W3C home > Mailing lists > Public > public-webappsec@w3.org > April 2017

Re: Breaking the `opener` relationship.

From: Mike West <mkwst@google.com>
Date: Fri, 28 Apr 2017 16:24:36 +0200
Message-ID: <CAKXHy=fteVVPQxBsVvxgCd_u71__1Uf2+YNo91iuZ6vxuGJUmA@mail.gmail.com>
To: Artur Janc <aaj@google.com>
Cc: Alex Russell <slightlyoff@google.com>, Emily Stark <estark@google.com>, Jonathan Watt <jwatt@mozilla.com>, Anne van Kesteren <annevk@annevk.nl>, "public-webappsec@w3.org" <public-webappsec@w3.org>
On Fri, Apr 28, 2017 at 2:12 PM, Artur Janc <aaj@google.com> wrote:

> #2 in https://wicg.github.io/isolation/#threat of Emily's doc spells out
>> the model in broad strokes. Basically, if someone has a handle to your
>> window, it's not really your window anymore, not fully. Locking down access
>> to `w.postMessage()`, access to frames (`w.length`, `w.frames['name']`,
>> `w.frames[1]`, etc), `focus()`/`blur()`, and `w.location` stand out as the
>> things I'm concerned about. Mostly in the context of Emily's Isolation
>> proposal, but hopefully in a generally applicable way.
> A couple of other things: `window.stop()`

I don't think this is exposed cross-origin today.

> getting load timings from cross-origin frames, and the ability to perform arbitrary
> CSS/SVG transforms <https://arturjanc.com/xo-frame.html> of cross-origin
> content, all of which enabled interesting attacks in the past. I'm not sure
> if the latter two would be addressed by this proposal, though.

They would not. At the moment, I'm strictly considering things that hang
off the `WindowProxy` object. The vectors you mention here both seem like
interesting things to defend against, but somewhat distinct from what I'm
suggesting. Do you see them as intertwined-enough to justify tying them

FWIW I also don't think postMessage is a huge concern here given that
> developers already have a mechanism that lets them decide which messages
> they trust.

I agree with you that it should be straightforward to defend against
`postMessage`-based attacks. As noted elsewhere, however, the `postMessage`
API is pretty easy to get wrong, and has lead to practical attacks in the
past. Maybe Google's doing a better job with this internally than other

On Fri, Apr 28, 2017 at 4:15 AM, Devdatta Akhawe <dev.akhawe@gmail.com>
>> wrote:
>>> Even though I agree with Artur about risk priority, I am ok with
>>> lumping iframe and window.open together for this directive.
>> I think it would be strange to treat these distinctly: both offer
>> third-parties access to your `WindowProxy` object. It's not clear that
>> trusting one is any different than trusting the other, as they offer the
>> same capabilities.
> To clarify, I don't think we should treat window.open and iframes
> differently, but that we should distinguish restrictions based on whether
> (1) the document is being embedded/opened by external content, and (2) the
> document itself decides to embed/open external content. I can see a lot of
> benefit to (1) and would use it in many applications; I don't see much
> benefit to (2) over existing mechanisms, and suspect it might make it less
> deployable if we bundle these two types of restrictions.

There's a practical challenge here, in that the `WindowProxy` checks don't
currently have any notion of how one window got access to another.

I'm sure we could wire it through somehow, so let's assume we could make
the distinction. In general, `a.com` framing `b.com` does seem like an
indication that `a.com` trusts `b.com` to some extent. I'm not sure the
same can be said for opening `b.com` in a new window. In fact, most of the
time I see new-window navigations, the opposite is the intent (e.g. "We
have nothing to do with this page, we just think it might be interesting to
you. Please come back to us when you're done? Please?").

And, really, one of the wonderful things about `iframe` is that it
separates out a chunk of content on the page. I'll bet that there are folks
out there (maybe even on this list!) that use frames for isolation because
they don't actually trust the content they're loading. Look at the old
implementation of `images.google.com`, for instance, which framed the
destination page for context.

> But, both for window.open and iframes, I think there is value to
>>> allowing post-messages. Post messages have a security model: the
>>> target page can check event.origin and ignore the messages; unlike
>>> window.opener navigation scenarios. Seconding Artur, I feel like
>>> restricting post message will significantly affect ability to
>>> effectively adopt this directive.
>> Artur raised similiar concerns. *shrug* If you do the right thing with
>> source and origin checks, then I agree with you that `postMessage()` isn't
>> terribly dangerous. That said, we consistently see folks do the wrong thing
>> with these checks. https://labs.detectify.com/2017/02/28/hacking-slack-
>> using-postmessage-and-websocket-reconnect-to-steal-your-precious-token/
>> is the most recent example I can remember of this kind of attack vector.
>> That said, I recognize the suggestion in both your response and Artur's
>> that blocking all cross-origin access might be overly draconian. I was
>> trying to avoid it, but one of the suggestions that Jonathan Watt made in
>> https://github.com/w3c/webappsec-csp/issues/194 was to turn this into a
>> source list such that the cross-origin check would turn into a list of
>> acceptable origins. I'm a little bit worried about the expense of injecting
>> such an access check into our bindings, but it's an option if a flat
>> lockdown isn't something we can work with. I'm also a little bit worried
>> that you'll next ask for a separation between different types of
>> cross-origin access (e.g. `postMessage` vs `location`) which I'm not
>> excited about creating (both because of implementation complexity and
>> developer confusion (where does named-access to frames go, for instance? Or
>> `blur()`?). "Cross-origin stuff" seems like a narrow enough bucket on its
>> own.
>> WDYT?
> I'm perfectly happy with a flat lockdown if we can apply it to case (1)
> but not (2) ;-)

Can you talk about the use cases you'd have for Google properties? If you
could have unicorns and ponies, what would user agents be doing for you
with regard to properties on window handles?

Received on Friday, 28 April 2017 14:25:31 UTC

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