Egor Homakov: HTML5 Sandbox - a bad idea

I don't like both idea and implementations of the Sandbox feature (part of so-called HTML5). Most of the articles about it are lame copy-pastes of the spec. Straight question WHY WE NEED IT AT ALL remains not answered. Furthermore, nobody writes that implementation is a little bit crazy — by fixing small vulnerabilities it introduces huge ones.

Why?

When we embed frames with external content we are still open for navigational and/or phishing attacks (obviously cookies/inner HTML cannot be stolen because of the same origin policy). The iframe code can change top.location or open a phishing popup window prompt('your password?'). ... that's it. That's all we needed to fix.

Besides allow-popup and allow-top-navigation options W3C introduced allow-scripts, allow-forms and allow-same-origin. They wanted best you know the rest.

Javascript... malicious shit.

W3C needed a really good reasonto introduce such a crazy feature: "Javascript from a frame can do malicious actions, now you can turn it off".
Malicious what? Tomorrow they will call Javascript a Remote Code Execution Vulnerability and ask us to remove the browser? Javascript cannot be malicious on it's own!

Untrusted code on the same origin.

Another announced killer-feature of Sandbox - running untrusted code on your origin (remark: USE A SUBDOMAIN DONT BE STUPID). People tend to believe it: "Cool, I will set up a javascript playground on my website!". This?

Now think again, what stops an attacker to simply use 'SITE/playground?code=alert(1)' as an XSS hole?

You cannot rely on it out-of-box, thus you need a way more comprehensive solution — you have to make sure that current page is loaded under sandbox (request is authentic), not in a new window

Now you are secure from arbitrary code reflection. Anyway, don't use it.