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Re 2: Javascript Security for Dummies

From: Ivan Herman <ivan@w3.org>
Date: Fri, 05 Mar 2010 09:22:29 +0100
Message-ID: <4B90BF45.3060500@w3.org>
To: Toby Inkster <tai@g5n.co.uk>
CC: RDFa WG <public-rdfa-wg@w3.org>
Sorry to have cut that into two mails, I shouldn't have...

You say that most modern browsers already implement or will implement
CORS. In other words, we could expect that all browsers implementing
HTML5 will have CORS included, right? If so, at least for the HTML5+RDFa
case, the whole issue may be solved, right?


On 2010-3-4 20:37 , Toby Inkster wrote:
> Apologies for cutting out of the telecon a few minutes early. As I left
> I volunteered to write a quick summary of how Javascript/JSON/JSONP/CORS
> relate to each other and the various security issues involved.
> Let's start with the basics: Javascript (more properly known as
> ECMAScript these days) is a scripting language with various
> implementations, the best known being the ones that are embedded in most
> modern, graphical browsers. Javascript, as implemented in browsers, runs
> in a sandbox to prevent maliciously crafted web pages from doing any
> damage to the visitor's machine.
> A good number of years ago, Microsoft implemented a proprietary
> extension to Javascript which allowed scripts to perform HTTP requests
> and make use of the responses. Mozilla implemented some fairly similar
> functionality, and eventually other browsers followed (using the Mozilla
> syntax). This feature is called XmlHttpRequest or XHR - it's a bit of a
> misnomer as it's not restricted to retrieving XML.
> For security reasons, XHR requests are only allowed to be performed to
> URLs on the same domain as the script itself was loaded from. This is a
> good thing because you don't want http://evil.example/badpage.html to be
> able to perform an XHR to <http://bank.example/account-statement.html>
> especially given that the XHR would be sent with all the applicable
> cookies in the browser's cookie jar.
> (Aside: technically it's not cross-domain requests that are disallowed,
> but cross-origin requests. An origin is a slightly wooly concept:
> foo.example.com and bar.example.com are considered to be the same
> origin, foo.co.uk and bar.co.uk are different origins, despite the fact
> that from a DNS viewpoint, they're both third-level domain names. This
> is all being standardised currently.)
> Douglas Crockford "discovered" JSON. He maintains that he didn't invent
> it, just realise that Javascript contained a useful bit of syntax that
> could be standardised on. JSON is a restricted subset of Javascript's
> notation for objects and arrays. (JSON = Javascript Object Notation.)
> JSON is a data format that allows strings, numbers, booleans, arrays and
> associative arrays to be represented. In many ways it can be considered
> a competitor to XML. It's also pretty similar to YAML (though the
> oft-quoted statement that it's a subset of YAML is an urban myth).
> Here's how a person might be represented in JSON:
> 	{
> 		"name"     : "Toby Inkster" ,
> 		"homepage" : "http://tobyinkster.co.uk/" ,
> 		"mbox"     : "mailto:mail@tobyinkster.co.uk"
> 	}
> Getting back to XHR, often people want to be able to request data from
> other origins, circumventing the same-origin policy enforced by
> browsers. With a little bit of extra syntax, JSON can be useful for
> this. This extra syntax is called JSONP. (JSONP = JSON plus Payload.)
> The way that JSONP works is that instead of supplying a JSON response,
> the server responding with the data sends a Javascript response, like
> this (usually the name of the callback function is configurable as a
> query string):
> 	callback_function({
> 		"name"     : "Toby Inkster" ,
> 		"homepage" : "http://tobyinkster.co.uk/" ,
> 		"mbox"     : "mailto:mail@tobyinkster.co.uk"
> 	});
> How does this circumvent XHR's same-origin policy? Answer: it doesn't.
> But it eliminates the need to use XHR at all. The page requesting the
> data doesn't need to perform an XHR request, it just defines a function
> called callback_function to deal with the data, then it loads the JSONP
> file using a standard HTML <script src> element. The browser downloads
> and executes the script, and calls the function with the data as a
> parameter.
> However, this opens up a big security hole. Suppose that the server
> supplying a JSONP response is compromised, or its owner just decides to
> turn to the dark side. The server can send arbitrary Javascript (i.e.
> not JSONP) and the browser will execute it unquestioningly. This
> Javascript could be used to steal cookies, passwords and other
> privileged information from the page it was included in. Not nice.
> CORS is another way around the same-origin policy, but this time it's
> not a hack. It's a set of HTTP headers that a URL can respond with to
> indicate that it's safe to be retrieved in cross-origin requests. So if,
> say, http://bank.example/homepage.html contains no private data and is
> perfectly safe for other sites to have access to, then it could set a
> CORS header to allow http://evil.example/badpage.html to try its worst.
> http://bank.example/account-statement.html wouldn't send the CORS header
> so would be protected by the default same-origin policy.
> So how does this apply to RDFa vocabularies/profiles?
> If vocabularies are hosted on a separate server to the pages making use
> of them, then Javascript implementations of RDFa would need to make a
> cross-origin request to read them. (Actually they could make a
> same-origin request to a proxying script, but that's not an especially
> elegant solution.)
> If we want such Javascript implementations of RDFa to be possible, this
> allows two solutions:
> 	1. Serve up the vocabulary document as JSONP; or
> 	2. Serve it up as something else plus CORS headers.
> #1 is problematic because as I said, JSONP is not nice, safe JSON,
> despite the similar names. JSONP is Javascript.
> #2 is problematic because CORS is a very new feature. Many of the newest
> browsers support it (including IE8), but if you want your script to work
> in downlevel browsers, this is not your solution.
> In my next message I'll outline how my RDFa vocab proposal (which is
> slightly different to Manu's) makes this a moot point by saying that
> retrieval of the vocab document is optional - a SHOULD requirement
> rather than a MUST - and provides a fallback in the case, e.g. of
> browsers which don't implement CORS.


Ivan Herman, W3C Semantic Web Activity Lead
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Received on Friday, 5 March 2010 08:22:17 GMT

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