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Re: [rtcweb] Notes on security for browser-based screen/application sharing

From: Cullen Jennings (fluffy) <fluffy@cisco.com>
Date: Fri, 22 Mar 2013 13:50:37 +0000
To: Eric Rescorla <ekr@rtfm.com>
CC: "rtcweb@ietf.org" <rtcweb@ietf.org>, "public-webrtc@w3.org" <public-webrtc@w3.org>
Message-ID: <C5E08FE080ACFD4DAE31E4BDBF944EB11342CB58@xmb-aln-x02.cisco.com>

One comment on this from a requirements point of view…

Clearly sharking the "desktop" has far more security concerns that sharing a single applications such as PowerPoint. All the use cases I am interested in only need to share an application not a desktop. I think we should separate the handling of permissions along these lines. So I would be fine with "share desktop" needed an explicit grant of permission every time it was invoked (preferably by the user selecting this as part to choosing what to share in a browser chrome window). On the other hand, when sharing an application I might be OK with a persistent permission based on an install model but when I think about the real uses cases, I'm not sure that is needed if we have a good browser based dialog box to pick what will be shared. 

When the applications being shared is the browser there are also the additional problems as you point out. My view of the best way to solve these would be to scope the "application" being shared to the origin. What I mean by this is assume that I have my browser open to two webpages, one with an origin of gmail.com and the other to github.com and I am also running powerpoint and word. When the browser pops up a dialog box asking me what I wanted to share, it would give me 5 choices "Firefox (Gmail.com)", "Firefox (github.com), "Word", "PowerPoint", and "Everything" and let me pick. 


On Mar 11, 2013, at 2:53 PM, Eric Rescorla <ekr@rtfm.com> wrote:

> 1. INTRODUCTION
> WebRTC [0][1] already contains facilities for JavaScript applications
> to acquire to the user's camera and microphone and either to directly
> access the media or to send it elsewhere over a voice/video call.
> This obviously presents security issues [2][3] and the consensus
> approach is that any access to camera or microphone must only occur
> with user consent. Current versions of Chrome obtain this consent once
> and persist it indefinitely for a given site. Firefox obtains consent
> for every request but will likely eventually add a persistent consent
> feature.
> 
> One of the major applications of WebRTC-style technology is
> videoconferencing and most videoconferencing applications offer either
> "screen sharing" or "application sharing" or both. Unfortunately,
> while the security properties of camera/microphone access are fairly
> obvious to the user--though the properties of persistent permissions
> may not be--the security properties of screen/application sharing are
> far less obvious. It has been suggested by Adam Barth among others that
> permissions should be stricter for screen sharing than for
> camera/microphone access. This note provides an overview of the
> relevant security issues and of the potential permissions/consent
> mechanisms.
> 
> 
> 2. GENERAL SECURITY PROPERTIES OF SCREEN/APPLICATION SHARING
> Technically, screen/application sharing is relatively simple. In
> screen sharing, the conference sees whatever is on the user's display;
> if there are multiple monitors, typically only one is shared. In
> application sharing, the conference gets access to all the windows in
> an application.
> 
> Because existing conferening products (e.g., WebEx) require some sort
> of download/install experience, they end up with the permissions of a
> native application. Thus, it doesn't really make sense to worry about
> misuse of the sharing permissions specifically because the application
> has free run of the user's machine. The security question then becomes
> whether the user wishes to run the application at all, not whether he
> trusts the application to see his screen specifically.
> 
> Even so, there are known security risks to this type of sharing,
> mostly due to the user's misunderstanding/lack of thought about
> the security properties. For instance:
> 
> * The desktop often contains icons that the user has forgotten
>   about, including the names of files. These themselves can
>   be confidential.
> 
> * Desktop notifications such as Growl for incoming messages,
>   IMs, etc. can be get shared. These can also contain confidential
>   information.
> 
> * Users often think of "application sharing" as "window sharing"
>   and will have other sensitive documents open at the same
>   time as the document they intend to be sharing. 
> 
> I have heard reports of all of these issues (the first two are also
> often seen in settings when the user is projecting their screen at
> conferences and the like). Fundamentally, these are examples of user
> error, though possibly combined with confusing interfaces. The users
> generally understand that they have given the downloaded application
> wide permissions. Because users' expectation for Web applications are
> that they are safer, there is yet more space for confusion.
>     
> 
> 3. SECURITY OF SCREEN/APPLICATION SHARING IN THE WEB ENVIRONMENT
> 3.1. Threat Model
> Huang et al.[4] describe the Web security guarantee as:
> 
>    Users can safely visit arbitrary web sites and execute scripts
>    provided by those sites.
> 
> More generally, users expect that the browser will protect them from
> malicious sites and that sites are isolated from each other.  (More on
> the technical mechanisms below).
> 
> Obviously, granting permission to see the desktop breaches this
> guarantee to some extent, since the user is granting the site (via the
> browser) some very dangerous capabilities. Generally the intent is
> that the user can understand the security impact of the permission he
> is granting. As should be clear from the discussion above, this is
> already not entirely so. However, in the Web environment the problem
> is much worse because the user likely thinks that he is assuming
> *just* the screen/application sharing risks without the corresponding
> "full application privileges" risks. This is unfortunately less true
> than one would like.
> 
> 
> 3.2. Background: Same Origin Policy
> In order to isolate sites from each other, browsers implement what's
> called the "Same Origin Policy" (SOP). The basic idea is that content
> (scripts, HTML, etc.) that runs on one site cannot get access to
> content from another site, except under very limited conditions.  For
> instance, site A can:
> 
> - Run scripts from site B.
> - IFRAME HTML from site B but not look at the content or output.
> - Display images and videos from site B but not examine their
>   contents.
> 
> [Note: I am ignoring CORS and WebSockets for the moment.]
> 
> The basic idiom here is that site A can cause content from site B to
> be *displayed* but it can't access the content itself. This allows for
> the construction of some kinds of composite Web sites (i.e., mash-ups)
> but still allows for site isolation.
> 
> Many important Web security mechanisms depend explicitly on these
> guarantees. For instance, consider a Web mail site which bases its
> authentication on cookies and will therefore service any HTTP request
> which contains the right cookie. Content from any other site can cause
> the browser to emit the right HTTP request, but because of the
> same-origin policy, it can't see the responses. This prevents random
> sites from accessing your web mail.
> 
> 
> 3.3. Web-Specific Risks
> At this point, the risk of combining screen sharing with the Web
> environment should be obvious. SOP protection depends on denying
> Web content access from site A to content from site B, but
> because site A can cause content from site B to be displayed
> on the screen, if A can see the user's screen then he can
> close the loop and bypass SOP. (We assume below that either
> the site is sharing the user's screen or that the browser is
> the application being shared.)
> 
> 
> 3.3.1. SOP Violations for Visible Content
> The most obvious attack vector is that the the site can see any
> content that the user can see. All he needs to do is to open a window
> with the URL of the relevant content and put it in view of the screen
> sharing system. Note that the content only needs to be briefly visible
> (long enough to be captured by the sharing code). Potential attacks
> include:
> 
> - Capture the user's webmail (and potentially individual messages).
> - Capture the user's "sitekey" anti-phishing picture. [6]
> - View any confidential documents that the user has access to
>   on the Internet or on their own computer.
> 
> In general, any resource which can be opened in a browser window
> (if the browser is being shared) or in an external application
> (if screen sharing is in use) can be accessed in this fashion.
> 
> 
> 3.3.2. SOP Violations for "Hidden" Content
> The SOP issue extends beyond visible content. For instance, many sites
> use secret tokens in HTML content to prevent Cross-Site Resource
> Forgery (CSRF) attacks. The idea is that the token is available to
> same-origin JS which then embeds it in any XHR requests it
> performs. The site checks for presence of the token, thus preventing
> content from other sites from performing operations which might have
> side effects (even if they cannot see the response). While embedded in
> the HTML, these tokens are hidden to avoid annoying the user.
> 
> Similar techniques to those described above can be used to bypass this
> type of CSRF protection. Instead of loading the content directly in a
> window, the attacker loads the HTML in a source view window (using the
> view-source: scheme, which both Chrome and Firefox support). Since the
> HTML source contains the CSRF token, the attacker can simply read it
> off, and use it to mount CSRF attacks.
> 
> 
> 4. CONSENT/PERMISSIONS OPTIONS
> A number of possible permissions models have been proposed.
> 
> 1. The same permissions model as audio/video, namely a consent
>    dialog with (optional?) persistent content. [the
>    natural default.]
> 
> 2. A similar permissions dialog to audio/video but with *only*
>    one-time consent. [proposed by Cullen Jennings]
> 
> 3. A "sysapps" [5] API in which the user had to go through an
>    app store type install experience to enable sharing for
>    each specific site. [proposed by Adam Barth and others]
> 
> There have also been proposals for hybrid designs, such as a
> sysapps-style API that also requires an in-chrome permissions dialog
> for every sharing instance. Another possible design would be a
> "preferred site" desing in which certain sites could directly ask for
> permission without an application install but other sites would need
> to do an app store install experience.  (Like the Firefox Social API).
> 
> The argument for the less onerous permissions models is of course
> reduced user friction. The argument for the more restrictive models is
> that the set of permissions that is being granted to the web site is
> really more similar to those of a Web application install (even though
> the user does not know it) and that therefore the barrier to entry
> should be more like that of an application (i.e., a curated,
> authenticated app store).
> 
> 
> 
> 
> ACKNOWLEDGEMENT
> Much of this material came out of discussions with Adam Barth,
> Cullen Jennings, and Randell Jesup but I may well have mangled it.
> 
> 
> [0] http://dev.w3.org/2011/webrtc/editor/webrtc.html
> [1] http://dev.w3.org/2011/webrtc/editor/getusermedia.html
> [2] http://tools.ietf.org/html/draft-ietf-rtcweb-security
> [3] http://tools.ietf.org/html/draft-ietf-rtcweb-security-arch
> [4] http://w2spconf.com/2011/papers/websocket.pdf
> [5] http://www.w3.org/2012/09/sysapps-wg-charter
> [6] https://www.bankofamerica.com/privacy/online-mobile-banking-privacy/sitekey.go
> 
> 
> 
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Received on Friday, 22 March 2013 13:51:08 UTC

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