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Re: Isolate-Me explainer

From: Artur Janc <aaj@google.com>
Date: Tue, 20 Sep 2016 12:04:37 +0200
Message-ID: <CAPYVjqpTGn04PJ7kpLta8NY_G_qJzOvPo9DqLrE6Dn5f5i6VCA@mail.gmail.com>
To: Tanvi Vyas <tanvi@mozilla.com>
Cc: "Emily Stark (Dunn)" <estark@google.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>, Mike West <mkwst@google.com>, Joel Weinberger <jww@google.com>
On Tue, Sep 20, 2016 at 12:11 AM, Tanvi Vyas <tanvi@mozilla.com> wrote:

> This is great!  Thank you for putting it together!  I have added some
> comments on individual sections below.
> *Section 2, Example 2 and 3*
> You make a good point about window.opener!  In the Containers feature, we
> check to ensure that the referrer is stripped when opening a link in a
> different type of container, but I'm not sure we disable the
> window.opener() and open() references.  I'll check that out and be sure to
> fix it if we don't.
> *Section 2, Example 6* (and Section 4, Policy 2)
> If a website says "isolate-me", is the website essentially also setting
> the X-Frame-Options to SameOrigin?  In the Containers model (and in Tor's
> First Party Isolation), there are no framing restrictions.
> For example, if foo.com told the browser to "isolate-me", any top level
> requests made to foo.com would be isolated with their own cookie jar.  If
> foo.com was framed by bar.com, then framed foo.com wouldn't have access
> to the same set of cookies they would have had as a top level request.
> Instead, they would start with a fresh cookie jar, that could then be
> populated.
> The above method reduces breakage; perhaps foo.com has unauthenticated
> content that they want framed.  On the other hand, if framed content did
> have access to a fresh cookie jar, the user could end up logging into
> foo.com via the iframe and then exposing themselves, despite foo.com's
> attempt to request isolation.  So another option would be to allow framed
> content, but not give that content access to any cookie jars (i.e.
> sandboxed frames).
> What about other types of subresources - ex: non-same origin image or
> script loads from the isolated domain?
> *Section 3, Protection 1*
> It is difficult to prevent XSS via navigations without restricting
> navigations.  Artur brought this up to the Containers team as well; if the
> browser isolates bank.com, a user could still click on a maliciously
> crafted bank.com link that could send their data to an attacker.  Hence,
> I understand the reason to restrict navigations.  But in practice, this may
> prompt the user to just copy/paste the link into the url bar.  If they see
> a link to an interesting article on isolated news.com, they don't want to
> visit news.com and then search for that article, they want to get to the
> article immediately.  So if clicking the link doesn't work, they are likely
> to just copy/paste it.  So I wonder if restricting navigations is really
> going to prevent XSS, or just act as an unnecessary hurdle for users to
> jump through.  Perhaps we could brainstorm to see if there are other
> alternatives.

The solution we've been talking about is to make navigation opt-in for the
application. In that model, a user entering a link in the URL bar wouldn't
navigate directly to that URL, but would instead tell the application the
desired destination in a way that would require explicit agreement from the
application (it could could happen via a client-side message, in an HTTP
header as Craig is suggesting below, or in some other way). The server
could then have logic to decide if the request should be allowed, i.e. if
it matches some application-dependent criteria then it would accept the

There are two difficulties here:
1) Developers could shoot themselves in the foot by allowing all
navigations, removing the security benefit of isolation. This would be a
bit better than the current state because the developer would have to make
two mistakes (have the XSS/CSRF bug in the first place, and write code to
allow external navigations to arbitrary parts of their app), but it would
still be possible to shoot yourself in the foot.

This could likely be solved by adding constraints in the API which sends
the "Navigate-Me" messages. For example, maybe the browser only allows is a
list of hardcoded URLs defined by the isolated app, or allows only paths
(no query parameters), or something more reasonable.

2) Having this used to break hotlinking (which I think was raised as a
concern with EPR). I believe Mike and Emily's solution to this -- which
seems reasonable to me -- is to make isolation sufficiently powerful that
an application which wants to break hotlinking to its resources would have
to agree to a lot of other constraints on its behavior, making opting into
isolation unattractive. Since breaking hotlinking is already pretty easy,
this would "protect" isolation from being used for this purpose, simply
because it would require more work on part of the developer.


> *Section 3, Protection 5* (and Section 4, policy 4)
> Consider this scenario:
> Top level - a.com
> Frame[0] - b.com
> Frame[1] - c.com
> Frame[1][0] - c.com creates a grandchild frame to b.com
> Should Frame[0] and Frame[1][0] share cookies?  Or each have their own
> isolated cookies?  In the Containers approach, they would share cookies.
> In Tor's First Party Isolation approach, they would have separate cookies.
> *Section 4, Policy 1*
> If isolation is done properly, is SameSite a given?  Is SameSite included
> as a policy here just to be explicit, or does SameSite provide some
> additional benefits over the isolation described?
> *Section 4, Policy 3*
> What is this policy aiming to protect?  Is it trying to prevent a third
> party from navigating the top level page, or something else?
> *Section 4, Policy 6*
> What if the new window is same origin?  Should two isolated windows from
> the same domain have access to each other?  Perhaps this should say:
> "When the isolated origin opens a new window to a different origin,
> disown/neueter the opened page’s window.opener."
> *Section 4, Policy 8*
> How could this happen?  Is this section meant to handle the
> foo.example.com and bar.example.com case, where one is isolated and
> another is not?
> As part of our work on Containers, we've had a lot of questions come up
> about what should and shouldn't be isolated.  We try to weigh the benefits
> and risks when making these decisions, and have changed our minds a number
> of times.  We should be specific about what isolate-me isolates i) always,
> ii) never, iii) at the discretion of the user agent.  Examples below.
> (Note that if framing and subresource loads from the isolated site are
> disabled, as proposed, some of these are not applicable):
> Permissions
> OCSP Responses
> Security Exceptions (ex: cert overrides)
> Passwords saved by the Password Manager
> User Certificates
> Saved Form Data
> Cache
> Thanks!
> ~Tanvi
> On 9/16/16 8:15 AM, Emily Stark (Dunn) wrote:
> Hi webappsec! Mike, Joel, and I have been discussing an idea for a
> developer facing opt-in to allow highly security- or privacy-sensitive
> sites to be isolated from other origins on the web.
> We wrote up the idea here to explain what we're thinking about, why we
> think it's important, and the major open questions: https://mikewest.gi
> thub.io/isolation/explainer.html
> Please read and comment/criticize/etc. Thoughts welcome, either here in
> this thread or as GitHub issues. Especially interested to hear from Mozilla
> folks as it relates to and is heavily inspired by containers.
> Thanks!
> Emily
Received on Tuesday, 20 September 2016 10:05:29 UTC

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