Re: [secdir] sec-dir review of draft-ietf-stox-presence-07

[Peter Saint-Andre <[e-mail]>]

On 2/6/14, 11:16 AM, Ben Laurie wrote:
> On 6 February 2014 00:00, Peter Saint-Andre <[e-mail]> wrote:
>> On 2/4/14, 8:11 PM, Peter Saint-Andre wrote:
>>>
>>> Hi Ben, thanks for the review. Comments inline.
>>>
>>> On 2/3/14, 10:50 AM, Ben Laurie 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.
>>>>
>>>> Summary: ready with nits
>>>>
>>>> Detail: the I-D documents a mapping between two fairly well-matched
>>>> presence protocols, and hence does not introduce much danger. The one
>>>> area of concern is that SIP presence subscriptions are short-lived and
>>>> XMPP's are long-lived, thus opening the possibility of an
>>>> amplification attack against SIP via XMPP.
>>>>
>>>> The suggested mitigation is weak:
>>>>
>>>> "To help prevent such an attack, access to an XMPP-
>>>>      SIP gateway that is hosted on the XMPP network SHOULD be restricted
>>>>      to XMPP users associated with a single domain or trust realm (e.g., a
>>>>      gateway hosted at simple.example.com ought to allow only users within
>>>>      the example.com domain to access the gateway, not users within
>>>>      example.org, example.net, or any other domain)"
>>>>
>>>> Many XMPP servers allow open registration and so this defence is no
>>>> defence at all in that case.
>>>
>>>
>>> Don't allow open registration. :-)
>>>
>>> Further, I think it would be irresponsible to offer a generalized
>>> gateway for use by any domain, so the foregoing text might be misleading.
>>>
>>>> Perhaps some kind of rate limitation
>>>> would be useful?
>>>
>>>
>>> All self-respecting XMPP servers include rate limiting, but it's unclear
>>> whether that would really help much in practice since you don't any sort
>>> of amplification in order to attack users at another domain even in the
>>> absence of a gateway (just normal XMPP server-to-server will do).
>>>
>>>> Also, this part:
>>>>
>>>> "Furthermore, whenever an XMPP-SIP gateway seeks to refresh
>>>>      an XMPP user's long-lived subscription to a SIP user's presence, it
>>>>      MUST first send an XMPP <presence/> stanza of type "probe" from the
>>>>      address of the gateway to the "bare JID" ([e-mail]) of the
>>>>      XMPP user, to which the user's XMPP server MUST respond in accordance
>>>>      with [RFC6121]; however, the administrator of an XMPP-SIP gateway MAY
>>>>      (based on local service provisioning) exempt "known good" XMPP
>>>>      servers from this check (e.g., the XMPP server associated with the
>>>>      XMPP-SIP gateway as described above)."
>>>>
>>>> is unclear: how does it help?
>>>
>>>
>>> This check at least places a burden on the XMPP server itself and
>>> verifies if the XMPP user has an active presence session before updating
>>> the presence subscription on the SIP side of the gateway.
>>>
>>> I'll think about the matter more deeply and formulate some better text.
>>
>>
>> How is this?
>
> Better, but...
>
>>
>>     The mismatch between long-lived XMPP presence subscriptions and
>>     short-lived SIP presence subscriptions introduces the possibility of
>>     an amplification attack launched from the XMPP network against a SIP
>>     presence server (since each long-lived XMPP presence subscription
>>     would typically result in multiple subscription refresh requests on
>>     the SIP side of a gateway).  Therefore, access to an XMPP-SIP gateway
>>     SHOULD be restricted in various ways; among other things, only an
>>     XMPP service that carefully controls account provisioning ought to
>>     host such a gateway (e.g., not a service that offers open account
>>     registration) and a gateway ought to be associated only with a single
>>
>>     domain or trust realm (e.g., a gateway hosted at simple.example.com
>>     ought to allow only users within the example.com domain to access the
>>     gateway, not users within example.org, example.net, or any other
>>     domain).
>
> ...I suspect the criterion you are really trying to capture is that
> their should be some kind of administrative link between the XMPP
> server and its users (i.e. abusers can be permanently kicked off, if
> needed). Perhaps better to say that directly?

Yes, that's part of it but not all of it. For instance, I run the 
jabber.org IM service, which has 1.4 million users or so. The other 
admins and I definitely have the ability to permanently disable user 
accounts, we have user registration locked down, we have rate limiting 
in place, etc. -- but there's no way we'd ever offer a gateway of the 
kind described in this document because we have too many users to know 
them all or have any truly effective control over them. If I were 
running an XMPP service at a company (even a company with 100,000 
employees) I'd be comfortable offering a gateway, because the company 
could always fire someone who does something wrong.

I'll try again to rework the text to capture some of these ideas.

Peter

-- 
Peter Saint-Andre
https://stpeter.im/