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Re: Comment on Content Security Policy 1.1, Draft of Dec 12 2012

From: Adam Barth <w3c@adambarth.com>
Date: Sat, 5 Jan 2013 13:16:14 -0800
Message-ID: <CAJE5ia-2tXcjqC_8dTFhCdW-b_yu6maK24g3ptsKz-UE7AyzCw@mail.gmail.com>
To: Tanvi Vyas <tanvi@mozilla.com>
Cc: Mike West <mkwst@google.com>, Florian Lasinger <florian@lasinger.org>, "public-webappsec@w3.org" <public-webappsec@w3.org>
At Google, we did some prototype implementations of CSP in several
large web applications, and we found that the "or" behavior was very
useful for whitelisting one or two critical inline script blocks that
couldn't be moved out-of-line for performance reasons.

As similar issue arises when using Google Analytics.  The script
Google Analytics asks you to include in your page is roughly the
following:

var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-XXXXXX-X']);
_gaq.push(['_trackPageview']);

(function() {
  var ga = document.createElement('script'); ga.async = true;
  ga.src = 'https://ssl.google-analytics.com/ga.js';
  var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(ga, s);
})();

The first stanza sets up an array that you can push commands onto, and
then the actual script that executes scripts loads asynchronously (and
then executes the commands once it loads).

When using CSP, you still want to create the _gaq using an inline
script block so that you can start pushing command immediately while
still avoiding blocking the page to load an external script.  The
script-nonce directive lets you do this without giving up your XSS
protections.

It's not really a problem to add https://ssl.google-analytics.com to
your script-src directive.  However, it is annoying to need to add the
nonce to every other script block, even the ones that you've already
whitelisted via script-src (e.g., 'self').  With the "or" semantics,
you can use CSP as you would before and just use the nonce to
whitelist the one performance-critical inline script block.

Adam


On Tue, Dec 18, 2012 at 10:38 AM, Tanvi Vyas <tanvi@mozilla.com> wrote:
> I think the "or" behavior would be helpful to sites that have sources they
> don't know about until the last minute (i.e. ad placement).  The server
> could inject a script nonce into the script tag and the csp header.  Then it
> can later fill in the src attribute of the script tag once it knows which ad
> is going to be placed on the site.  With the "and" behavior, the server
> would have to wait until it has the advertisement before generating the csp
> header (specifically the script-src directive).
>
> ~Tanvi
>
> On Dec 18, 2012, at 6:44 AM, Mike West <mkwst@google.com> wrote:
>
> Hi Flo, thanks for the feedback. I'll add an example to make the section
> more clear.
>
> Regarding the behavior in general, I do think there's room for argument
> about the interaction between the two directives. The currently specified
> "and" behavior makes sense to me, and seems to have the best security
> properties as it asks the developer to explicitly whitelist all possible
> sources of script for a page via 'script-src', and then specifically allow
> each in a given context via the nonce.
>
> It does seem to be surprising, however. You're certainly not the first to
> note that the current behavior doesn't match your expectations.
>
> Changing the directive to more "or"ish behavior would mean that, given a
> nonce, script from untrusted origins could be loaded. I don't think there's
> a way to exploit that without already having script access to the page, but
> I haven't thought about it enough to be sure.
>
> I'm interested in others' opinions. :)
>
> -mike
>
> --
> Mike West <mkwst@google.com>, Developer Advocate
> Google Germany GmbH, Dienerstrasse 12, 80331 München, Germany
> Google+: https://mkw.st/+, Twitter: @mikewest, Cell: +49 162 10 255 91
>
>
>
> On Wed, Dec 12, 2012 at 4:51 AM, Florian Lasinger <florian@lasinger.org>
> wrote:
>>
>> @chapter „4.12.2 Interaction with the script-src directive“
>>
>>
>>
>> The document contains one example for the case
>>
>> „nonce provided and correct / src not allowed by script-src directive“.
>>
>>
>>
>> There should be an example for the inverse case
>>
>> „no nonce provided / src allowed by script-src directive“.
>>
>>
>>
>> As it currently stands, the second case script would be rejected because
>> it doesn’t have a nonce.
>>
>> Intuitively I would assume the script to be safe because it comes from a
>> whitelisted origin.
>>
>>
>>
>> Therefore I would propose to restrict the relevant enforcing rule to only
>> script tags with content.
>>
>>
>>
>>
>>
>> Sincerely,
>>
>> Flo
>>
>>
>
>
Received on Saturday, 5 January 2013 21:17:22 GMT

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