Re: [secdir] review of draft-nottingham-site-meta-04

Mark Nottingham <mnot@mnot.net> Thu, 03 December 2009 05:49 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Thu, 3 Dec 2009 16:48:57 +1100
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To: Sandra Murphy <sandy@sparta.com>
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Cc: apps-discuss@ietf.org, eran@hueniverse.com, secdir@ietf.org
Subject: Re: [secdir] review of draft-nottingham-site-meta-04
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Hi Sandra,

Responses inline. 

On 02/12/2009, at 12:41 PM, Sandra Murphy wrote:
> 
>   Note that this specification defines neither how to determine the
>   authority to use for a particular context, nor the scope of the
>   metadata discovered by dereferencing the well-known URI; both should
>   be defined by the application itself.
> 
> I'm not sure what "authority to use for a particular context", but I presume that it means that each application should consider the authorization model of who should have the authority to use the well-known URI at each host/site.  This sounds lke a general security concern, but it is not verbatim reflected in the security considerations section (the scope part is mentioned, not the "authority to use".)  Note: given that I say below that it would be impossible to be complete in the security concerns that might arise in any particular application, this is NOT a recommendation that the text should change.

Not quite. It's basically saying that, given a particular application context using arbitrary network resources, it's up to you to determine what the appropriate URI authority (e.g., 'example.com' in 'http://www.example.com/.well-known/foo') should be.

> The second possibility mentioned is DNS rebinding:
> 
>   Because most URI schemes rely on DNS to resolve names, they are
>   vulnerable to "DNS rebinding" attacks, whereby a request can be
>   directed to a server under the control of an attacker.
> 
> My understanding is that DNS rebinding allows the attacker to rebind a name it controls to a local address.  So it is the directing to a server that is under the control of the attacker, not the server itself.  I'm not sure that is what the text here is saying.  DNS rebinding here would be a concern if the well-known URI provided some access that would be useful to an attacker.  That would be a subject for the application to consider, so I'm not saying that it needs to be mentioned here.
> 
> Recommendations for protection against DNS rebinding have to do with the browser or the enterprise, not the application, so I don't think they need to be mentioned here.

I agree; DNS rebinding was brought up as a concern during review, but AIUI it's more of a concern for applications using well-known locations, if they choose to try to address that problem. It may be that they just pass a warning upstream to their implementers/users.


> I could see that there might be other ways that the existence of a well-known URI could be a concern, depending on how the application used that file (DDOS if the use caused transmission, exposure if the use caused access to sensitive data, whatever).  But I don't think that this document could possibly be complete in discussing all the security concerns these unknown applications with their unknown uses of the URI could have.
> 
> In general, I think this section could be replaced with just guidelines about what the specification of a new well-known URI should discuss or consider.  Consider the authorization model, consider corruption, exposure, etc. of the URI file, consider vulnerability to DNS rebinding attacks, etc.

I think that's a good suggestion. 


> IANA considerations section
> 
> The draft mentions several things that a specification of a new well-known URI should discuss or include. Is the IANA resonsible for ensuring that a specification for a new well-known URI meets the stipulations made here? Or maybe the Designated Expert does that?


The designated expert.

Cheers and thanks for the review,

--
Mark Nottingham     http://www.mnot.net/