Caja (pronounced "KAhah"[2]) is a Google project for "virtual iframes" based on the principles of object-capabilities. Caja takes JavaScript, 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.
The word "caja" is Spanish for “box” or "safe" (as in a bank), the idea being that Caja can safely contain JavaScript programs as well as being a capabilities-based JavaScript.
Caja emulates ECMAScript 5 strict mode (called "Valija" in the Caja documents) on top of a much smaller subset of JavaScript, named "Cajita". Cajita removes all the parts of JavaScript that are insecure or that make defensive programming impractical, resulting in an object-capability language. Then Valija simulates the parts that are incompatible with defensive programming (such as monkey patching) while maintaining isolation. Cajita is much faster than Valija and more secure than JavaScript, but typically incompatible with existing code.
Caja is being adopted by MySpace[3][4] and Yahoo!.[5]
References
See also
- Joe-E, an object-capability subset of Java
- ADsafe, a similar subset of Javascript
- E
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