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11.1.2 Subsets for Security

11.1.2 Subsets for Security


The Good Parts is a language subset designed for aesthetic reasons and with a desire to improve programmer productivity. There is a larger class of subsets that have been designed for the purpose of safely running untrusted JavaScript in a secure container or “sandbox.” Secure subsets work by disallowing all language features and APIs that can allow code to break out of its sandbox and affect the global execution environment. Each subset is coupled with a static verifier that parses code to ensure that it conforms to the subset. Since language subsets that can be statically verified tend to be quite restrictive, some sandboxing systems define a larger, less restrictive subset and add a code transformation step that verifies that code conforms to the larger subset, transforms it to use a smaller language subset, and adds runtime checks where static analysis of the code is not sufficient to ensure security.

In order to allow JavaScript to be statically verified to be safe, a number of features must be removed:

11.1 JavaScript Subsets | 267

properties—these functions perform runtime checks to ensure that they aren’t used to access forbidden properties.

Some of these restrictions, such as forbidding the use of eval() and the with statement, are not much of a burden for programmers, since these features are not commonly used in JavaScript programming. Others, such as the restriction on the use of square brackets for property access are quite onerous, and this is where code translation comes in. A translator can automatically transform the use of square brackets, for example, into a function call that includes runtime checks. Similar transformations can allow the safe use of the this keyword. There is a tradeoff, of course, between the safety of these runtime checks and execution speed of the sandboxed code.

A number of secure subsets have been implemented. Although a complete description of any subset is beyond the scope of this book, we’ll briefly describe some of the most important:

ADsafe ADsafe ( http://adsafe.org ) was one of the first security subsets proposed. It was created by Douglas Crockford (who also defined The Good Parts subset). ADsafe relies on static verification only, and it uses JSLint ( http://jslint.org ) as its verifier. It forbids access to most global variables and defines an ADSAFE variable that provides access to a secure API, including special-purpose DOM methods. ADsafe is not in wide use, but it was an influential proof-of-concept that influenced other secure subsets.

dojox.secure The dojox.secure subset ( http://www.sitepen.com/blog/2008/08/01/secure-mashups -with-dojoxsecure/ ) is an extension to the Dojo toolkit ( http://dojotoolkit.org ) that was inspired by ADsafe. Like ADsafe, it is based on static verification of a restrictive language subset. Unlike ADsafe, it allows use of the standard DOM API. Also, it includes a verifier written in JavaScript, so that untrusted code can be dynamically verified before being evaluated.

Caja Caja ( http://code.google.com/p/google-caja/ ) is Google’s open-source secure subset. Caja (Spanish for “box”) defines two language subsets. Cajita (“little box”) is a narrow subset like that used by ADsafe and dojox.secure. Valija (“suitcase” or “baggage”) is a much broader language that is close to regular ECMAScript 5 strict mode (with the removal of eval()). Caja itself is the name of the compiler that transforms (or “cajoles”) web content (HTML, CSS, and JavaScript code) into secure modules that can be safely hosted on a web page without being able to affect the page as a whole or other modules on the page.

Caja is part of the OpenSocial API ( http://code.google.com/apis/opensocial/ ) and has been adopted by Yahoo! for use on its websites. The content available at the portal http://my.yahoo.com , for example, is organized into Caja modules.

FBJS FBJS is the variant of JavaScript used by Facebook ( http://facebook.com ) to allow untrusted content on users’ profile pages. FBJS relies on code transformation to ensure security. The transformer inserts runtime checks to prevent access to the global object through the this keyword. And it renames all top-level identifiers by adding a module-specific prefix. Any attempt to set or query global variables or variables belonging to another module is prevented because of this renaming. Furthermore, any calls to eval() are transformed by this identifier prefixing into calls to a nonexistent function. FBJS emulates a safe subset of the DOM API.

Microsoft Web Sandbox Microsoft’s Web Sandbox ( http://websandbox.livelabs.com/ ) defines a broad subset of JavaScript (plus HTML and CSS) and makes it secure through radical code rewriting, effectively reimplementing a secure JavaScript virtual machine on top of nonsecure JavaScript.

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