Re: [secdir] SecDir review of draft-ietf-nsis-tunnel-11

Charles Shen <charles@cs.columbia.edu> Sun, 20 June 2010 03:52 UTC

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From: Charles Shen <charles@cs.columbia.edu>
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Cc: Henning Schulzrinne <hgs@cs.columbia.edu>, iesg@ietf.org, draft-ietf-nsis-tunnel.all@tools.ietf.org, secdir@ietf.org
Subject: Re: [secdir] SecDir review of draft-ietf-nsis-tunnel-11
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Hi Yaron, thank you for your careful review. Please see comments inline.

On Sun, Jun 13, 2010 at 3:58 AM, Yaron Sheffer <yaronf.ietf@gmail.com> wrote:
> I have reviewed this document as part of the security directorate's
> ongoing effort to review all IETF documents being processed by the IESG.
> These comments were written primarily for the benefit of the security
> area directors.  Document editors and WG chairs should treat these
> comments just like any other last call comments.
>
> This draft discusses the problem of NSIS messages (particularly, QoS
> reservation flows) being encapsulated into various IP tunneling protocols,
> which prevent the correct QoS setup from being performed. The draft proposes
> a solution for NSIS tunnel-aware tunnel endpoints, which basically adds an
> NSIS signaling flow between the tunnel endpoints, but outside of the tunnel.
>
> General
>
> The draft presents the problem, and the solution, reasonably well.
>
> The draft goes for the "no new security issues" approach. I think this is
> incorrect, and in fact a number of security issues should be analyzed and
> possibly resolved. In addition, as a complete outsider to NSIS, I have
> identified one major unspecified piece, leading me to believe that the draft
> has not had enough review.
>
> Security
>
> The main security issue is that the draft fails to consider
> security-oriented tunnels. IPsec tunnels (and the commonly used
> GRE-over-IPsec) provide security services: normally encryption and integrity
> protection with ESP, less commonly integrity-protection only with AH, ESP
> with null encryption, or the new WESP (RFC 5840). The proposed solution
> raises at least three major security issues related to these tunnels:
>
> 1. A so-called covert channel that results from NSIS flows in the protected
> networks directly triggering NSIS protocol exchanges in an unprotected
> network (i.e. between the tunnel endpoints). Please see Appendix B.1 of
> draft-ietf-tsvwg-ecn-tunnel-08 for treatment of a similar issue.
>

With regard to this specific draft, I see the problem as a more
generic issue which exists also for other protocols (e.g., RSVP)
requiring per-hop processing to interact with IPSec. E.g., RFC4302
mentions that "NOTE: Use of the Router Alert option is potentially
incompatible with use of IPsec. Although the option is immutable, its
use implies that each router along a packet's path will "process" the
packet and consequently might change the packet.". I think that
mentioning of this potential incompatibility will be beneficial. But I
don't quite see how "limiting the bandwidth of the covert channel" as
discussed in Appendix B.1 of draft-ietf-tsvwg-ecn-tunnel-08 can be
applied here. Please correct me if I were wrong.

> 2. A more serious interaction in the other direction: unprotected NSIS flows
> outside the tunnel interact with NSIS flows in the protected networks and
> inside the tunnel, and so, an attacker in the unprotected network can
> possibly influence QoS behavior in protected networks.
>
> 3. A practical result of (2) is that the NSIS protocol stack on the tunnel
> endpoint is now exposed to unprotected networks and therefore suddenly
> becomes security-critical.
>

IMHO, if we have a segment of the path which is compromised, the QoS
of the rest of the path segments (and the end-to-end QoS) can be
easily affected anyway, whether you have a tunnel segment in the path
or not. Therefore, it doesn't seem to me as a new threat introduced by
this document per se. But it will certainly also be helpful to mention
this scenario in the security considerations section.

> Non-Security
>
> The draft defines extra UDP encapsulation in some cases ("the tunnel
> entry-point inserts an additional UDP header"), but the format
> (specifically, the port number) is not specified. This omission is strange,
> because the protocol cannot be implemented in the absence of this
> information!
>

To me this is an intended feature. The mechanism does not require a
pre-allocated fixed UDP port, but allows the port to be dynamically
chosen and conveyed during the tunnel flow/session binding operations.

Thanks

Charles