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Re: Finalizing the shape of CSP ‘unsafe-dynamic’

From: Devdatta Akhawe <dev.akhawe@gmail.com>
Date: Wed, 8 Jun 2016 05:26:17 -0700
Message-ID: <CAPfop_1Zg4rd6jc_UxVMiphjGS2CVvDN_PrnBty1Zc49r2FpTA@mail.gmail.com>
To: Artur Janc <aaj@google.com>
Cc: Mike West <mkwst@google.com>, Brad Hill <hillbrad@gmail.com>, WebAppSec WG <public-webappsec@w3.org>, Christoph Kerschbaumer <ckerschbaumer@mozilla.com>, Daniel Bates <dabates@apple.com>, Devdatta Akhawe <dev@dropbox.com>
Hi

I agree with the trade-off that Artur outlined. My general inclination is
that specifications/browsers should make more flexible, low-level
primitives and then linters, stackoverflow, frameworks to warn about unsafe
uses or provide safe high level abstractions. But, as Artur points out, it
is not clear this has worked for CSP. So, now this is almost a meta
question. I am ok with whatever Mike, as editor, prefers :)

--dev

On 6 June 2016 at 10:10, Artur Janc <aaj@google.com> wrote:

> On Mon, Jun 6, 2016 at 5:48 PM, Mike West <mkwst@google.com> wrote:
>
>> On Mon, Jun 6, 2016 at 4:45 PM, Devdatta Akhawe <dev.akhawe@gmail.com>
>> wrote:
>>>
>>> Are there use-cases for these separately? I'm all for adding things to
>>>> the platform if they're useful, but I'm not convinced from this thread that
>>>> these keywords add anything other than complexity. That is, Brad can
>>>> accomplish the things he's interested in with two policies, which I think
>>>> actually turns out to be a more powerful primitive than splitting the
>>>> keywords.
>>>>
>>>
>>> hmm .. the use case I am interested in is script-src
>>> https://www.dropbox.com/script/require.js 'allow-dynamic'
>>>
>>> Right now, this would force me to use a nonce. The nonce then is a
>>> leakable token present in the HTML of the page. Not a huge risk, but still.
>>>
>>
>> I see. That makes sense to me: you want the loader functionality, but you
>> don't want the added surface area of the capability token.
>>
>> I think I agree that there are two orthogonal concerns here (loading an
>> initial script via a nonce, hash, or URL on the one hand, and allowing that
>> initial script to load other things on the other), and splitting them up
>> makes sense. I worry a bit that folks would simply slap the dynamic keyword
>> onto an already weak policy and call it done, but I suppose that doesn't
>> make things any worse than they are today.
>>
>
> This idea (splitting unsafe-dynamic into something like 'allow-dynamic'
> and 'drop-whitelist'; the names are for illustrative purposes only!) is
> very interesting to discuss, and I think it's a little more complicated
> than it seems.
>
> What we gain:
> 1. It's possible to apply CSP in places that can't or don't want to have
> nonces. For example for cache-able static HTML pages we couldn't easily use
> 'unsafe-dynamic' because we wouldn't have the capability to load external
> scripts via <script#src> -- hashes can whitelist inline scripts, but not
> external ones. If we made the split, pages could combine a whitelist and
> 'allow-dynamic' and be safe from DOM XSS if the whitelist isn't bypassable.
> 2. It's easier to understand the behavior of the policy -- we decouple the
> two aspects of unsafe-dynamic from each other.
>
> What we lose:
> 1. Policies with 'allow-dynamic' but without 'drop-whitelist' aren't
> backwards compatible and would require user-agent sniffing. That's because
> any dynamically loaded scripts would be blocked in older browsers (which
> don't support allow-dynamic), or they would have to be explicitly
> whitelisted, resulting in an insecure policy even in new browsers. Unless
> developers pay attention, it will be easier to create a policy that works
> in new browsers but breaks on older ones.
> 2. We depart from an approach that promises easier CSP adoption and also
> offers better security. If we decouple the adoption benefit (easier loading
> of dynamic scripts) from the security benefit (dropping the whitelist) then
> people will likely choose the adoption benefit without realizing that their
> policy offers no protection against XSS -- we already know that >95% of
> policies are in that boat and this is a *very* conservative estimate. I
> worry that we'd abandon a big chance to improve the CSP ecosystem.
> 3. Arguably, having more keywords substitutes one kind of complexity
> (understanding of the behavior of unsafe-dynamic) for another: there will
> be more switches in CSP and realistically 'allow-dynamic' and
> 'drop-whitelist' would almost always have to be used together in order to
> provide a security benefit.
> 4. (Maybe this doesn't matter as much) We introduce keywords which are
> still coupled and need to be implemented simultaneously, e.g. if a
> user-agent implements drop-whitelist but not allow-dynamic, pages will
> break.
>
> Based on my experience, drawbacks #1 and #2 are fairly substantial, and
> I'm worried that risking them to enable the nonce-less/static content case
> (#1) -- which seems to be the main benefit of the split -- could be the
> wrong trade-off. Could we enable the static content case by allowing hashes
> to whitelist external scripts, as Mike suggested below? This way we could
> build policies without nonces so the nonce-leaking concern would disappear
> (unless there's another case that I'm not thinking about?). Even absent
> that, I'd generally prefer to prioritize the "dynamic application" case
> where it's possible to use nonces, because this is where CSP has the
> highest potential benefit against XSS.
>
>
>> 'allow-dynamic' (or just 'dynamic', maybe?) and 'nonce-only'?
>> 'drop-whitelist' is a little too broad, since nonces are part of the
>> whitelist, and we're not dropping those. 'ignore-hosts'?
>>
>> Concretely, would Dropbox use one (or both?) of these keywords if we
>>>> implemented them?
>>>>
>>>
>>> Not really sure yet. But, I am inclined towards the script-src example
>>> above.
>>>
>>
>> Cool, good to hear, thanks!
>>
>> While we're on the topic, I'd like to harden that example via
>> externalized hashes (e.g. `sha256-abc...` would allow `<script
>> integrity="sha256-abc..." ...>` to load). I'd like to find a mechanism to
>> do so in a backwards compatible way. We discussed it briefly at our last
>> meeting. Anyone have any good ideas? :)
>>
>
> To properly discuss it, I'd suggest doing it on another thread, maybe? ;)
> FWIW my preference would be to allow hashes to whitelist script URLs rather
> than contents, and keep SRI as a mechanism to enforce integrity...
> Otherwise, the "static content" case will be difficult to achieve with
> hashes because any changes to the external scripts will break the policy,
> since the digest will no longer match.
>
Received on Wednesday, 8 June 2016 12:27:05 UTC

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