evel.js

evel comes between eval and evil (code)

[see caveats below]

Usage

<script src="evel.js">
<script>
    var result = evel("42");
    console.log("Result: ", result);
</script>
<script>
    var fn = evel.Function('test', "console.log("Hello ", test)");
    fn("world");    // would work, but `fn` can't access `console` :-)
</script>

Basically, evel providesCANNOT PROVIDE (see caveats/issue tracker) an evel function that works like a eval and a evel.Function that works like Function — except access to the global environment is somewhat prevented.

Load evel.js in a page a try out each of these lines in the JS console for funsies:

// regular `eval` allows access to shared prototypes
eval("({}).__proto__") === Object.prototype;

// `evel` doesn't
evel("({}).__proto__") === Object.prototype;

// this returns `undefined`
evel("eval('alert')");

// but this doesn't!
evel("eval")('alert');

Caveats**

evel only works where ES5 strict mode does

In older browsers, evel will always throw an exception rather than running code.

JavaScript builtins still available

While evel masks out all other globals, untrusted code will still have access to JavaScript builtins of a "clean" iframe. This should usually be fine, so long as leaking the user's local and current time is okay for your application, but if a poorly-written browser plugin/extension adds more functionality to JS core this could also be a concern.

The Halting Problem

Credit: Dominic Tarr

A malicious script could while(true); and freeze the page. There's not a lot we could do about this while still allowing syncronous return values. This a denial of service attack: it doesn't directly give the attacker much, but it does break the user experience.

To avoid this, you could design your code to work asyncronously and maybe use evel from within a communicating worker or something — but at that point consider using even more robust alternatives on the server-side if it makes sense for your application.

[** still] Unproven

While I can't think of any other [new] ways to subvert it … maybe someone else will think of more? See the list of known bypasses in the issue tracker.

MIT license

Copyright (c) 2013 Nathan Vander Wilt

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.