the fix is being prepared...

• A Node.js subprocess is created by the Jailed library;

• the subprocess (down)loads the file containing an untrusted code as
  a string (or, in case of DynamicPlugin, simply uses the provided
  string with code)

• then "use strict"; is appended to the head of that code (in order
  to prevent breaking the sandbox using arguments.callee.caller);

• finally the code is executed using vm.runInNewContext() method,
  where the provided sandbox only exposes some basic methods like
  setTimeout(), and the application object for messaging with the
  application site.

--

follow me on twitter: https://twitter.com/asvd0

Source: https://github.com/asvd/jailed