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Re: webRTC and Content Security Policy connect-src

From: Peter Thatcher <pthatcher@google.com>
Date: Fri, 12 Jan 2018 16:44:45 +0000
Message-ID: <CAJrXDUHLYoChPO2qc3-bca6Nq_ZTm8u_E-E_SvVfdpVYFNnDZw@mail.gmail.com>
To: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
Cc: T H Panton <thp@westhawk.co.uk>, Cullen Jennings <fluffy@iii.ca>, Iñaki Baz Castillo <ibc@aliax.net>, "public-webrtc@w3.org" <public-webrtc@w3.org>
Actually, as I pointed out on the other threads (why are there so many?),
maybe domain-name-only remote ICE candidates and TURN servers and STUN
servers with whitelisted domains might work.  And that, while heavily
disabled, would at least allow PCs to whitelisted servers.

On Fri, Jan 12, 2018, 8:30 AM Sergio Garcia Murillo <
sergio.garcia.murillo@gmail.com> wrote:

> I agree.
> That is why my original proposal was to disable webrtc if CSP it's in use.
> We could add an "webrtc" token for connect-src to enable it back knowing
> the risks.
> For what it seems, anything more fine grained would be difficult to get it
> working.
> Best regards
> Sergio
> El 12/1/2018 17:16, "Peter Thatcher" <pthatcher@google.com> escribió:
>> Why is this ICE-lite specific?   A controlling ICE agent will begin
>> sending data as soon as it had received a check response, regardless of
>> whether or not it has received a check yet.   And that happens for full
>> clients as well.
>> I think you need to go further and say something like "don't send data
>> until you get a check", which is like a reverse consent check.
>> But even that might not be good enough.   You could, for example, convey
>> information to the server simply by controlling the ports to which you send
>> checks, which is driven by the adding of remote candidates, which is
>> controlled by JS.
>> Even if we could prevent that, there is probably a way to convey
>> information by controlling the IP or port used for gathering server
>> reflexive or relay addresses via STUN or TURN.
>> How far down the rabbit hole of disabling things you want to go?
>> On Fri, Jan 12, 2018, 3:04 AM T H Panton <thp@westhawk.co.uk> wrote:
>>> In the call yesterday, I said I'd try and summarize the concerns that
>>> have been raised about 'Drive-by webRTC CSP' attacks.
>>> Content Security Policy on web pages allows a site developer to
>>> proscribe what their page can do.
>>> This is intended to mitigate the risks of XSS and other injection
>>> attacks.
>>> https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/connect-src
>>> This reduces exposure to included 3rd party scripts being tampered with
>>> at source
>>> e.g. if someone hacks https://webrtc.github.io/adapter/adapter-latest.js
>>>  - the page may not function correctly, but it should not be able to
>>> send sensitive
>>> data to domains that aren't whitelisted in the CSP connect-src header.
>>> This prevents a web-font supplier from capturing the credit card data
>>> from your
>>> e-commerce site shopping forms.
>>> The connect-src explicitly covers websockets. It does not mention
>>> DataChannels or webRTC.
>>> In principle you might not think that matters, since in order to set up
>>> a data-channel
>>> you need to perform an SDP exchange, and that SDP exchange would have to
>>> go through
>>> a whitelisted server.
>>> It turns out in the case of ice-lite the browser does not verify that
>>> the remote party has
>>> ever seen it's SDP - ICE responses do not require the requester's ufrag
>>> or pass.
>>> This means that the malicious javascript does not need to send an answer
>>> to a
>>> cooperating server.
>>> So it would be possible to bury static SDP for an ice-lite offer in
>>> malicious javascript.
>>> The offer would point to a malicious server that implemented ice-lite on
>>> a fixed port
>>> (for example) and accepted data channels without checking the DTLS
>>> fingerprint.
>>> The javascript would apply this to a peerconnection and drop the
>>> generated answer in the
>>> bit-bucket.
>>> The malicious javascript can now inspect the page DOM and send all the
>>> form values it
>>> finds over a datachannel to the malicious server. Despite the fact that
>>> the conscientious developer
>>> had configured connect-src to mitigate this risk.
>>> At the heart of this is that ice-lite breaks the conceptual linkage
>>> between the 5 tuple and the
>>> page origin.
>>> Proposal:
>>> a) Ban ice-lite on pages with any CSP set
>>> c) add a allow-ice-lite CSP
>>> b) add a CSP turn-servers whitelist (to prevent leakage via the
>>> credentials)
>>> c) test plain ICE to make sure it fails if the far side sends no valid
>>> requests.
>>> d) ensure that any new ICE api's don't make this mistake (worse)
>>> Thanks to ibc@aliax.net for starting the discussion and
>>> sergio.garcia.murillo@gmail.com for pointing me at CSP connect-src:
>>> https://twitter.com/ibc_tw/status/949993145978245120
>>> P.S.
>>> As you will see in the twitter thread, this isn't specific to the
>>> datachannel - one can exfiltrate data over DTMF or G711 perfectly easily.
>>> Tim.
Received on Friday, 12 January 2018 16:45:23 UTC

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