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

Yaron Sheffer <> Tue, 06 July 2010 12:57 UTC

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Date: Tue, 06 Jul 2010 15:57:28 +0300
From: Yaron Sheffer <>
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Cc: Henning Schulzrinne <>,,
Subject: Re: [secdir] SecDir review of draft-ietf-nsis-tunnel-11
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Looks good to me.


On 07/06/2010 03:59 AM, Charles Shen wrote:
> Hi Yaron, I have just submitted a revised version. please take a look
> and let me know if you have any further comments.
> Thanks again.
> Charles
> On Sun, Jul 4, 2010 at 9:13 AM, Yaron Sheffer<>  wrote:
>> Hi Charles,
>> I have been assigned your draft for a secdir re-review. The draft has not
>> been revised since my last review, so I will just reiterate: I recommend
>> that the Security Considerations be improved before the document goes into
>> IESG telechat.
>> Thanks,
>>         Yaron
>> On 06/21/2010 06:15 PM, Charles Shen wrote:
>>> Thanks Yaron, please see inline.
>>> On Sun, Jun 20, 2010 at 7:46 AM, Yaron Sheffer<>
>>>   wrote:
>>>> [Removed the IESG.]
>>>> Hi Charles,
>>>> please see my comments inline.
>>>> Thanks,
>>>>     Yaron
>>>> On 06/20/2010 06:52 AM, Charles Shen wrote:
>>>>> Hi Yaron, thank you for your careful review. Please see comments inline.
>>>>> On Sun, Jun 13, 2010 at 3:58 AM, Yaron Sheffer<>
>>>>>   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.
>>>> You can say this solution is incompatible with IPsec and be done with it.
>>>> Otherwise, there is a "covert channel
>>>> <>" - someone can create
>>>> spurious
>>>> NSIS signaling flows within the protected network, just to create
>>>> signaling
>>>> in the outside network, which then someone else is monitoring. For highly
>>>> secure networks, this would be seen as a way to smuggle information out
>>>> of
>>>> the network, and you would want to rate-limit this channel.
>>> That makes sense. My understanding is that the rate-limit does not
>>> complete solve the problem, but does reduce the potential harm.
>>>>>> 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.
>>>> What I'm saying in #3 is that any security vulnerability (e.g. buffer
>>>> overflow) in the NSIS stack is suddenly exposed to the big bad Internet,
>>>> even when the customer may have expected all their traffic to be
>>>> protected
>>>> by a VPN gateway, where the VPN software is normally the only software
>>>> that
>>>> needs to be hardened.
>>> I agree. What I had been thinking is that compromise of other nodes
>>> (non-tunnel end-points) may similarly affect end-to-end QoS signaling,
>>> even if the end-to-end path includes a secure tunnel.
>>>>>> 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.
>>>> Sure, I missed this point. Can you please mention it explicitly.
>>> Sure!
>>> Thanks
>>> Charles