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Local Eval

Evaluate a string of JS code without access to the global object.

Always use that instead of eval(). Always.


localeval(code :: String, sandbox :: Object) :: Object.

localeval(code :: String,    sandbox :: Object,
          timeout :: Number, cb :: Function)

The code is a string of JS code. The sandbox contains objects which are going to be accessible in the JS code. It returns the last evaluated piece of JS code in code, if no timeout is given. Otherwise, after at most timeout milliseconds, the callback gives that result as a parameter: function(error, result) {…}.

Node example:

var localeval = require('localeval');
localeval('console.log("Do I have access to the console?")');  // Throws. 
localeval.clear();  // Kills processes used internally. 

Browser example:

<!doctype html><title></title>
<script src='localeval.js'></script>
<!-- Alerts "32". -->
<script> alert(localeval('a + b', {a: 14, b: 18})) </script> 

You may find an example of use in browser code in main.html.


Trying to find a reasonable cross-environment ES5 sandbox evaluation function.

Offering a process-separated timeout-ed VM for node.


If no timeout is given, it doesn't protect your single-threaded code against infinite loops. Additionally, the following leak:

  • ({}).constructor.getOwnPropertyNames = function(){return 'leak';}
  • Function(" = 'leak'")()

If a timeout is given, an attacker can still use XHR:

  • Function("this.XMLHttpRequest(…); …")()

That said, it protects against any security leak.

  1. All local and global variables are inaccessible.

  2. Variables defined while evaluating code don't pollute any scope.

  3. Evaluated code cannot fiddle with global object's properties. Think localeval('([]).__proto__.push = function(a) { return "nope"; }').

Things to try

In comments are what should be executed outside the sandbox.

String.prototype.slice = function() { return 'leaked'; };
// 'nice'.slice(1) === 'ice' 
String.fromCharCode = function() { return 'leaked'; };
// String.fromCharCode(42) === '*' 
// var foo = 1 
foo = 7 = 7 = 7
eval('foo = 7')
// foo === 1 
delete Number.parseInt
// Number.parseInt('1337') === 1337 
String.prototype.leak = function() { return 'leak'; }
// try { ''.leak() } catch(e) { /not a function/.test(e.message) } 

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