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[whatwg] Re: Cross Domain Policies

From: Malcolm Rowe <malcolm-what@farside.org.uk>
Date: Tue, 27 Jul 2004 11:57:14 +0100
Message-ID: <courier.4106350A.00007B35@mail.farside.org.uk>
Jim Ley writes:
> Most programming toolkits aren't run in a browser!  This _DOES_ make
> Mozilla SOAP in secure, I have SOAP services running on this intranet
> which are protected purely by the fact they're behind the firewall,
> any machine here can access them, those outside cannot - However,
> www.anydomain.org can use them easily, if someone happens to drop an
> XML file in the root.

Sorry, but I don't see a problem here. 

You have an internal webserver that contains important data, and accepts 
SOAP calls; let's call it important.example.com. That server doesn't require 
authentication, but that's ok, because you trust all your users. 

Your browser is capable of making scripted SOAP calls. It's also capable of 
downloading scripts from the internet. You don't want your browser to make 
calls on your behalf using 'untrusted' scripts, but it's ok for it to make 
calls using 'trusted' scripts. 

There's no way to determine 'trusted' from 'untrusted' automatically, for 
all organisations, since the degree of 'trust' may be different for each 
service and organisation. For example, internal-phonebook.example.com may 
trust any scripts downloaded from *.example.com, while important.example.com 
may not trust scripts downloaded from anywhere else. Particularly (for this 
example), it doesn't trust www.anydomain.com. 

Note that this issue of 'trust' is only really relevant in an intranet 
scenario, where you can trust your users; in the internet at large, a SOAP 
service has to contend with malicious clients, so the server (and not the 
client) must handle access control if necessary. 

Mozilla's current implementation (ignoring some unimportant details) means 
that scripts downloaded from important.example.com are the only ones that 
can access SOAP services on that server. 

If the server operator then decides that scripts at important-2.example.com 
can also be trusted, they can create a file at 
http://important.example.com/web-scripts-access.xml that contains a record 
for important-2.example.com. The server operator is the one who manages the 
data; they also get to set the policy. 

When any script attempts to make a SOAP call to important.example.com, the 
XML file will be downloaded by the client. If the file doesn't exist, or if 
the script's origin does not appear in the file (and the service isn't 
marked as public in the file), then the SOAP call is prohibited. [The design 
also allows for delegation, but I'm ignoring that for now.] 

Incidentally, if (for example) Google decided to allow any script to make 
SOAP calls to api.google.com, they could carry out the same procedure and 
mark the service as public. 

I don't understand what your objection is to this design. It allows the 
server operator to decide which scripts can be trusted to make SOAP calls to 
their service. The server operator, and not the user, is the one who gets to 
set the policy, and is also the person responsible for securing the data. 

The only feasible attack that I can see would be if www.anydomain.com could 
persuade the server operator to permit access from their domain, either 
explicitly, or by marking the service as public. However, the server 
operator effectively controls all the data on the server anyway (even in an 
ASP scenario), so I don't see that this makes that hole any larger. That 
would be like arguing that Apache's httpd.conf file allows the administrator 
to disable .htpasswd checks sitewide. Sure, it does, but why does that give 
you a realistic attack? 

Do you have a better idea that still permits public (or semi-public) SOAP 
services? After all, we don't restrict scripts from POSTing to different 
domains, which is, essentially, the same thing. 

Now, I do have *some* reservations about this design: 

* It pollutes the URI space, which should be completely under the control of 
the server operator. This is essentially the TAG issue that I originally 
mentioned. 

* It allows the server operator to permit access to all services held on 
their server (with no override possible). This assumes that the server 
operator is the stakeholder, which may not be the case. However, as 
mentioned above, the server operator *does* have effective control over all 
services running on their server, so this should not introduce any *new* 
attack vectors. 

I also have some issues with the Mozilla implementation: 

* It allows a preference to be set that can be used to mark any service as 
public. I'm sure that this is significantly faster in an intranet setup with 
mass-deployed clients, since it removes the requirement for all users to 
check the permissions file on the server. However, as an attacker, I now 
only need to control that pref value on one intranet client machine in order 
to be able to access any intranet service. 

* As you pointed out, it doesn't appear to be possible to disable this model 
and revert to just using the same-origin check. I would have thought that it 
would be prudent to allow this, if only so that we can switch it off for a 
security fire-drill. Fortunately, this doesn't look like it would be too 
hard to implement. Doron, what do you think? Would this be worthwhile? 

Regards,
Malcolm 
Received on Tuesday, 27 July 2004 03:57:14 UTC

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