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Re: Isolate-Me explainer

From: Emily Stark (Dunn) <estark@google.com>
Date: Tue, 20 Sep 2016 17:19:23 -0700
Message-ID: <CAPP_2SYL9B_WZVqekYB8o4NCwDjj8oVoGexBt4iVghYTH2ZdAA@mail.gmail.com>
To: Artur Janc <aaj@google.com>
Cc: Tanvi Vyas <tanvi@mozilla.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>, Mike West <mkwst@google.com>, Joel Weinberger <jww@google.com>
Hi Tanvi, thanks for the detailed feedback! Thoughts inline.

On Tue, Sep 20, 2016 at 3:04 AM, Artur Janc <aaj@google.com> wrote:

> 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).
I was thinking that an isolated site should be treated as if it had
X-Frame-Options set to SAMEORIGIN. However, I could also get behind your
suggestion that an isolated site can be framed cross-origin but then does
not get access to cookie jars... with the caveat that it should also not
get access to localStorage, etc. (I'd like if it Isolate-Me protected sites
that store auth tokens in localStorage just as well as it protects sites
that store auth tokens in cookies.)

[Note: I changed my mind about this below, at the very end of my email.]

>> What about other types of subresources - ex: non-same origin image or
>> script loads from the isolated domain?
If all cookies on isolated origins are SameSite (and I think that for an
isolated origin, all cookies should be SameSite by default), then I think
we can safely allow these types of requests; do you agree? (Discussed more

If we decide that isolated origins should be allowed to have non-SameSite
cookies... then we probably need to rethink this.

>> *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
> navigation.
> 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.

Oh, good point about the temptation to copy/paste, Tanvi; I hadn't thought
of that. I had previously been mildly opposed to making navigations opt-in,
because of the first difficulty that Artur listed: if the developer can
make a mistake and introduce an XSS, why do we trust the developer to
correctly allow/disallow navigation requests? However, given the copy/paste
risk and Artur's idea about making the opt-in API constraining to be as
safe as possible, I think this might be the best option we have so far.

> Cheers,
> -A
>> *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.
I'm thinking that, to get the security properties we want, Frame[0] and
Frame[1][0] should *not* share cookies. The double-keyed storage is to
address Section 2 item 7: evil.com shouldn't be able to attack the isolated
site's third-party dependencies, which may have ambient authority granted
by the isolated site. Suppose that upon logging into a.com, a.com
communicates with b.com in some way that sets a b.com cookie authenticating
the user. I think that a.com framing c.com should not allow c.com to attack
the b.com cookie, just as a.com framing c.com should not allow c.com to
attack any of a.com's 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?
Hrm... I'm not sure I understand this question. By "done properly", do you
mean if the user agent implements it properly? My intention was that when
an app isolates itself, the browser automatically treats all its cookies as
if the SameSite flag were set. The goal is to address Section 2, item 1 and
item 6: a malicious site performing XSS, CSRF, HEIST, etc. by loading
authenticated cross-origin resources on the isolated site.

>> *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?
I was thinking of vulnerable postMessage APIs. Artur had also pointed me to
some other examples which I forgot to reference in the doc: for example, a
cross-origin site could traverse and count frames in the frame tree and
potentially learn something useful from that information.

>> *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."
Ah, thanks, just fixed that.

>> *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?
Yep, that's right. Or foo.example.com is isolated and example.com 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
Now that I see this long scary list written out, I'm leaning back towards
restricting cross-origin framing entirely (that is, Isolate-Me implies
X-Frame-Options: SAMEORIGIN). Up above I mentioned that I could get behind
allowing cross-origin framing of isolated sites as long as there's no
access to localStorage or cookie jars. But now I'm thinking that the framed
isolated content also shouldn't have access to permissions or saved form
data or any number of other things that I can't think of right now. As you
noted, it sure would simplify things to just not allow framing isolated
sites. If a site wants to opt in to Isolate-Me, it's probably easy enough
for them to host any unauthenticated content that they want to be
frame-able on a separate origin; that's probably the least burdensome thing
that they have to do to make sure that their site still works after turning
on isolation.

>> 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 Wednesday, 21 September 2016 00:20:16 UTC

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