Re: [secdir] [Simple] secdir review of draft-ietf-simple-msrp-sessmatch

[Adrian Georgescu <[e-mail]>]

My two cents. Having implemented both models in Blink client (Blink is a free download if someone cares and wants to experiment with both MSRP models), I can comment that I do not like the acm model. The relay model is simply better, cleaner and more secure.

Adrian


On Oct 14, 2010, at 3:27 PM, Cullen Jennings wrote:

> 
> The new draft is clearer but I still don't think it addresses my concerns. I would say at this point they could be summarized as 
> 
> 1) The draft is very hard to review without doing the diffs to 4975. To try and help instead of just complain, I'm willing to go back patch these changes into the last XML for 4975 and provide a draft that we can actually read to see what this all means. I can't do that till after the -01 deadline. 
> 
> 2) As far as I can tell, this does change the security of 4975 in a pretty significant way in that this allows a MITM to masquerade with the wrong to and from path that may be in the cert. It is not clear how it work when the end points are not using self signed certs and changes the preferred deployment mode from using certs rooted in a trust anchor to self signed. All this seems to significantly weaken the security of 4975 which concerns me and I have not seen relevant discussion of all this. I am open to the idea that it does not making this much worse than they currently are in 4975 and that it is a reasonable trade off but I'd like to see concrete discussion of the issues and tradeoffs. How bad is it? how much worse is it? People says it is no worse but I and several others remain unconvinced that it is the same as 4975. I'd rather see a very explicitly discussion with people like the security reviewer about how much this changes things and if it is acceptable. It's not easy to sort this all out - it actually might be acceptable - I'm just not convinced yet and the "there is no problem because there is not change" form of argument is not convincing me - clearly there is a a change and at causal glance the point of that change seems to be to insert a MITM. 
> 
> 3) The backwards comparability issue seems huge. Some people have said an endpoint using this draft will not talk with one that only does 4975. Yet if this draft if published as an RFC would basically depreciate the 4975 and replace it with a the result of applying this diff to 4795. So if one person implements the pre update version, and another person the post - it's not clear to me how we migrate from old to new on the existing deployments. A flag day is obviously not going to work. The more I look at this, the more I think this draft needs to be  recast as a backwards compatible extension to 4975 and not a draft that update 4975. When I look at how this changes 4975 it seems to mostly relax the existing security but not disallow things that used to work so I think it should be possible to do this. On a side note, I phoned a few people who I know that have MSRP implementation and none of them had any plans to implement this and were surprised to hear there was a draft that would update in 4975 with a change like this. To me this combined with the no backwards compatibility issue argues strongly for figuring out how to make this an extension instead of a change to MSRP. 
> 
> 4) When I search the email lists, I find more more people who see significant problems with this than I find people that seem to think it is OK. I don't think it has consensus -I think it just has people who stopped care.  The changes that needed to happen in IETF LC to fix this draft so it had any chance of working at all more or less convinced me the WG did not read this draft. The [e-mail] list is not an ideal location for discussion that rewrites pretty much all of the normative text of this draft (which is what is happening here). 
> 
> Cullen
> 
> 
> 
> On Oct 5, 2010, at 1:33 AM, Gonzalo Camarillo wrote:
> 
>> Hi,
>> 
>> Christer has submitted a new revision of this draft:
>> 
>> https://datatracker.ietf.org/doc/draft-ietf-simple-msrp-sessmatch/
>> 
>> Those of you who sent IETF LC comments on this draft, could you please
>> have a look at the new version and let Christer know if he has addressed
>> your concerns?
>> 
>> Thanks,
>> 
>> Gonzalo
>> 
>> 
>> On 31/08/2010 8:39 PM, Christer Holmberg wrote:
>>> Hi,
>>> 
>>> The purpose of this e-mail is to address the secdir comments given by Richard
>>> Barnes and Ted Hardie. Due to summer vacations, standardization meetings
>>> etc it took a while to put the e-mail together, and we appologise for that.
>>> 
>>> GENERAL
>>> =======
>>> 
>>> First, the draft does NOT propose any changes to the TLS authentication
>>> procedures – that will be clarified. The changes are only related to the
>>> procedure for matching an incoming MSRP message to an MSRP session that
>>> has been negotiated using SDP – once any TLS authentication procedure has
>>> already taken place.
>>> 
>>> So, in case of TLS and name based authentication, if an SBC/ALG modifies
>>> the a=path MSRP URI, the TLS authentication WILL fail. That is the current
>>> behavior, and sessmatch doesn’t change that.
>>> 
>>> We understand that this fact needs to be clearly indicated in the draft.
>>> 
>>> Basically sessmatch allows so that, when using peer to peer MSRP, SIP SBCs
>>> and SIP aware firewalls can be in the SIP signaling path without acting as
>>> MSRP B2BUAs. But, for an SBC or ALG to interwork correctly with MSRP relays
>>> the SBC/ALG needs to act as MSRP B2BUA, as today.
>>> 
>>> Sessmatch aims to extend the usability of MSRP peer to peer communication to
>>> scenarios where simple ALGs/SBCs are used, and at least in our experience
>>> customer interest for standard MSRP has grown (from more or less zero)
>>> dramatically due to sessmatch. And, OMA, which previously used a *non-standard*
>>> version of MSRP (with no interoperability with standard MSRP), has also agreed
>>> to switch to sessmatch (even if it required a number of changes in their
>>> specifications).
>>> 
>>> Second, the intention of sessmatch is not to modify the MSRP URI matching rules,
>>> but rather to not use MSRP URI matching for session matching.
>>> 
>>> Please also note that when we talk about SBCs/ALGs, we refer to entities that
>>> normally do NOT have the capability to act as MSRP B2BUAs.
>>> 
>>> We will comment the individual comments based on the assumptions above.
>>> 
>>> 
>>> Comments from Richard
>>> =====================
>>> 
>>>> 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 document changes the URI matching algorithm used in MSRP.  MSRP sessions
>>>> are typically initiated using SDP bodies in SIP.  These SDP
>>>> bodies contain MSRP URIs that the peers use to contact each other.
>>>> When one peer receives a request to initiate a session, he verifies that the
>>>> URI being requested is one that he initiated in SDP, thereby using the URI as a
>>>> shared secret to authenticate that the originator of the session actually
>>>> received the SDP body in question.
>>>> 
>>>> According to the current SDP specification, this comparison is performed over
>>>> the whole URI; this document restricts the comparison to the "session-id"
>>>> component, omitting the host, port, and transport components.  The goal of the
>>>> document is to facilitate a certain class of man-in-the-middle attack, namely
>>>> to allow a signaling intermediary to insert a media intermediary.  The
>>>> restriction on the URI comparison is needed in order for the media intermediary
>>>> not to have to modify URIs in MSRP packets to reflect the modifications to URIs
>>>> in SDP bodies performed to redirect traffic through the media intermediary.
>>> 
>>> When an MSRP UA receives an MSRP packet it performs msrp session matching in order
>>> to verify that the msrp packet belongs to an existing SDP negotiated msrp session
>>> at the UA. RFC4975 prescribes that URI matching should be used for session matching.
>>> We argue that the namespace scoping of the session-id values that use of URI matching
>>> brings is unnecessary. The 80-bit randomness of the session-id and the fact that it
>>> was the UA itself that decided on the session-id value and can ensure that it is
>>> unique within the UA makes the session-id sufficiently unique for session matching.
>>> Sessmatch is not changing the MSRP URI matching algorithm, it is changing the session
>>> matching algorithm not to use MSRP URI matching.
>>> 
>>>> I have a few significant reservations about this document:
>>>> 
>>>> 1) This extension makes it more difficult for MSRP entities to secure their
>>>> communications against attackers in the signaling path.  The current model
>>>> provides a basic integrity protection, in that signaling intermediaries cannot
>>>> redirect traffic to an arbitrary third party; they must at least advise the
>>>> third party about how to modify MSRP packets. The proposed modification would
>>>> remove even this cost.
>>> 
>>> If we do not introduce the sessmatch change then the only alternative for MSRP
>>> connections to be able to be set up when SBCs or SIP aware firewalls are in the
>>> SIP signaling path is for these to introduce MSRP B2BUA support. This is probably
>>> not feasible for most SBCs and SIP aware firewalls, and if it actually were
>>> feasible then it would mean as big a security problem, or even bigger, than
>>> sessmatch. The choice is thus to not use MSRP at all in presence of such devices
>>> or to introduce sessmatch. Not to fix this probably would mean that use of MSRP
>>> will be marginalized.
>>> 
>>>> 2) Moreover, it raises the cost of providing integrity protection to messages,
>>>> since Alice must now employ both integrity and confidentiality protections on
>>>> an end-to-end basis; if her messages are only integrity-protected, then a proxy
>>>> can remove the integrity protection and redirect traffic without it being
>>>> observable to Alice.
>>>> 
>>>> The document needs to clarify what the impacts are for authentication in secure
>>>> modes of MSRP.  In particular:
>>>> -- The distinction between "self-signed" and "public" certificates is
>>>> inappropriate.  The proper distinction is between the name-based authentication
>>>> in Section 14.2 of RFC 4975 and the fingerprint-based authentication in Section
>>>> 14.4.
>>> 
>>> We cannot find the terminology “name-based” authentication in RFC 4975. The RFC talks
>>> about TLS authentication using either certificates from well-known certificate
>>> authorities, or using self-signed certificates together with certificate fingerprints.
>>> 
>>> Having said that, however, we DO agree that the terminology you suggest is more
>>> appropriate than what we have used and we will introduce this terminology and explain
>>> it in the Convention section of the sessmatch draft.
>>> 
>>>> -- In either case, changing the host name need not result in an authentication
>>>> failure, since the media intermediary can simply authenticate as itself to both
>>>> endpoints, having changed the respective MSRP URIs appropriately.
>>> 
>>> A media intermediary can only do this if it is an MSRP B2BUA, and sessmatch was
>>> introduced just to avoid most SBCs and ALGs having to implement an MSRP B2BUA in order
>>> to allow standard MSRP deployment.
>>> 
>>>> -- There is currently no requirement that an endpoint identity in the To-Path
>>>> URI matches the endpoint identity authenticated at the TLS layer, because these
>>>> two are required to be the same.  This document changes that assumption, and
>>>> should note that these two identities can differ.
>>> 
>>> We will explicitly mention this.
>>> 
>>>> The document also precludes any name-based multiplexing, where a single MSRP
>>>> process (single IP address and port) directs requests to different virtual
>>>> recipients based on the domain name in the To-Path header. (In analogy to
>>>> Host-based multiplexing in HTTP, which is very widely deployed.) Since with
>>>> this extension, the domain in the To- Path is completely unpredictable from the
>>>> recipient's perspective, it is useless to the recipient.
>>> 
>>> That is correct, but there should be no problem for a single MSRP process (single
>>> IP address and port) to direct requests to different virtual recipients - based
>>> on the session-id instead. It is only needed for the different virtual recipients
>>> to inform the receiver process on which session-ids that are currently negotiated
>>> instead of informing it on which domain name the virtual recipient shall be
>>> associated with.
>>> 
>>>> The document has no backward-compatibility. MSRP implementations that do not
>>>> support this extension will not be able to receive MSRP sessions from
>>>> implementations that do. In that regard, this document seems more like a new
>>>> version of MSRP rather than an update.
>>> 
>>> It is not true that there is no backwards compatibility. If there are no
>>> SIP ALGs/SBCs in the SIP/SDP signalling path then there is no problem for MSRP
>>> implementations that do not support the sessmatch extension to receive MSRP
>>> sessions from implementations that do.
>>> 
>>> MSRP implementations that do not support the sessmatch extension are however not
>>> able to establish MSRP end to end conversations if there are ALGs/SBCs in the
>>> session path (unless these implement MSRP B2BUA) and sessmatch does not change this
>>> fact – it will not work disregarding if the other endpoint supports sessmatch or not.
>>> 
>>>>>> I also note that the security considerations, in addition to having
>>>>>> some fairly disingenuous language about the impact of this change,
>>>>>> seems to fail to mention MSRPS URIs and what, if any, impact this
>>>>>> would have on them.
>>>>> 
>>>>> There are no impacts to MSRPS URIs. I assumed it would be implicitly
>>>>> understood since MSRPS URIs are not mentioned in the draft.
>>>>> 
>>>>> However, we could explicitly make it clear by modifying the first
>>>>> sentences of section 5:
>>>>> 
>>>>> "The change of session matching procedure does not impact the
>>>>> format of MSRP URIs, disregarding if the "msrp" scheme or the "msrps" scheme
>>>>> is used. However, MSRP endpoints can only check that the session-id part of
>>>>> the MSRP URI..."
>>>> 
>>>> The conflict here is that with MRSPS authentication, the name in the
>>>> certificate is checked against the domain name in the URI, which was OK when
>>>> the URI in the message was required to be the same. By allowing the domain
>>>> name in the message to change, this draft removes man-in-the-middle protection
>>>> from MSRPS.
>>>> 
>>>> The document notes that a SIP MitM can already direct the user to another
>>>> destination.  However, if the peers use MSRPS with the current authentication
>>>> scheme, the MitM will not be able to be a part of the resulting MSRPS session,
>>>> since he can't authenticate as one of the endpoints. If only the session ID is
>>>> used in comparisons, the MitM can patch himself in by changing the domain in
>>>> the MSRPS URI. (Which actually seems to be the intended use case for this >draft.)
>>>> 
>>>> So the current document does introduce a new MitM vulnerability to MSRPS.
>>> 
>>> Sessmatch does not change the fact that name based TLS authentication for MSRPS
>>> will fail if an SBC or ALG has modified the hostname value in the MSRP URI in the
>>> SDP a=path attribute without also acting as MSRP B2BUA.
>>> 
>>> 
>>> Comments from Ted
>>> =================
>>> 
>>>> I join Richard in believing that this document makes changes beyond that which
>>>> could be understood as "updating" the MSRP URI scheme processing.
>>>> 
>>>> ...
>>>> 
>>>> I also note that the security considerations, in addition to having some fairly
>>>> disingenuous language about the impact of this change, seems to fail to mention
>>>> MSRPS URIs and what, if any, impact this would have on them.
>>> 
>>> We will clarify what impacts there are.
>>> 
>>> -------
>>> 
>>>>>> To highlight one particular aspect, RFC 4975 does not require
>>>>>> session-ids to be present, a fact noted both in the ABNF and in this
>>>>>> text:
>>>>>> 
>>>>>> 4. The session-id part is compared as case sensitive.  A URI without
>>>>>> a session-id part is never equivalent to one that includes one.
>>>>>> 
>>>>>> A matching scheme which relies on a URI section which is not
>>>>>> guaranteed to be present has some interesting problems ahead of it. If
>>>>>> this effectively makes their use mandatory, that requires a change to
>>>>>> the fundamental ABNF and text.
>>>>> 
>>>>> An MSRP URI in an SDP offer or answer for an MSRP session MUST include a
>>>>> session-id part, so I believe the comment is
>>>>> based on incorrect assumptions.
>>>> 
>>>> This is not indicated in the URI matching section
>>> 
>>> We will clarify that sessmatch conformant UAs do not use MSRP URI matching in
>>> order to perform MSRP session matching.
>>> 
>>>>> Section 6 of RFC 4975 says:
>>>>> 
>>>>> "The session-id part identifies a particular session of the
>>>>> participant. The absence of the session-id
>>>>> part indicates a reference to an MSRP host device, but does not refer to a
>>>>> particular session at that device."
>>>>> 
>>>> 
>>>> The full section from which that quote is taken is:
>>>> 
>>>> The MSRP URI authority field identifies a participant in a particular
>>>> MSRP session.  If the authority field contains a numeric IP address,
>>>> it MUST also contain a port.  The session-id part identifies a
>>>> particular session of the participant.  The absence of the session-id
>>>> part indicates a reference to an MSRP host device, but does not refer
>>>> to a particular session at that device.  A particular value of
>>>> session-id is only meaningful in the context of the associated
>>>> authority; thus, the authority component can be thought of as
>>>> identifying the "authority" governing a namespace for the session-id.
>>>> 
>>>> This proposal changes the concept of a namespace authority present in the URI
>>>> matching section of RFC 4975. I am sorry if my wry reference to this in my
>>>> previous message was hard to follow; I should have known better.
>>>> 
>>>> To be more plain, this proposal fundamentally changes the matching semantics of
>>>> the URI set out in RFC 4975, by requiring a match on only a portion of the URI.
>>>> At a bare minimum, this would require noting a normative update to section 6
>>>> and 6.1 of RFC 4975, which this draft does not do.  In reality, this is
>>>> unlikely to be sufficient, as URI matching semantics do not generally have the
>>>> concept of ignoring the authority in providing a match (at least in my reading
>>>> of the RFC 3986 "ladder of comparison" text).  That means you'd have to special
>>>> case the MSRP matching semantics, rather than have the URI be parsed and
>>>> compared using a standard library.
>>> 
>>> Sessmatch removes the URI matching as a means to do MSRP session matching, and
>>> replaces it with a pure session-id matching. There is no need to create a new
>>> URI scheme that does not re-use the authority component. We believe the minimum
>>> 80-bit randomness of the session-id, together with the fact that the UA itself
>>> generates the session-id it matches on, to be enough for the session-id to be
>>> unique in the scope of the sessions that are active at the UA.
>>> 
>>> Also, the security of the matching is not particularly decreased, since it is
>>> relatively easy to find out the authority name anyway.
>>> 
>>>>>> I also note that the security considerations, in addition to having
>>>>>> some fairly disingenuous language about the impact of this change,
>>>>>> seems to fail to mention MSRPS URIs and what, if any, impact this
>>>>>> would have on them.
>>>>> 
>>>>> There are no impacts to MSRPS URIs. I assumed it would be implicitly understood
>>>>> since MSRPS URIs are not mentioned in the draft.
>>>>> 
>>>>> However, we could explicitly make it clear by modifying the first sentences of
>>>>> section 5:
>>>>> 
>>>>> "The change of session matching procedure does not impact the format of MSRP
>>>>> URIs, disregarding if the "msrp" scheme or the "msrps" scheme is used.
>>>>> However, MSRP endpoints can only check that the session-id part of the MSRP
>>>>> URI..."
>>>>> 
>>>> This doesn't seem to me to actually work, based on Richard's comments, unless
>>>> what you mean to say is "if MSRPS is in use, nothing in this draft can be
>>>> used". That gives you different matching semantics for MSRPS (authority must
>>>> be present and must be matched using TLS semantics) vs MSRP (only session-id is
>>>> checked) which is at the very least a violation of the principle of least
>>>> surprise (no other foo over TLS protocol works that way that I know of ).
>>> 
>>> Session matching is done when receiving MSRP packets on an already established TCP
>>> or TCP/TLS connection, and there will not be any different session matching procedure
>>> depending on if the connection uses TLS or plain TCP.
>>> 
>>> Regards,
>>> 
>>> Christer
>>> 
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