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Re: In-browser sanitization vs. a “Safe Node” in the DOM

From: David Ross <drx@google.com>
Date: Fri, 22 Jan 2016 14:03:24 -0800
Message-ID: <CAMM+ux7k=L1tX9y6Ov1AmBz0fqUeVMo_KQOO6-UFSX4O-GKQ3Q@mail.gmail.com>
To: Chris Palmer <palmer@google.com>, Crispin Cowan <crispin@microsoft.com>
Cc: Craig Francis <craig.francis@gmail.com>, Conrad Irwin <conrad.irwin@gmail.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>
> How is CSP not sufficient?
CSP operates on a per-page basis.  Here's the canonical use case for
sanitization (and also Safe Node): Fetch bits of markup via XHR and
just plop them into the existing DOM in various places, safely.

I started with the assumption that client-side sanitization is coming
to the browser.  This is obviously not a given, but discussion about
the possibility is what initiated my train of thought.  The Safe Node
proposal attempts to achieve the same result but in a way that I argue
has certain advantages over client-side sanitization baked into the
browser.

Dave

On Fri, Jan 22, 2016 at 1:53 PM, David Ross <drx@google.com> wrote:
>> How, exactly, can we compute whether a given string is an anti-CSRF
>> defense token, and how, exactly, can we compute if CSS just leaked
>> it to the attacker? I don't immediately see how that security guarantee
>> is computable.
> We should probably talk about a specific version of the attack to make
> sure we're on the same page.  (Maybe this one:
> http://html5sec.org/webkit/test)
>
> I think that if you're only detecting what you're describing above,
> it's too late.  My expectation is that CSS defined within the Safe
> Node would have no affect on the DOM outside of the Safe Node.  Is
> there some reason why this is not possible to implement or that it
> would be ineffective at addressing the issue?
>
>> Basically, my argument is that the con — that a general purpose
>> filtering HTML parser would be useful — is huge, and also
>> sufficiently covers your intended goal (duct-taping an anti-pattern).
>> Thus, if we do anything, we should do that.
> Mmm, I lost you here...  How is that a con?  It sounds like just an
> assertion, and one that I wouldn't argue with.  And my intended goal
> is not to get rid of innerHTML, but I'm happy to help by removing
> innerHTML the design pattern I originally suggested.
>
>> I would rather deprecate innerHTML, yes. But at least I can easily grep for "assigns to innerHTML but there is no call to purify(...)".
> In any event the Safe Node idea is not dependent on innerHTML.  I'm
> happy to cut it out!
>
> Dave
>
> On Fri, Jan 22, 2016 at 1:32 PM, Chris Palmer <palmer@google.com> wrote:
>> On Fri, Jan 22, 2016 at 1:00 PM, David Ross <drx@google.com> wrote:
>>
>>> > For example, what is the actual mechanism/algorith/heuristic thsi API
>>> > would use to enforce the Safe CSS set of policies?
>>> My argument is that it's smarter to implement policy enforcement
>>> within Blink / WebKit than it is to implement the same thing reliably
>>> within custom sanitizer code stapled onto Blink / WebKit.  For
>>> example, consider the case of the policy that disables script.  The
>>> browser can quite definitively disable script execution initiated from
>>> within a particular DOM node.  However a sanitizer has to whitelist
>>> all the elements and attributes it suspects are capable of initiating
>>> script execution.  Pushing the policy enforcement to be implemented
>>> close to the code that is being regulated makes it less likely that
>>> new browser features will subvert the policy enforcement.  Some things
>>> that are simply difficult or impossible for sanitizers to regulate in
>>> a granular way (eg: CSS) are easier to handle with a Safe Node.
>>
>>
>> That doesn't answer the question. How, exactly, can we compute whether a
>> given string is an anti-CSRF defense token, and how, exactly, can we compute
>> if CSS just leaked it to the attacker? I don't immediately see how that
>> security guarantee is computable.
>>
>>>
>>> > element.innerHTML = purify(untrustworthyString, options...)
>>> > That seems convenient enough for callers?
>>> See the pros / cons in my writeup.
>>
>>
>> Basically, my argument is that the con — that a general purpose filtering
>> HTML parser would be useful — is huge, and also sufficiently covers your
>> intended goal (duct-taping an anti-pattern). Thus, if we do anything, we
>> should do that.
>>
>> But I remain skeptical of the goal.
>>
>>>
>>> And wait, didn't you just argue
>>> that we shouldn't make use of .innerHTML given it's an anti-pattern?
>>> =)
>>
>>
>> I would rather deprecate innerHTML, yes. But at least I can easily grep for
>> "assigns to innerHTML but there is no call to purify(...)".
Received on Friday, 22 January 2016 22:04:14 UTC

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