Re: [secdir] secdir review of draft-ietf-roll-admin-local-policy-02

Robert Cragie <robert.cragie@gridmerge.com> Wed, 07 January 2015 13:26 UTC

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Cc: iesg@ietf.org, draft-ietf-roll-admin-local-policy.all@tools.ietf.org, secdir@ietf.org
Subject: Re: [secdir] secdir review of draft-ietf-roll-admin-local-policy-02
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Steve,

My comments inline, bracketed by <rcc></rcc>.

Robert

On 02/01/2015 18:26, Steve.Hanna@infineon.com wrote:
> Peter,
>
> Thanks for your prompt response. I have added some more comments below,
> delimited with <steve>.
>
> Steve
>
> -----Original Message-----
> From: peter van der Stok [mailto:stokcons@xs4all.nl]
> Sent: Friday, January 02, 2015 5:19 AM
> To: Hanna Steve (IFNA CCS SMD AMR)
> Cc: secdir@ietf.org; iesg@ietf.org;
> draft-ietf-roll-admin-local-policy.all@tools.ietf.org
> Subject: Re: secdir review of draft-ietf-roll-admin-local-policy-02
>
> Dear Steve,
>
> thanks for the comments. I will respond below to the points you raise.
>
> Greetings, Peter
>
> Steve.Hanna@infineon.com schreef op 2014-12-30 13:55:
>
>> This document describes a technique for automatically managing the
>> boundaries of the admin-local multicast scope in a border router,
>> using MPL (Multicast Protocol for Low power and Lossy Networks).
> <peter>
> Correct
> </peter>
>> In my view, this document is Not Ready.
>>
>> The document is hard to understand. For example, the acronyms MPL and
>> MPL4 are used throughout the document but they are not defined.
> <peter>
> You are the third to remark on this point. Adrian did suggestions to
> improve the document with a definition of MPL, that we will take over.
> MPL4 concepts are defined in section 1.2.
> </peter>
> <steve>
> I'm happy to see that you'll be adding a definition of MPL. That's good.
>
> While it's true that section 1.2 defines several terms that include MPL4
> (e.g., "MPL4 message"), the term "MPL4" is never defined anywhere.
> I found that confusing.
> </steve>
>> After reading the document several times, I have concluded that
>> section 3.2 contains the most important part of the document: an
>> algorithm for automatically defining the boundaries of the admin-local
>> multicast scope using MPL. This section basically says that a border
>> router should periodically send an MPL message on all interfaces to
>> the ALL_MPL_FORWARDERS multicast address with admin-local scope. It
>> should also listen for such messages on all interfaces. If it receives
>> such a message, that interface should be marked as part of the
>> admin-local scope.
>>
>> This algorithm seems problematic from a security standpoint. Because
>> any device on a network can send an MPL message to the
>> ALL_MPL_FORWARDERS multicast address with admin-local scope, the
>> boundaries of the admin-local multicast scope can be expanded by
>> attackers fairly easily.
<rcc>I guess it depends what you mean by "fairly easily". An attacker in 
this case would have interfaces on more than one network and would have 
to be authenticated on all networks in question. The attack scenario may 
be just to attach to one network and then forward into other network(s) 
which would not originally be considered part of the admin-local scope 
(which is I think what you are saying below) but it still has to 
authenticate to the network being attacked and obtain the relevant key 
before its interface on that network can validly receive a MPL4 
message.</rcc>
>> Such manipulation may permit nodes that
>> should be outside a particular administrative domain to join that
>> domain and participate in receiving and sending multicast traffic
>> within that domain. The implications of such an attack are not clear
>> to me because I am not familiar with the protocols and devices that
>> use admin-local multicast scope. However, it seems likely that
>> expanding the admin-local multicast scope will permit external
>> attackers to more easily monitor multicast traffic that should be
>> private and to inject multicast messages into a relatively trusted
>> domain.
<rcc>Again, it has to have been authenticated to one or more networks 
and obtained the relevant key before it can do this.</rcc>
> <peter>
> In the discussion with Joel, we came to the conclusion that we were not
> very clear with respect to the administrative zone specification.
> We suggested to limit the mpl admin-local scope within one zone.
> The text will be extended to include this extra limit on the boundary of
> admin-local-scope.
> The extent of the scope does not specify anything about the contents of
> the MPL messages.
> It is expected that MPL messages are encrypted depending on the wanted
> security level.
> Monitoring by an attacker limits itself to counting messages, which is
> already possible on wireless links any way.
> To inject messages, the attacker should know the keys necessary to
> encrypt.
> When messages are not encrypted they are already readable on the
> wireless links, and the monitoring by extending the mpl-admin-local
> scope does not increase the vulnerability to snooping the messages.
> </peter>
> <steve>
> Please send me a copy of your new text limiting the admin-local scope
> to one zone. I don't see how this will help but maybe the new text will
> make it clear.
>
> You just stated that "It is expected that MPL messages are encrypted".
> However, this is never stated in draft-ietf-roll-trickle-mcast-11.txt or in
> draft-ietf-roll-admin-local-policy-02.txt. In fact, there's no mention of
> encryption in those documents at all. If the security of your design
> depends on this encryption, you'd better talk about it somewhere!
>
> Now I'd like to investigate the security provided by this encryption.
> Are you expecting that application-layer encryption will be used?
> I suppose not because that would require extensive description
> And you provided none. Probably you're thinking about link-layer
> encryption such as that provided by IEEE 802.11i. However, I don't
> think that such encryption will prevent the attacks that I described.
>
> A border router frequently spans networks that are part of multiple
> administrative domains. Your current design (even with link encryption)
> means that any device connected to any of these networks can dictate
> the boundaries of the admin-local scope. Is it really desirable to trust
> all those devices to that extent? I think not.
> </steve>
<rcc>
The concept of scope, network boundary and security boundary are all 
somewhat orthogonal. RFC7346 acknowledges this with additional text and 
draft-ietf-roll-admin-local-policy explains how automatic configuration 
for realm-local scope occurs in relation to a network identifier, which 
then links network boundary and realm-local scope. However, there isn't 
necessarily a one-to-one correspondence between security boundary and 
network boundary even though it often happens in practice, e.g. a WPA2 
key for an 802.11 WLAN or network key for ZigBee IP WPAN, for example. 
Assuming this link-layer encryption, I don't think the attacks can take 
place as an attacker would have to authenticate itself to the network 
being attacked first or else it would not be able to validly received 
the MPL4 message.

It is true that a border router configured with multiple MPL-enabled 
interfaces does indeed by implication expand the admin-local scope, 
however it is not obliged to have all its interfaces MPL-enabled; this 
is part of configuration as well, potentially controllable by some 
superordinate entity.

Perhaps we need some extra text to explain this.
</rcc>
>
>> In addition to this fundamental concern, I have a few minor concerns
>> about the readability of the document. However, I seem to have mislaid
>> my notes during the holidays. Because this review is already late and
>> I'm on vacation, I will send the review now and follow up with the
>> minor concerns at a later date if I can find them when I return to the
>> office.
> <peter>
> If you find terms which are not defined in this draft,
> ietf-rol-trickle-mcast or RFC7346, I will be happy to extend the
> definitions of section 1.2.
> </peter>
> <steve>
> OK, I'll look for my notes. However, the most important aspect of
> my review is the security issue described above.
> </steve>
>