W3C home > Mailing lists > Public > public-webappsec@w3.org > January 2016

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

From: David Ross <drx@google.com>
Date: Fri, 22 Jan 2016 13:53:36 -0800
Message-ID: <CAMM+ux5Brp=+d6woAPVurzv0eKa_i5MPpg_7y_RjLfDw=cqEDg@mail.gmail.com>
To: Chris Palmer <palmer@google.com>
Cc: Craig Francis <craig.francis@gmail.com>, Conrad Irwin <conrad.irwin@gmail.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>
> 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 21:54:25 UTC

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