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webRTC and Content Security Policy connect-src

From: T H Panton <thp@westhawk.co.uk>
Date: Fri, 12 Jan 2018 11:01:34 +0000
Message-Id: <248D5999-BB84-484F-AA14-4FB3F9FC95EB@westhawk.co.uk>
Cc: IƱaki Baz Castillo <ibc@aliax.net>, Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>, Cullen Jennings <fluffy@iii.ca>
To: "public-webrtc@w3.org" <public-webrtc@w3.org>
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 11:01:59 UTC

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