Re: (short version) Re: Last Call: <draft-faltstrom-uri-10.txt> (The Uniform Resource Identifier (URI) DNS Resource Record) to Proposed Standard

Eliot Lear <lear@cisco.com> Fri, 27 February 2015 16:11 UTC

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Date: Fri, 27 Feb 2015 17:11:44 +0100
From: Eliot Lear <lear@cisco.com>
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To: Sam Hartman <hartmans-ietf@mit.edu>
Subject: Re: (short version) Re: Last Call: <draft-faltstrom-uri-10.txt> (The Uniform Resource Identifier (URI) DNS Resource Record) to Proposed Standard
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Ok, I understand what you are saying.  The changing of the origin that
gets validated is the fundamental no no.

Eliot

On 2/27/15 4:18 PM, Sam Hartman wrote:
>>>>>> "Eliot" == Eliot Lear <lear@cisco.com> writes:
>
> I don't understand your statement.  What's a decision process?
>
> Your DNS libraries definitely have a different trust validation process
> than your application TLS libraries.
> Your application is probably not entirely aware of its use of DNS;
> that's how we write apps today.
>
> Let's take a concrete example.
> I felt greate unease when I heard Patrik talk about getting support for
> the URI record added to libcurl.
>
> Imagine that's done, and by default libcurl will follow a URI RR
> redirection if it's DNSSec validated.
>
> I am using libcurl to talk to some cloud service.
> I've carefully configured libcurl's TLS parameters because I know
> exactly what I'm expecting.
> Let's say that I've done so in a way that's site specific.
> If libcurl is fetching the URI https://cloud.example.com/ then my
> validation rules apply.
> (I believe that libcurl has sufficiently rich callback interfaces to
> support this, but I get the various HTTP library capabilities confused)
>
>
> I have a fairly strict set of trust anchors for that URI because I know
> exactly what to expect.
>
> however, an attacker has managed to steal example.com's DNS registry
> account password and has compromised their zone.
> With a version of libcurl that  does not support the URI rr, my
> application is vulnerable to a denial of service attack if they mess up
> the  DNS zone, but I am not vulnerable to an attack on integrity of my
> interactions with the cloud services.
>
> However, now that libcurl follows the URI RR, since the attacker has
> compromised example.com's DNS zone, they can now redirect me elsewhere.
> And since the URI RR draft specifies we're going to do our TLS
> validation based on the target of the redirection rather than the
> source, my validation rules for cloud.example.com no longer apply, and
> so my security is likely weak enough to attack if the attacker can get a
> cert for a domain controlled by the attacker.
>
> OK, so perhaps libcurl should not default to following URI RR redirects.
> Fine.  But we as standards authors need to help application authors
> understand the implications of turning on the URI RR.  You probably
> don't want to follow URI RR if you're supplying a non-default TLS trust
> anchor set, pinning to a specific certificate or the like unless you're
> somehow supplying specific acceptable DNSSec trust anchors for the zones
> in question and not trusting the root trust anchor.  Note, however that
> application TLS libraries ar much more customizable in their validation
> than application-layer DNSSec libraries.  In practice TLS allows
> application security policy to be specified, but DNSSec seems to use one
> global security policy.  I understand that's not inherent in the
> standards, although the way the two standards are written does
> contribute to it.  It does seem true in the code for TLS and DNSSec I'm
> aware of, and it is to the best of my understanding a practical reality
> for authors of applications.
>
>