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Re: [webappsec] CSP: are blob uri's really just origin='self'?

From: Ian Melven <ian.melven@gmail.com>
Date: Fri, 30 Aug 2013 14:26:53 -0700
Message-ID: <CA+0m=Ffe6Gt=5g=fydX3j59JbYwDVPE6hauBnmMuVyEiFGNkUg@mail.gmail.com>
To: Brad Hill <hillbrad@gmail.com>
Cc: "public-webappsec@w3.org" <public-webappsec@w3.org>
Hi,

interesting !

according to http://www.w3.org/TR/FileAPI/#originOfBlob :

The origin <http://www.w3.org/TR/FileAPI/#origin> of a Blob
URI<http://www.w3.org/TR/FileAPI/#url> must
be the origin <http://www.w3.org/TR/FileAPI/#origin> of the script that
called URL.createObjectURL<http://www.w3.org/TR/FileAPI/#dfn-createObjectURL>
.. Blob URIs <http://www.w3.org/TR/FileAPI/#url> must only be valid within
this origin <http://www.w3.org/TR/FileAPI/#origin>.

Things get more interesting when blobs are passed cross origin via
postMessage, for example. Gecko can't quite do this
(see https://bugzilla.mozilla.org/show_bug.cgi?id=722126) but from that bug
it sounds like other browsers do support this.

In the cross origin/postMessage case especially, 'self' seems like the
wrong thing to be doing.

On first thought at least, linking blobs to 'unsafe-eval' seems like a
reasonable thing to do - as you say
the code is coming from unsafe strings...

thanks,
ian



On Fri, Aug 30, 2013 at 2:05 PM, Brad Hill <hillbrad@gmail.com> wrote:

> I started writing CSP tests for workers, and realized that the blob:
> scheme can be used to circumvent inline-script and eval protections. You
> can grab text out of the DOM or any string, use createObjectURL() and run
> it as script, so long as 'self' is in the policy.
>
> Example here:
>
> http://webappsec-test.info/web-platform-tests/CSP/script-src/CSP_1_11.php
>
>
> Mozilla and Chrome both treat blob: as equivalent to 'self'.  They block
> it if it the policy disallows 'self':
>
> http://webappsec-test.info/web-platform-tests/CSP/script-src/CSP_1_11_1.php
>
>
> In another test, the script does the equivalent of an eval using the same
> blob construction:
>
> http://webappsec-test.info/web-platform-tests/CSP/script-src/CSP_1_11_2.php
>
> I wonder if this is the right treatment.  It seems that blob: data could
> come from anywhere, and that using it as the source of a script or worker
> is creating code from unsafe strings.  I wonder if we shouldn't link it to
> unsafe-inline, unsafe-eval, or both rather than to 'self'?  Otherwise it
> seems like an obvious bypass.
>
> (source at: https://github.com/hillbrad/CSP/)
>
> :(
>
> -Brad
>
Received on Friday, 30 August 2013 21:58:16 UTC

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