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I just read about Caja, which is a "sanitized" version of JavaScript. But I'm wondering - what is the big problem with JavaScript(it seems so widely used )? Just how dangerous is it?

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    It appears to me the main difference here (security wise) is it does not give direct exposure to the DOM. Which will (in theory I suppose) negate silly errors introduced by some developers.
    – rlemon
    Mar 29, 2012 at 23:17

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The changes it makes vs. regular javascript are pretty well described here.

From that reference:

Caja takes JavaScript (technically, ECMAScript 5 strict mode code), HTML, and CSS input and rewrites it into a safe subset of HTML and CSS, plus a single JavaScript function with no free variables. That means the only way such a function can modify an object is if it is given a reference to the object by the host page. Instead of giving direct references to DOM objects, the host page typically gives references to wrappers that sanitize HTML, proxy URLs, and prevent redirecting the page; this allows Caja to prevent certain phishing attacks, prevent cross-site scripting attacks, and prevent downloading malware. Also, since all rewritten programs run in the same frame, the host page can allow one program to export an object reference to another program; then inter-frame communication is simply method invocation.

And, Google's site on Caja.

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