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Re: Whitelisting external resources by hash (was Re: Finalizing the shape of CSP ‘unsafe-dynamic’)

From: Artur Janc <aaj@google.com>
Date: Wed, 8 Jun 2016 16:04:39 +0200
Message-ID: <CAPYVjqpCK5BfZ1Uv=kt=_gjr2wGgZ0ZtUoSe_jzSmWkAFBKg1w@mail.gmail.com>
To: Mike West <mkwst@google.com>
Cc: Brad Hill <hillbrad@gmail.com>, Devdatta Akhawe <dev.akhawe@gmail.com>, WebAppSec WG <public-webappsec@w3.org>, Christoph Kerschbaumer <ckerschbaumer@mozilla.com>, Daniel Bates <dabates@apple.com>, Devdatta Akhawe <dev@dropbox.com>
On Wed, Jun 8, 2016 at 3:14 PM, Artur Janc <aaj@google.com> wrote:

> On Tue, Jun 7, 2016 at 9:59 PM, Mike West <mkwst@google.com> wrote:
>> On Tue, Jun 7, 2016 at 8:27 PM, Artur Janc <aaj@google.com> wrote:
>>> - You could whitelist specific URLs for script-src without risking
>>> redirect-based whitelist bypasses. For example `script-src 'self'
>>> ajax.googleapis.com/totally/safe.js` is an ineffective policy if there
>>> is an open redirect in 'self' due to the ability to load other scripts from
>>> ajax.googleapis.com caused by CSP's path-dropping behavior. A hash
>>> would avoid this problem.
>> I think you might have something in mind other than just hashing the URL?
>> It's not clear to me how a different spelling of the URL would mitigate the
>> issues that lead to the path-dropping-after-redirect behavior. Denying
>> redirects entirely, perhaps?
> Surprisingly, allowing hashes to bless script URLs would actually solve
> this problem. (For clarity, the strawman proposal I'm talking about is to
> allow the loading of external scripts if the digest of the `src' attribute
> is present as a hash in the policy: <script src="
> https://example.org/foo.js"></script> would be permitted by a CSP of
> script-src 'sha256-8wKZoJZ5SgqL4cU079oehMJ9lwrGSV9gBLjuY30aM3Q=').

Something that I neglected to mention in the summary above is that this
would be a sufficient condition for loading the external script. That is,
no matter what redirects happen when https://example.org/foo.js is
requested, the script would be allowed to load if the digest of the
original URL (before any redirects) is present in the policy.

> The direct reason is that in the hash case it will be acceptable to follow
> any redirects when fetching the script, whereas for host-source the UA
> needs to check the location of any redirect to make sure it's present in
> the whitelist -- which is what enables revealing targets of redirects and
> leads to the privacy leak. Why is following redirects okay in the hash
> case, but not for host-source, you ask? Because by definition a hash allows
> only a single URL trusted by the developer and exactly matching the digest
> in the policy, so it is very unlikely to redirect to attacker-controlled
> data; for regular whitelists we couldn't do this because any open redirect
> in a whitelisted host-source would bypass CSP.
> Using hashes to whitelist specific script#src values sacrifices some
> flexibility (i.e. you'd have to explicitly hash every script URL on your
> page, rather than do it with a single host or path as with whitelists), but
> in exchange it solves the path-dropping problem. Since we're talking about
> this in the context of static content which might want to use
> {unsafe,allow}-dynamic, and where we can't use nonces, it would allow only
> expected scripts to be loaded without whitelist-related concerns.
> (An alternative, inferior answer to your question is that we could just
> ban redirects when fetching external scripts whitelisted by a hash because
> it's a new mechanism and we could enforce this without breaking existing
> users. I don't like it, but it's another way to handle this.)
>>> - It would allow more flexibility in whitelisting exact script URLs.
>>> Using a traditional URL whitelist it's not possible to have a safe policy
>>> in an application which uses JSONP (script-src /api/jsonp can be abused by
>>> loading /api/jsonp?callback=evilFunction). With hashes you could allow
>>> SHA256("/api/jsonp?callback=goodFunction") but an attacker could not use
>>> such an interface to execute any other functions.
>> Is hashing important here? Would extending the source expression syntax
>> to include query strings be enough?
> Possibly, but now you'd have all the original concerns about revealing
> redirects, potentially with more worrying consequences if we support query
> strings; and if we keep using source expressions for this then the
> path-dropping behavior would remain a problem unless we handled that
> somehow.
>> - It would work with a policy based on 'unsafe-dynamic' /
>>> 'drop-whitelist' -- even if the host-source is dropped, the hash would
>>> offer a way to include specific external scripts.
>>> For CSP to become a useful XSS protection we will almost certainly have
>>> to move away from the whitelist-based model.
>> I think we agree that Google will certainly need to move away from the
>> whitelist-based model. Though I agree with you that a nonce-based model is
>> simpler to deploy for many sites, GitHub seems to be a reasonable
>> counter-example to general necessity.
> Without picking on GitHub, I would disagree with your counter-claim ;-) It
> is certainly possible to build CSP whitelists that will allow an
> application to function properly, but the overwhelming majority of such
> policies offer no benefit against XSS; I shared some data on the parallel
> thread and would be happy to share more if you'd like ;) The problem,
> however, is that this is fairly difficult to see because the failure mode
> of CSP with an unsafe whitelist is hidden -- only when someone attempts to
> exploit an XSS does it turn out that a policy was ineffective.
> It's not that I dislike the concept of whitelists, they made sense when
> they were proposed. It's just that we have convincing data -- both from
> Google applications and from a large survey of policies used in the wild --
> that in the current state of the Web they just aren't effective in
> practice. (In the comment above I meant "we" as in web application
> developers, rather than just Google.)
>>> Dynamic applications can often use nonces instead, but for static
>>> content, or situations where using nonces would be difficult, I think
>>> hashes are a good solution -- one of their main benefits is that they're
>>> already in the spec and any expansion of their capabilities would be a
>>> relatively small change. (Another upside is that they can be used in a
>>> backwards-compatible way alongside a whitelist.)
>> I still don't understand why hashing a URL is useful. :(
> Here are the benefits I see:
> - We could handle the static content case with the current shape of
> 'unsafe-dynamic', without splitting it into separate keywords. A developer
> could set a script-src policy composed only of hashes (for inline and
> external scripts) and 'unsafe-dynamic', and the page would have a working,
> safe policy.
> - We would have a way to allow the loading of specific external URLs
> without nonces, and without risking path-dropping CSP bypasses; my guess is
> that this is simpler than adding query parameters to host-source.
> - It would not require any changes to the document and a policy could be
> built just by statically inspecting the markup -- a tool could parse the
> page, calculate digests of internal scripts and of the URLs of external
> ones.
> - Pages would be less likely to break than with the current SRI-based
> approach because this would allow the returned content to change; it would
> also handle the case of static pages loading non-static JS (various JS
> widgets and APIs). Of course it could still be used in combination with SRI
> if the developer so desires.
> - It is consistent with the behavior of nonces (which can whitelist both
> inline and external scripts), and conceptually it's easy to understand if
> you're used to the idea of hashes (calculate the digest of the script#src
> just like you'd do it for an inline script).
> An obvious drawback is that this is ugly and offers functionality that is
> similar to the existing whitelist; there are also some performance concerns
> raised by Dan related to the proliferation of policies with hashes. The
> question is about the cost/benefit ratio; I think in this case the benefits
> can be quite compelling...
> Cheers,
> -A
Received on Wednesday, 8 June 2016 14:05:32 UTC

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