Re: [Ecrit] Benjamin Kaduk's Discuss on draft-ietf-ecrit-data-only-ea-21: (with DISCUSS and COMMENT)

[Brian Rosen <[e-mail]>]

It will be very common for the entity sending the message to not be the entity hosting the referenced document.  For example, a device may send the MESSAGE, but some service that the device’s mfg operates hosts the CAP.  So we make NO assumptions that there is any connection between them.  The same is true for the Additional Data from 7852. 

Proposed text (revises the text I previously suggested to Roman)
This document describes potential retrieval of information by dereferencing URIs found in a Call Info header of a SIP MESSAGE.  These may include a CAP message as well as other Additional Data (RFC7852) blocks.  The domain of the device sending the SIP MESSAGE, the domain of the server holding the CAP message, if sent by reference, and the domain of other Additional Data blocks, if sent by reference, may all be different.  No assumptions can be made that there are trust relationships between these entities.  Recipients MUST take precautions in retrieving any Additional Data blocks passed by reference, including the CAP message, because the URI may point to a malicious actor.  The considerations in handling URIs in RFC3986 apply.  


With respect to your DISCUSS on timestamps.  Can I add:
Use of timestamps to prevent replay is subject to the availability of accurate time at all participants.  Because emergency event notification via this mechanism is relatively low frequency and generally involves human interaction, implementations may wish to consider messages with times within small number of seconds of each other to be effectively simultaneous for the purposes of detecting replay.  Implementations may also wish to consider that most deployed time distribution protocols likely to be used by these systems are not presently secure.

With regard to DoS, the only thing that makes this different from any emergency call situation is that there is no two way media with a human, and so you immediately assume a bot rather than a human, and therefore probably more likely to be a DoS source.

OTOH, there isn’t anything we can do with that information.  If I can generate large numbers of malicious SIP transactions, whether they are INVITE or MESSAGE doesn’t matter.  If I can compromise a device, whether it’s a phone that generates INVITE or an IOT device that generates MESSAGE, then I can misuse it.  I’m at a loss to know what to say.  I can supply the advice that the emergency authorities have always offered: automated devices have increased probability to generate false positive emergency calls/messages.  This is a burden on the emergency services which has to respond to all calls/messages.  In some jurisdictions, there are legal restrictions on automated emergency calls/messages, and in others, there are penalties for repeated false positive calls/messages.

I’ll work on the comments and reply if I have any issues as I update in response to them. 

Brian


> On Mar 2, 2020, at 8:16 PM, Benjamin Kaduk via Datatracker <[e-mail]> wrote:
> 
> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-ecrit-data-only-ea-21: Discuss
> 
> When responding, please keep the subject line intact and reply to all
> email addresses included in the To and CC lines. (Feel free to cut this
> introductory paragraph, however.)
> 
> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
> 
> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-ecrit-data-only-ea/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> I support Roman's Discuss (and comment) and will expound on his first
> point some more.
> 
> The use of URIs to refer to remote content, potentially hosted on a
> third party, has numerous security considerations, some better known
> than others.  We need to reference the security considerations of RFC
> 3986 to get some coverage of these, such as the "reliability and
> consistency" point.
> 
> One of the more subtle security considerations of using a URI to a
> remote resource as part of a request in cases such as this, is that the
> binding between the identity of the entity referring to the remote URI
> and the identity of the site providing the corresponding resource (i.e.,
> the authority compoent of the URI) is not always clear.  Although RFC
> 7852 requires TLS mutual authentication and HTTPS transport for
> information provided by reference, it makes no statement requiring the
> TLS server certificate to correspond to the SIP initiator, or any other
> indication of authorization that the indicated resource makes sense as
> part of the indicated request.
> 
> I also have two other Discuss-level points:
> 
> We discuss the use of timestamps in protocol elements for detecting
> replay; this requires that all participants have (secure and) accurate
> time.  We need to document this dependency on accurate time, and
> optionally point out that common internet time protocols such as NTP are
> not particularly secure at present.
> 
> I'm also not sure where there's existing treatment of using SIP MESSAGEs
> as a DoS attack vector; that seems particularly pronounced in the
> emergency-services case since emergency messages may receive
> preferential treatment from the network.
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Introduction (and Abstract?)
> 
>   medical monitors.  These messages may be one-shot alerts to emergency
>   authorities and do not require establishment of a session.  These
>   type of interactions are called 'non-interactive emergency calls'.
> 
> Is this an existing/widespread definition or one new to this document?
> It may be worth providing a reference or rewording to clarify that it is
> a new definition.
> 
> Section 2
> 
>   CID is Content InDirection [RFC2392]
> 
> The string "InDirection" does not appear in RFC 2392.
> 
> Section 4.1
> 
>   A CAP message may be sent in the initial message of any SIP
>   transaction.  However, this document only addresses sending a CAP
>   message in a SIP MESSAGE transaction for a one-shot, non-interactive
>   emergency call.  Behavior with other transactions is not defined.
> 
> I suggest rephrasing to "A CAP message is sent in the initial message of
> a SIP transaction" to avoid potential concerns about inconsistency
> between "may be sent" and "not defined".
> 
>   the CAP message.  Alternatively, the Call-Info header field may
>   contain a Content Indirect url [RFC2392] and the CAP message included
> 
> nit: "Indirect" here vs. "InDirection" in Section 2
> 
> Section 4.2
> 
>      field.  If there is a need to copy the PIDF-LO structure
>      referenced by 'geolocation' to <area>, implementers must be aware
> 
> When might this occur?  (It seems almost out of scope for this document
> based on my limited understanding.)
> 
>      that <area> is limited to a circle or polygon, and conversion of
>      other shapes will be required.  Points SHOULD be converted to a
>      circle with a radius equal to the uncertainty of the point.  Arc-
>      bands and ellipses SHOULD be converted to an equivalent polygon.
>      3D locations SHOULD be converted to their equivalent 2D forms.
> 
> nit: what is an "equivalent" polygon to an ellipse or arc-band?  In a
> mathematical sense it might mean "strict equivalence" or a large-n limit
> of an n-gon, or perhaps there is a corresponding uncertainty on the
> ellipse/arc-band that the polygon must stay within?  Rewording to
> "corresponding" might be an easy fix.
> 
> Section 5.1
> 
>   The 425 response code is a rejection of the request due to its
>   included alert content, indicating that it was malformed or not
>   satisfactory for the recipient's purpose.
> 
>   A SIP intermediary can also reject an alert it receives from a User
>   Agent (UA) when it detects that the provided alert is malformed.
> 
> (I assume there's an implicit "reject with this code" in here.)
> 
>   It is only appropriate to generate a 425 response when the responding
>   entity has no other information in the request that is usable by the
>   responder.
> 
> I'm not sure I'm parsing this sentence correctly.  My best guess is that
> it's saying "if there's anything in the request that you can respond to,
> don't use 425, even if there's some malformed CAP", but that doesn't
> really seem to be what the actual words are saying.  [Section 5.2 seems
> to support my guess.]
> 
> Section 5.2
> 
>      message-header   /= AlertMsg-Error
>                              ; (message-header from 3261)
> 
> nit: mybe "RFC3261" to match the later comments?
> 
>   wrong with the alert in the request.  This error code has a
> 
> pedantic nit: the ABNF has 1-to-3-digit
> 
>   standardized -- meaning an operator can change the strings -- but
>   MUST NOT change the meaning of the error code.  Similar to how RFC
>   3261 specifies, there MUST NOT be more than one string per error
>   code.
> 
> RFC 3261 is ... pretty long.  Maybe throw us a bone with a section
> reference?  Also, I assume this "MUST NOT be more than one" is within a
> single AlertMsg-Error construction, since a truly global interpretation
> would mean that (since this document assigns strings) there cannot
> actually be changes made to the strings by an operator, which we just
> said is possible.
> 
>   This document defines an initial list of AlertMsg-Error values for
>   any SIP response, including provisional responses (other than 100
>   Trying) and the new 425 response.  There MUST be no more than one
>   AlertMsg-Error code in a SIP response.  AlertMsg-Error values sent in
> 
> Is this redundant with the "MUST NOT" I was asking about above?
> 
>   Additionally, if an entity cannot or chooses not to process the alert
>   message from a SIP request, a 500 (Server Internal Error) SHOULD be
>   used with or without a configurable Retry-After header field.
> 
> I don't think I understand when it's recommended to use 500 vs 425 (and
> vice versa).
> 
> Section 7
> 
> It's probably worth mentioning RFC 3428's note about:
> 
> %  The size of MESSAGE requests outside of a media session MUST NOT
> %  exceed 1300 bytes, unless the UAC has positive knowledge that the
> %  message will not traverse a congestion-unsafe link at any hop, or
> %  that the message size is at least 200 bytes less than the lowest MTU
> %  value found en route to the UAS
> 
> since that seems to still apply here.
> 
> Section 8
> 
> I'd consider using a more recent date than 2008-11-19 for the example
> <sent> time.  Also, why is the timestamp in the additional information
> 2010-11-14, almost two years later?
> 
>                     <gml:pos>32.86726 -97.16054</gml:pos>
> 
> Is that ... someone's house?  Maybe some different coordinates would be
> more appropriate.
> 
> Figure 3 has "Content-Disposition: by-reference;handling=optional" but
> Figure 4 does not; is that significant?
> 
> Section 9
> 
>   In any case, for alerts initiated by sensors, the identity could play
>   an important role in deciding whether to accept or ignore an incoming
>   alert message.  With the scenario shown in Figure 1 it is very likely
>   that only authenticated sensor input will be processed.  For this
>   reason, it needs to be possible to refuse to accept alert messages
>   from an unknown origin.  Two types of information elements can be
>   used for this purpose:
> 
>   1.  SIP itself provides security mechanisms that allow the
>       verification of the originator's identity, such as P-Asserted-
>       Identity [RFC3325] or SIP Identity [RFC8224].  The latter
>       provides a cryptographic assurance while the former relies on a
>       chain of trust model.  These mechanisms can be reused.
> 
>   2.  CAP provides additional security mechanisms and the ability to
>       carry further information about the sender's identity.
>       Section 3.3.4.1 of [cap] specifies the signing algorithms of CAP
>       documents.
> 
> I'd consider adding a note that "the specific policy and mechanisms used
> in a given deployment are out of scope for this document", since the
> actual requirements in practice are so broad that we cannot really make
> detailed recommendations here.
> 
>   In addition to the desire to perform identity-based access control,
>   the classic communication security threats need to be considered,
>   including integrity protection to prevent forgery or replay of alert
>   messages in transit.  To deal with replay of alerts, a CAP document
>   contains the mandatory <identifier>, <sender>, <sent> elements and an
>   optional <expire> element.  Together, these elements make the CAP
>   document unique for a specific sender and provide time restrictions.
>   An entity that has already received a CAP message within the
>   indicated timeframe is able to detect a replayed message and, if the
>   content of that message is unchanged, then no additional security
> 
> Is *able* to detect, sure.  But is it recommended to do so?  Required?
> 
>   vulnerability is created.  Additionally, it is RECOMMENDED to make
>   use of SIP security mechanisms, such as the SIP Identity PASSporT
>   [RFC8225], to tie the CAP message to the SIP message.  To provide
> 
> Not required?  Please describe the risks of not doing so (whether
> directly or by reference).
> 
>   protection of the entire SIP message exchange between neighboring SIP
>   entities, the usage of TLS is REQUIRED.
> 
> Just to check: this is a 100% always, for everyone, "REQUIRED", not just
> in the subset of cases where protection of the entire exchange is
> desired?
> 
>   Note that none of the security mechanism in this document protect
>   against a compromised sensor sending crafted alerts.  Privacy
>   provided for any emergency calls, including non-interactive messages,
>   is subject to local regulations.
> 
> I suggest using "confidentiality protection" rather than "privacy"
> (unless it's a term of art in this field), to avoid confusion with...
> Privacy considerations for alerts (e.g., generated by sensors) that
> should be discussed.  RFC 7852 has some extensive privacy considerations
> that can be referenced to do the bulk of the work here.
> 
> I'd also consider reiterating the opportunistic nature of cryptographic
> protection on emergency messages, as discussed in 6443 (since insecure
> emergency messaging is preferred to no emergency messaging), but this is
> not strictly speaking required.
> 
> Section 12.2
> 
> RFC 8225, as a RECOMMENDED feature, is more properly listed as a
> normative reference, per
> https://www.ietf.org/about/groups/iesg/statements/normative-informative-references/
> 
> 
>