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Re: [SRI] To trust or not to trust a CDN

From: Mike West <mkwst@google.com>
Date: Wed, 29 Oct 2014 08:26:57 +0100
Message-ID: <CAKXHy=dVDW1+JtuKjy5P8FzYtiOcvrrZ5oaCoy9W=8D5To7drA@mail.gmail.com>
To: Brian Smith <brian@briansmith.org>
Cc: Joel Weinberger <jww@chromium.org>, Frederik Braun <fbraun@mozilla.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>
I think a future iteration of SRI could certainly tie CSP hashes to SRI
digests, with some syntax that forced both to be present. That is, given:

    CSP: script-src 'sha256-kh....lkhf'

    <script src='http://whatever.com/script.js'
integrity='ni:///kh...lkhf'> <!-- loads! -->
    <script src='http://whatever.com/script.js'> <!-- doesn't! -->

-mike

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On Wed, Oct 29, 2014 at 6:37 AM, Brian Smith <brian@briansmith.org> wrote:

> On Joel Weinberger <jww@chromium.org> wrote:
>
>> On Tue, Oct 28, 2014 at 1:02 PM, Frederik Braun <fbraun@mozilla.com>
>> wrote:
>>
>>> If I want to use CSP I am saying "I trust this whole origin". If I want
>>> to use SRI I say "I do not trust this origin, but that file is fine".
>>>
>>> With SRI in place means we will put an untrusted origin (it's untrusted
>>> - otherwise we wouldn't use SRI?) into the CSP whitelist. Meaning that
>>> every file from this untrusted origin is fine.
>>>
>> Not necessarily. With paths in CSP, it's certainly possible that you'd
>> only list the one file.
>>
>
> I suspect that for usability reasons, most websites are going to have
> pretty course-grained CSP policies, whitelisting whole domains or fairly
> non-specific path prefixes.
>
>
>>
>>> This is a weird contradiction. Assuming XSS and thinking the CDN may
>>> indeed become evil, an attacker may just include <script
>>> src="https://cdn.example/evil.js"> instead of <script
>>> src="https://cdn.example/lib.js integrity=valid> and it will work :(
>>>
>> I don't see this as a contradiction at all. Perhaps a better way to
>> describe CSP and SRI is:
>>    * CSP is for protecting your web application from client-side
>> compromise
>>    * SRI is for protecting your web application from third-party server
>> compromise
>>
>
> I mostly agree with your classification. But, I also agree with Frederik
> that it seems strange that there's no way to combine SRI with CSP while
> honoring the principle of least privilege.
>
> Consider this:
>
>     Content-Security-Policy: script-src http://example.com/foo.js
>
>     <script src='http://example.com/foo.js' integrity='ni:[digest]'>
>
>     <script src='http://example.com/foo.js'>
>
> The first <script> is an intended part of the page. The second script is
> injected with XSS. Thus, an XSS attack is able to undo the protection of
> SRI even when CSP is in place, and there is nothing you can do about it
> with CSP, *even* if you use a full path. That seems wrong. On the other
> hand, your server-side development framework should never have let the XSS
> happened in the first place, and CSP is the last line of defense, not the
> first.
>
> Cheers,
> Brian
>
Received on Wednesday, 29 October 2014 07:27:46 UTC

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