[whatwg] Security thoughts

Ian Hickson ian at hixie.ch
Tue Aug 3 14:39:52 PDT 2010

On Sun, 9 May 2010, Perry Smith wrote:
> In HTML5 6.3.1 Relaxing The Same Origin Restriction [1] bullet 3, sub 
> bullet 3 there is a clause that says that if the domain is reduced down 
> to something that is on the Public Suffix List, the new value is 
> rejected.  That phrase caused me to pause.
> I was wondering about internal attacks.  First, we need to assume a 
> couple of things but they are relatively easy to assume.  The first is 
> that the relaxing of the restriction has a valid use.  This seems easy 
> or it would not be in the spec.  The second is that an internal domain 
> can effectively be a public suffix list to users on the internal 
> intranet.  For example, at the place I work, I connect my laptop to the 
> wifi, it grabs an address and also registers the name.  Even if the name 
> was not registered, it would still have some DNS entry.  The point is 
> that all DNS entries within this subdomain are not trusted.
> If we have a site like official_site.area_subdomain.big.com which 
> relaxes the restriction to area_subdomain.big.com, it is now exposed to 
> the potential of an attack from any of the systems within the same 
> area_subdomain including laptops connected via wifi.  The wifi is 
> secure.  The place I work at trusts me to some degree but with a large 
> corporation, they very often try to restrict information on the "need to 
> know" basis.  And, corporate espionage is a real threat.
> I don't know how common it is for internal corporate sites to relax the 
> same origin restriction but I could see it becoming more and more common 
> as they try to take advantage of various technologies.
> The corporations could take steps of course to secure the sites.  They 
> could put all official web sites in their own subdomain and then relax 
> to this more trusted subdomain.
> The purposed of this email is to ask if a warning should be added in the 
> 3rd bullet to advise web developers of internal sites to be careful in 
> assuming that all the hosts on their internal subdomain are trusted.

I would be happy to add such a warning, but I'm not sure I understand the 
attack you had in mind.

Is this the scenario you have in mind?:

   User A controls a laptop within Example Corp's firewall and has a host 
   name of laptop1.corp.example.com.

   User B is also within the firewall.

   Service V is at service.corp.example.com, and it uses document.domain 
   to relax its same-domain restrictions to "example.com".

   User A tricks User B into visiting a file hosted on his laptop.

   That file relaxes its same-domain restriction to "example.com", loads 
   service V in an iframe, and uses the DOM to perform an attack on V 
   using B's credentials.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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