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Secure Telephone Identity Threat Model

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7375.
Author Jon Peterson
Last updated 2014-07-10 (Latest revision 2014-06-11)
Replaces draft-peterson-stir-threats
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Robert Sparks
Shepherd write-up Show Last changed 2014-06-13
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Responsible AD Richard Barnes
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Network Working Group                                        J. Peterson
Internet-Draft                                             NeuStar, Inc.
Intended status: Informational                             June 11, 2014
Expires: December 13, 2014

                 Secure Telephone Identity Threat Model


   As the Internet and the telephone network have become increasingly
   interconnected and interdependent, attackers can impersonate or
   obscure calling party numbers when orchestrating bulk commercial
   calling schemes, hacking voicemail boxes or even circumventing multi-
   factor authentication systems trusted by banks.  This document
   analyzes threats in the resulting system, enumerating actors,
   reviewing the capabilities available to and used by attackers, and
   describing scenarios in which attacks are launched.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 13, 2014.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and Scope  . . . . . . . . . . . . . . . . . . .   2
   2.  Actors  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Endpoints . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Intermediaries  . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Attackers . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Voicemail Hacking via Impersonation . . . . . . . . . . .   6
     3.2.  Unsolicited Commercial Calling from Impersonated Numbers    7
     3.3.  Telephony Denial-of-Service Attacks . . . . . . . . . . .   8
   4.  Attack Scenarios  . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Solution-Specific Attacks . . . . . . . . . . . . . . . .  10
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction and Scope

   As is discussed in the STIR problem statement
   [I-D.ietf-stir-problem-statement], the primary enabler of
   robocalling, vishing, swatting and related attacks is the capability
   to impersonate a calling party number.  The starkest examples of
   these attacks are cases where automated callees on the PSTN rely on
   the calling number as a security measure, for example to access a
   voicemail system.  Robocallers use impersonation as a means of
   obscuring identity; while robocallers can, in the ordinary PSTN,
   block (that is, withhold) their calling number from presentation,
   callees are less likely to pick up calls from blocked identities, and
   therefore appearing to calling from some number, any number, is
   preferable.  Robocallers however prefer not to call from a number
   that can trace back to the robocaller, and therefore they impersonate
   numbers that are not assigned to them.

   The scope of impersonation in this threat model pertains solely to
   the rendering of a calling telephone number to a callee (human user
   or automaton) at the time of call set-up.  The primary attack vector
   is therefore one where the attacker contrives for the calling
   telephone number in signaling to be a chosen number.  In this attack,
   the number is one that the attacker is not authorized to use (as a
   caller), but gives in order for that number to be consumed or
   rendered on the terminating side.  The threat model assumes that this

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   attack simply cannot be prevented: there is no way to stop the
   attacker from creating call setup messages that contain attacker-
   chosen calling telephone numbers.  The solution space therefore
   focuses on ways that terminating or intermediary elements might
   differentiate authorized from unauthorized calling party numbers, in
   order that policies, human or automatic, might act on that

   Securing an authenticated calling party number at call set-up time
   does not entail any assertions about the entity or entities that will
   send and receive media during the call itself.  In call paths with
   intermediaries and gateways (as described below), there may be no way
   to provide any assurance in the signaling about participants in the
   media of a call.  In those end-to-end IP environments where such
   assurance is possible, it is highly desirable.  However, in the
   threat model described in this document, "impersonation" does not
   consider impersonating an authorized listener after a call has been
   established (e.g., as a third party attempting to eavesdrop on a
   conversation).  Attackers that could impersonate an authorized
   listener require capabilities that robocallers and voicemail hackers
   are unlikely to possess, and historically such attacks have not
   played a role in enabling robocalling or related problems.

   In SIP and even many traditional telephone protocols, call signaling
   can be renegotiated after the call has been established.  Using
   various transfer mechanisms common in telephone systems, a callee can
   easily be connected to, or conferenced in with, telephone numbers
   other than the original calling number once a call has been
   established.  These post-setup changes to the call are outside the
   scope of impersonation considered in this model.  Furthermore, this
   threat model does not include in its scope the verification of the
   reached party's telephone number back to the originator of the call.
   There is no assurance to the originator that they are reaching the
   correct number, nor any indication when call forwarding has taken
   place.  This threat model is focused only on verifying the calling
   party number to the callee.

   In much of the PSTN, there exists a supplemental service that
   translates calling party numbers into names, including the proper
   names of people and businesses, for rendering to the called user.
   These services (frequently marketed as part of 'Caller ID') provide a
   further attack surface for impersonation.  The threat model described
   in this document addresses only the calling party number, even though
   presenting a forged calling party number may cause a chosen calling
   party name to be rendered to the user as well.  Providing a
   verifiable calling party number therefore improves the security of
   calling party name systems, but this threat model does not consider
   attacks specific to names.  Such attacks may be carried out against

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   the databases consulted by the terminating side of a call to provide
   calling party names, or by impersonators forging a particular calling
   party number in order to present a misleading name to the user.

2.  Actors

2.1.  Endpoints

   There are two main categories of end-user terminals relevant to this
   discussion, a dumb device (such as a 'black phone') or a smart

      Dumb devices comprise a simple dial pad, handset and ringer,
      optionally accompanied by a display that can render a limited
      number of characters.  Typically the display renders enough
      characters for a telephone number and an accompanying name, but
      sometimes fewer are rendered.  Although users interface with these
      devices, the intelligence that drives them lives in the service
      provider network.

      Smart devices are general purpose computers with some degree of
      programmability, and with the capacity to access the Internet and
      to render text, audio and/or images.  This category includes smart
      phones, telephone applications on desktop and laptop computers, IP
      private branch exchanges, etc.

   There is a further category of automated terminals without an end
   user.  These include systems like voicemail services, which may
   provide a different set of services to a caller based solely on the
   calling party's number, for example granting the (purported) mailbox
   owner access to a menu while giving other callers only the ability to
   leave a message.  Though the capability of voicemail services varies
   widely, many today have Internet access and advanced application
   interfaces (to render 'visual voicemail,' [refs.OMTP-VV] to
   automatically transcribe voicemail to email, etc.).

2.2.  Intermediaries

   The endpoints of a traditional telephone call connect through
   numerous intermediary devices in the network.  The set of
   intermediary devices traversed during call setup between two
   endpoints is referred to as a call path.  The length of the call path
   can vary considerably: it is possible in VoIP deployments for two
   endpoint entities to send traffic to one another directly, but, more
   commonly, several intermediaries exist in a VoIP call path.  One or
   more gateways also may appear on a call path.

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      Intermediaries forward call signaling to the next device in the
      path.  These intermediaries may also modify the signaling in order
      to improve interoperability, to enable proper network-layer media
      connections, or to enforce operator policy.  This threat model
      assumes there are no restrictions on the modifications to
      signaling that an intermediary can introduce (which is consistent
      with the observed behavior of such devices).

      A gateway is a subtype of intermediary that translates call
      signaling from one protocol into another.  In the process, they
      tend to consume any signaling specific of the original protocol
      (elements like transaction-matching identifiers) and may need to
      transcode or otherwise alter identifiers as they are rendered in
      the destination protocol.

   This threat model assumes that intermediaries and gateways can
   forward and retarget calls as necessary, which can result in a call
   terminating at a place the originator did not expect; this is a
   common condition in call routing.  This observation is significant to
   the solution space, because it limits the ability of the originator
   to anticipate what the telephone number of the respondent will be
   (for more on the "unanticipated respondent" problem, see

   Furthermore, we assume that some intermediaries or gateways may, due
   to their capabilities or policies, discard calling party number
   information, in whole or in part.  Today, many IP-PSTN gateways
   simply ignore any information available about the caller in the IP
   leg of the call, and allow the telephone number of the PRI line used
   by the gateway to be sent as the calling party number for the PSTN
   leg of the call.  For example, a call might also gateway to a multi-
   frequency network where only a limited number of digits of automatic
   numbering identification (ANI) data are signaled.  Some protocols may
   render telephone numbers in a way that makes it impossible for a
   terminating side to parse or canonicalize a number.  In these cases,
   providing authenticated calling number data may be impossible, but
   this is not indicative of an attack or other security failure.

2.3.  Attackers

   We assume that an attacker has the following capabilities:

      An attacker can create telephone calls at will, originating them
      either on the PSTN or over IP, and can supply an arbitrary calling
      party number.

      An attacker can capture and replay signaling previously observed
      by it.

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      An attacker has access to the Internet, and thus the ability to
      inject arbitrary traffic over the Internet, to access public
      directories, etc.

   There are attack scenarios in which an attacker compromises
   intermediaries in the call path, or captures credentials that allow
   the attacker to impersonate a caller.  Those system-level attacks are
   not considered in this threat model, though secure design and
   operation of systems to prevent these sorts of attacks are necessary
   for envisioned countermeasures to work.

   This threat model also does not consider scenarios in which the
   operators of intermediaries or gateways are themselves adversaries
   who intentionally discard valid identity information (without a user
   requesting anonymity) or who send falsified identity; see
   Section 4.1.

3.  Attacks

   The uses of impersonation described in this section are broadly
   divided into two categories: those where an attacker impersonates an
   arbitrary identity in order to disguise its own, and those where an
   attack will not succeed unless the attacker impersonates a specific
   identity.  At a high level, impersonation encourages targets to
   answer attackers' calls and makes identifying attackers more
   difficult.  This section shows how concrete attacks based on those
   different techniques might be launched.

3.1.  Voicemail Hacking via Impersonation

   A voicemail service may allow users calling from their phones access
   to their voicemail boxes on the basis of the calling party number.
   If an attacker wants to access the voicemail of a particular target,
   the attacker may try to impersonate the calling party number using
   one of the scenarios described in Section 4.

   This attack is closely related to attacks on similar automated
   systems, potentially including banks, airlines, calling-card
   services, conferencing providers, ISPs, and other businesses that
   fully or partly grant access to resources on the basis of the calling
   party number.  It is analogous to an attack in which a human is
   encouraged to answer a phone, or to divulge information once a call
   is in progress, by seeing a familiar calling party number.

   The envisioned countermeasures for this attack involve the voicemail
   system treating calls that supply an authenticated calling number
   data differently from other calls.  In the absence of that identity
   information, for example, a voicemail service might enforce some

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   other caller authentication policy (perhaps requiring a PIN for
   caller authentication).  Asserted caller identity alone provides an
   authenticated basis for granting access to a voice mailbox only when
   an identity is claimed legitimately; the absence of calling identity
   here may not be evidence of malice, just of uncertainty or a
   limitation imposed by the set of intermediaries traversed for a
   specific call path.

   If the voicemail service could learn ahead of time that it should
   expect authenticated calling number data from a particular number,
   that would enable the voicemail service to adopt stricter policies
   for handling a request without authentication data.  Since users
   typically contact a voicemail service repeatedly, the service could
   for example remember which requests contain authenticated calling
   number data and require further authentication mechanisms when
   identity is absent.  Alternatively, issuers of credentials or other
   authorities could provide a service that informs verifiers that they
   should expect identity in calls from particular numbers.

3.2.  Unsolicited Commercial Calling from Impersonated Numbers

   The unsolicited commercial calling, or for short robocalling, attack
   is similar to the voicemail attack, except that the robocaller does
   not need to impersonate the particular number controlled by the
   target, merely some "plausible" number.  A robocaller may impersonate
   a number that is not an assignable number (for example, in the United
   States, a number beginning with 0), or an unassigned number.  A
   robocaller may change numbers every time a new call is placed, e.g.,
   selecting numbers randomly.

   A closely related attack is sending unsolicited bulk commercial
   messages via text messaging services.  These messages usually
   originate on the Internet, though they may ultimately reach endpoints
   over traditional telephone network protocols or the Internet.  While
   most text messaging endpoints are mobile phones, increasingly,
   broadband residential services support text messaging as well.  The
   originators of these messages typically impersonate a calling party
   number, in some cases a "short code" specific to text messaging

   The envisioned countermeasures to robocalling are similar to those in
   the voicemail example, but there are significant differences.  One
   important potential countermeasure is simply to verify that the
   calling party number is in fact assignable and assigned.  Unlike
   voicemail services, end users typically have never been contacted by
   the number used by a robocaller before.  Thus they can't rely on past
   association to anticipate whether or not the calling party number
   should supply authenticated calling number data.  If there were a

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   service that could inform the terminating side that it should expect
   this data for calls or texts from that number, however, that would
   also help in the robocalling case.

   When a human callee is to be alerted at call setup time, the time
   frame for executing any countermeasures is necessarily limited.
   Ideally, a user would not be alerted that a call has been received
   until any necessary identity checks have been performed.  This could
   however result in inordinate post-dial delay from the perspective of
   legitimate callers.  Cryptographic and network operations must be
   minimized for these countermeasures to be practical.  For text
   messages, a delay for executing anti-impersonation countermeasures is
   much less likely to degrade perceptible service.

   The eventual effect of these countermeasures would be to force
   robocallers to either block their caller identity, in which case end
   users could opt not to receive such calls or messages, or to force
   robocallers to use authenticated calling numbers traceable to them,
   which would then allow for other forms of redress.

3.3.  Telephony Denial-of-Service Attacks

   In the case of telephony denial-of-service (or TDoS) attacks, the
   attack relies on impersonation in order to obscure the origin of an
   attack that is intended to tie up telephone resources.  By placing
   incessant telephone calls, an attacker renders a target number
   unreachable by legitimate callers.  These attacks might target a
   business, an individual or a public resource like emergency
   responders; the attacker may intend to extort the target.  Attack
   calls may be placed from a single endpoint, or from multiple
   endpoints under the control of the attacker, and the attacker may
   control endpoints in different administrative domains.  Impersonation
   in this case allows the attack to evade policies that would block
   based on the originating number, and furthermore prevents the victim
   from learning the perpetrator of the attack, or even the originating
   service provider of the attacker.

   As is the case with robocalling, the attacker typically does not have
   to impersonate a specific number in order to launch a denial-of-
   service attack.  The number simply has to vary enough to prevent
   simple policies from blocking the attack calls.  An attacker may
   however have a further intention to create the appearance that a
   particular party is to blame for an attack, and in that case, the
   attacker might want to impersonate a secondary target in the attack.

   The envisioned countermeasures are twofold.  First, as with
   robocalling, ensuring that calling party numbers are assignable or
   assigned will help mitigate unsophisticated attacks.  Second, if

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   authenticated calling number data is supplied for legitimate calls,
   then Internet endpoints or intermediaries can make effective policy
   decisions in the middle of an attack by deprioritizing unsigned calls
   when congestion conditions exist; signed calls, if accepted, have the
   necessary accountability should it turn out they are malicious.  This
   could extend to include, for example, an originating network
   observing a congestion condition for a destination number and perhaps
   dropping unsigned calls that are clearly part of a TDoS attack.  As
   with robocalling, all of these countermeasures must execute in a
   timely manner to be effective.

   There are certain flavors of TDoS attacks, including those against
   emergency responders, against which authenticated calling number data
   is unlikely to be a successful countermeasure.  These entities are
   effectively obligated to attempt to respond to every call they
   receive, and the absence of authenticated calling number data in many
   cases will not remove that obligation.

4.  Attack Scenarios

   The examples that follow rely on Internet protocols including SIP
   [RFC3261] and WebRTC [I-D.ietf-rtcweb-overview].

   Impersonation, IP-IP

   An attacker with an IP phone sends a SIP request to an IP-enabled
   voicemail service.  The attacker puts a chosen calling party number
   into the From header field value of the INVITE.  When the INVITE
   reaches the endpoint terminal, the terminal renders the attacker's
   chosen calling party number as the calling identity.

   Impersonation, PSTN-PSTN

   An attacker with a traditional PBX (connected to the PSTN through
   ISDN) sends a Q.931 SETUP request with a chosen calling party number
   which a service provider inserts into the corresponding SS7
   [refs.Q764] calling party number (CgPN) field of a call setup message
   (IAM).  When the call setup message reaches the endpoint switch, the
   terminal renders the attacker's chosen calling party number as the
   calling identity.

   Impersonation, IP-PSTN

   An attacker on the Internet uses a commercial WebRTC service to send
   a call to the PSTN with a chosen calling party number.  The service
   contacts an Internet-to-PSTN gateway, which inserts the attacker's
   chosen calling party number into the SS7 [refs.Q764] call setup
   message (the CgPN field of an IAM).  When the call setup message

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   reaches the terminating telephone switch, the terminal renders the
   attacker's chosen calling party number as the calling identity.

   Impersonation, IP-PSTN-IP

   An attacker with an IP phone sends a SIP request to the telephone
   number of a voicemail service, perhaps without even knowing that the
   voicemail service is IP-based.  The attacker puts a chosen calling
   party number into the From header field value of the INVITE.  The
   attacker's INVITE reaches an Internet-to-PSTN gateway, which inserts
   the attacker's chosen calling party number into the CgPN of an IAM.
   That IAM then traverses the PSTN until (perhaps after a call
   forwarding) it reaches another gateway, this time back to the IP
   realm, to an H.323 network.  The PSTN-IP gateway takes the calling
   party number in the IAM CgPN field and puts it into the SETUP
   request.  When the SETUP reaches the endpoint terminal, the terminal
   renders the attacker's chosen calling party number as the calling

4.1.  Solution-Specific Attacks

   Solution-specific attacks are outside the scope of this document.
   There are a few points which future work on solution-specific threats
   must acknowledge.  The design of the credential system envisioned as
   a solution to this threats must for example limit the scope of the
   credentials issued to carriers or national authorities to those
   numbers that fall under their purview.  This will impose limits on
   what (verifiable) assertions can be made by intermediaries.

   Some of the attacks that should be considered in the future include
   the following:

   Attacks Against In-band

      Token replay

      Baiting a call to a chosen number with a REFER

      Removal of in-band signaling features

   Attacks Against Out-of-Band

      Provisioning Garbage CPRs

      Data Mining

   Attacks Against Either Approach

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      Attack on directories/services that say whether you should expect
      authenticated calling number data or not

      Canonicalization attacks

5.  Acknowledgments

   Sanjay Mishra, David Frankel, Penn Pfautz, Stephen Kent, Brian Rosen,
   Alex Bobotek, Henning Schulzrinne, Hannes Tschofenig, Cullen Jennings
   and Eric Rescorla provided key input to the discussions leading to
   this document.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   This document provides a threat model and is thus entirely about

8.  Informative References

              Alvestrand, H., "Overview: Real Time Protocols for Brower-
              based Applications", draft-ietf-rtcweb-overview-09 (work
              in progress), February 2014.

              Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              draft-ietf-stir-problem-statement-05 (work in progress),
              May 2014.

              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements", draft-peterson-sipping-
              retarget-00 (work in progress), February 2005.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

              OMTP, , "Visual Voice Mail Interface Specification", URL:
              OMTP_VVM_Specification_1_3.pdf, May 1998.

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              ITU-T, , "Signaling System No. 7; ISDN User Part Signaling
              procedure", ITU-T URL:
    , September

              ITU-T, , "ISDN user-network interface layer 3
              specification for basic call control", ITU-T URL:
    , May 1998.

Author's Address

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520


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