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Re: [discovery-api] Mitigating real-world device compromise

From: Jean-Claude Dufourd <jean-claude.dufourd@telecom-paristech.fr>
Date: Thu, 24 Oct 2013 13:45:23 +0200
Message-ID: <52690853.9060204@telecom-paristech.fr>
To: public-device-apis@w3.org
Le 24/10/13 03:05 , Rich Tibbett a écrit :
> I want us to work through a real-world UPnP compromise that has
> publicized recently. The specific compromise we can focus on was made
> against Netgear routers [1].
> the tl;dr version of [1] is that, by visiting a specific URL on your
> router, you can gain persistent access to it and all its settings
> without passing through any authentication process.
> We could do some hand-wringing on how this happened in the first place
> but it would be more productive to detail how the Network Service
> Discovery API is capable of mitigating against such attacks. I will
> summarize some initial thoughts here and would welcome other input on
> this topic.
> Firstly, all existing router-level devices (including the compromised
> Netgear devices) would not be exposed to web pages via this API
> because they do not provide CORS support. To opt in to web sharing,
> UPnP devices need to implement CORS on specific UPnP service
> endpoints. It should be obvious that router-level devices and other
> sensitive services should not expose CORS HTTP headers and thus should
> never be exposed to web pages in the first place.
> A secondary attack vector arising here is if a web page gains access
> to a service residing on the same device as router functionality (e.g.
> I could request and be granted access to media services via CORS on a
> device that also implements router services).
> An exploit that takes advantage of this could look as follows:
> navigator.getNetworkServices('serviceA').then(function(networkServices) {
>    var serviceUrl = new URL(networkServices[0].url); //
> http://url.spec.whatwg.org/#api
>    var exploitUrl = serviceUrl.protocol + '//' + serviceUrl.host;
>    exploitUrl += (serviceUrl.port ? ':' + serviceUrl.port : '');
>    exploitUrl += '/secondaryService/maliciousMethodCall';
>    // Use XHR to interact with exploitUrl...
> });
> According to the current specification this attack vector would fail
> if 'secondaryService' did not implement CORS or did not provide CORS
> to the requesting origin. However, to initial a simple GET request to
> a particular URL as detailed in this particular exploit, a web page
> could simply do the following:
> var imgEl = new Image();
> imgEl.src = exploitUrl;
> document.body.appendChild(imgEl);
> That would have the effect of requesting this URL via HTTP GET
> regardless of whether that resource is an image or not (which it
> clearly isn't and that device, according to [1], is now pwned).

None of the attacks above would work with my suggestion of removing the 
URLs from the NSD APIs altogether, and dealing with the messaging inside 

> I have briefly mentioned a way we could avoid this exploit where local
> network urls will be obfuscated from their original host [2] thereby
> avoiding a leak of local network IP addresses and ports.
> Let's say in the example above that serviceUrl returned
> 'app://<uuid>/'. The original service endpoint url for this service
> was actually '' and app://<uuid>/ simply
> acts as a proxy to that URL. What this proxy does is make
> '' the _root_ path of 'app://<uuid>/'.
> Developers cannot call anything above '/myService' in the www
> hierarchy and developers cannot infer paths to any other potential
> services on the same device from the provided url. The 'app://<uuid>/'
> serviceUrl acts as a full HTTP proxy to
> and any sub-resources therein (e.g. app://<uuid>/logo.png maps to
>, etc).
> Having such a concept removes the ability for a web developer to infer
> other service endpoint URLs from any other shared service endpoint
> URLs.
> A remaining challenge with that approach is that services, once
> connected to web pages via app:// based URLs. may still leak their IP
> address in service messaging data calls. E.g. a service providing a
> pointer to a media resource may transfer data via XHR to a web page
> that looks as follows
> ''. Exposing this URL to
> web pages would allow them to infer other well-known services on the
> same device by running the exploit code I included above.

This is the only attack vector mentioned in this email that still works 
with no URLs and messaging in NSD.

> A full solution should would obfuscate both the initial service
> endpoint url and monitor service messaging itself with a view to
> obfuscating in-messaging urls via XHR. For this I propose we add an
> additional user agent processing step whenever a web page calls
> xhr.response, xhr.responseText or xhr.responseXML on an app:// URL.
> Primarily, the additional XHR processing step would be that if the XHR
> url requested begins with 'app://' then parse any response body for
> matches to the original URL host+port combination and replace with any
> matches with an app:// URL based reference to that same resource. The
> finer details of this process would be defined fully within the NSD
> API spec (I'm just presenting the main proposal for now).

If all messaging passes through NSD, and is restricted to UPnP messaging 
in the case of UPnP services, then such a processing step can be applied 
to the content of UPnP messages.

What you are suggesting is actually very close to proxying XHR through 
NSD anyway.

You are still open to attacks on any other communication protocol that 
may be usable, if any.

Best regards

> When we are obfuscating local network URLs at both the service sharing
> and service communication levels we will be able to fully isolate
> network services from each other. Also, the only network services that
> can be shared with web pages are CORS-enabled services and sensitive
> network resources should not expose such headers (thereby excluding
> them from any NSD API sharing). At this point we have successfully
> mitigated any exposure to the kind of attack in [1].
> Other thoughts and proposals on this would be welcome, specifically
> mitigating or avoiding altogether the attack vectors detailed in [1].
> Showing we can avoid/mitigate such attacks is an essential step in
> demonstrating APIs suitability.
> Best regards,
> Rich
> [1] http://shadow-file.blogspot.com.au/2013/10/complete-persistent-compromise-of.html
> [2] http://lists.w3.org/Archives/Public/public-device-apis/2013Oct/0129.html

Télécom ParisTech <http://www.telecom-paristech.fr> 	*Jean-Claude
DUFOURD <http://jcdufourd.wp.mines-telecom.fr>*
Directeur d'études
Tél. : +33 1 45 81 77 33 	37-39 rue Dareau
75014 Paris, France

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Received on Thursday, 24 October 2013 11:45:55 UTC

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