W3C home > Mailing lists > Public > public-webappsec@w3.org > June 2014

Re: [MIX]: Expand scope beyond TLS/non-TLS (Re: "Mixed Content" draft up for review.)

From: Katharine Berry <katharine@getpebble.com>
Date: Wed, 11 Jun 2014 05:32:35 -0700
Cc: Zack Weinberg <zackw@cmu.edu>, Brian Smith <brian@briansmith.org>, "public-webappsec@w3.org" <public-webappsec@w3.org>
Message-Id: <DBC6D545-1DE7-4B97-BAE7-8BCE58A4426F@getpebble.com>
To: Mike West <mkwst@google.com>
On 11 Jun 2014, at 02:40, Mike West <mkwst@google.com> wrote:
> The nice thing from my perspective is that users who don't fall into
> the category of developers who need to access local devices from a
> public website would retain all their status-quo protections

As a minor addendum, I would like to step back from “developers”; this
use case is more broadly something like “webapp on a public site would
like to configure a user’s private, network-accessible devices.” 

For us, “user” is (loosely) “developer” and “configure” is “run code
on”, but I doubt this is the only use for such a setup (even for us).

> (I realized while writing that bit that I don't actually understand
> (how Pebble's app opens a WebSocket connection in the first place. How
> (do you know what IP address to connect to?)

We have two paths for this:

a) The user looks at their phone, which displays the address. The user
types the address into the web app. We store the address locally to
save future typing.

b) 0) The phone registered itself with a Pebble service during
      onboarding, when the user logged in with the same credentials
      used on the webapp (OAuth both times). This registration includes
      the phone’s human-readable name and a push token.
   1) The webapp retrieves and presents the user’s registered devices
      in a dropdown list.
   2) The user selects the device from the list; the webapp arranges
      for a message containing a unique token to be pushed to the app
      via Google or Apple’s push messaging services.
   3) The app receives the message and responds with its local IP
      address and that token.
   4) The webapp temporarily stores that address locally because this
      dance is relatively slow.

After that the interaction between webapp and phone is the same in
each case. We have found that users very frequently fail the first
path (IP addresses are hard), but the second path is pretty reliable
and we default to it when available (it isn’t always).

The existence of the second path is also why we already have much
of the infrastructure required to support Zack’s suggestion.

> Note also that, because of the pharming attack, we really need
> mutual authorization here.
> 
> I do agree. That's one of the reasons I find it difficult to allow
> arbitrary access to pieces of an internal network without the user
> actively doing something in order to enable that ordinarily dangerous
> connection.

“Arbitrary” access, sure, but I don’t think anyone is suggesting that
- if both ends have explicitly indicated their interest in making the
connection, it’s not clear to me why the user needs to cast the
deciding vote. I’m not sure that you actually disagree with me here,
since we could (hypothetically) just kill TLS and then a CORS-like or
similar solution would work fine on its own.

> Manually installing a certificate that Pebble provides seems like the
> right solution for the TLS side of the problem. That was rejected as
> being too much work for the developer. Personally, I'd prefer that we
> reevaluate that objection.

I’m not actually rejecting it as “too much work” so much as some
combination of “they probably can’t do it” on their side and
“demanding this is embarrassing” on ours. At least anecdotally, I have
seen (proportionally) many users struggle with certificate
installation in an environment where an organisational root CA was
used universally, and few succeed.

If necessary we will consider this, but I think it is more likely we
will either attempt to subvert existing PKI (we have already discussed
this and are willing to try but are kinda disgusted by the idea) or
just throw our hands up and turn off TLS on the main site, isolating
that as much as possible from the rest of our services (we generally
dismiss this out of hand, but it can and may well be our final
fallback if necessary).

> Something CORS-like seems like the right solution to the private-IP
> side of the problem, but I'm quite open to other proposals.

While I prefer Zack’s suggestion in that it solves all of our problems
in one shot and in a manner that looks to provide security in both the
general case and the mixed public/private case, I cannot disagree with
the complexity (or “PITA”) comment. Failing that, the CORS-like
preflight seems pretty reasonable to me to specifically solve the
public/private issue.

> The two of those together would provide mutual authorization: the user
> expects connections to a device matching the cert they installed into
> their trust store, and the device expects to service connections from
> the outside world.

Again, not sure that the *user* is the right party to be performing
this authorisation; I imagine the device and webapp together know much
better than the user what they expect to happen. At best, the user is
blindly guided by one or both of those parties trying to encourage
them to do the things that make them happy, whatever those ultimately
end up being.

– Katharine
Received on Wednesday, 11 June 2014 12:33:08 UTC

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