W3C home > Mailing lists > Public > public-webrtc@w3.org > November 2013

Re: Why does screen sharing require a browser extension?

From: Harald Alvestrand <harald@alvestrand.no>
Date: Mon, 25 Nov 2013 20:46:47 +0100
Message-ID: <5293A927.1010208@alvestrand.no>
To: public-webrtc@w3.org
 From draft-ietf-rtcweb-security-05.txt:

4.1.1.  Threats from Screen Sharing

    In addition to camera and microphone access, there has been demand
    for screen and/or application sharing functionality. Unfortunately,
    the security implications of this functionality are much harder for
    users to intuitively analyze than for camera and microphone access.
    for a full analysis.)

    The most obvious threats are simply those of "oversharing". I.e.,
    the user may believe they are sharing a window when in fact they are
    sharing an application, or may forget they are sharing their whole
    screen, icons, notifications, and all.  This is already an issue with
    existing screen sharing technologies and is made somewhat worse if a
    partially trusted site is responsible for asking for the resource to
    be shared rather than having the user propose it.

    A less obvious threat involves the impact of screen sharing on the
    Web security model.  A key part of the Same Origin Policy is that
    HTML or JS from site A can reference content from site B and cause
    the browser to load it, but (unless explicitly permitted) cannot see
    the result.  However, if a web application from a site is screen
    sharing the browser, then this violates that invariant, with serious
    security consequences.  For example, an attacker site might request
    screen sharing and then briefly open up a new Window to the user's
    bank or Gmail account, using screen sharing to read the resulting
    displayed content.  A more sophisticated attack would be open up a
    source view window to a site and use the screen sharing result to
    view anti cross-site request forgery tokens.

    These threats suggest that screen/application sharing might need a
    higher level of user consent than access to the camera or microphone.

I think it would be good to formulate suggestions for changes here in a 
way that suggest changes to this text, if changes are warranted.

On 11/25/2013 05:56 PM, cowwoc wrote:
> Hi,
> In the WebRTC World conference Justin Uberti mentioned that Chrome 
> (and Firefox too?) will be moving screen sharing out of Javascript, 
> requiring developers to publish a browser extension per application 
> that wishes to screen-share. The logic behind it was that malicious 
> app could be banned from the app store.
> One thing I didn't understand (and was not explained) is why screen 
> sharing is substantially more security-sensitive than webcam sharing? 
> I get the fact that someone could use screen sharing to snoop on my 
> banking activity, but how is this any more security sensitive than 
> knowing what I look like and where I live? If the security dialog is 
> good enough for webcam sharing, why is it not good enough for screen 
> sharing?
> And finally, couldn't you simply require the use of SSL for this 
> feature and then ban malicious applications based on their 
> certificate? Requiring the download of an extension is almost like 
> requiring a browser plugin for WebRTC. I'd like to avoid it if at all 
> possible.
> Thanks,
> Gili
Received on Monday, 25 November 2013 19:47:16 UTC

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