1. RFC 7375
Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 7375                                 NeuStar, Inc.
Category: Informational                                     October 2014
ISSN: 2070-1721

                 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction and Scope ..........................................2
   2. Actors ..........................................................4
      2.1. Endpoints ..................................................4
      2.2. Intermediaries .............................................5
      2.3. Attackers ..................................................6
   3. Attacks .........................................................6
      3.1. Voicemail Hacking via Impersonation ........................7
      3.2. Unsolicited Commercial Calling from Impersonated Numbers ...8
      3.3. Telephony Denial-of-Service Attacks ........................9
   4. Attack Scenarios ...............................................10
      4.1. Solution-Specific Attacks .................................11
   5. Security Considerations ........................................11
   6. Informative References .........................................12
   Acknowledgments ...................................................12
   Author's Address ..................................................13

1.  Introduction and Scope

   As is discussed in the STIR problem statement [RFC7340] (where "STIR"
   refers to the Secure Telephone Identity Revisited working group), 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 Public Switched Telephone Network (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; therefore,
   appearing to call from some number, any number, is preferable.

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   However, robocallers prefer not to call from a number that can trace
   back to the them, so 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 setup.  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 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 information.

   Securing an authenticated calling party number at call setup 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: the motivating use
   cases of defeating robocalling, voicemail hacking, and swatting all
   rely on impersonation during the initial call setup.  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

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   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
   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

   o  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.

   o  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

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   widely, many today have Internet access and advanced application
   interfaces (to render 'visual voicemail' [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 Voice over IP (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.

   o  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).

   o  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 to 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 [SIP-SECURITY]).

   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 Primary Rate Interface
   (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

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   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

2.3.  Attackers

   We assume that an attacker has the following capabilities:

   o  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.

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

   o  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.  To date, robocallers and
   other impersonators do not resort to compromising systems but rather
   exploit the intrinsic lack of secure identity in existing mechanisms;
   remedying this problem lies within the scope of this threat model.

   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 attack will not succeed
   unless the attacker impersonates a specific identity and those where
   an attacker impersonates an arbitrary identity in order to disguise
   its own.  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.

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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 alone (rather than any shared secret or further identity
   check).  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 authenticated calling number data
   differently from other calls.  In the absence of that identity
   information, for example, a voicemail service might enforce some
   other caller authentication policy (perhaps requiring a PIN for
   caller authentication).  Asserted caller identity alone provides an
   authenticated basis for granting access to a voicemail box only when
   an identity is claimed legitimately; the absence of a verifiably
   legitimate 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.  The deployment of such a feature would be
   facilitated in many environments by the fact that the voicemail
   service is often operated by an organization that would be in a
   position to enable or require authentication of calling party
   identity (for example, carriers or enterprises).  Even if the
   voicemail service is decoupled from the number assignee, issuers of
   credentials or other authorities could provide a service that informs
   verifiers that they should expect identity in calls from particular

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3.2.  Unsolicited Commercial Calling from Impersonated Numbers

   The unsolicited commercial calling, or 'robocalling' for short,
   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.  This
   behavior is seen in the wild today.  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, broadband
   residential services are increasingly supporting 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 services.

   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 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 (a) block their caller identity, in which case
   end users could opt not to receive such calls or messages, or (b) use
   authenticated calling numbers traceable to them, which would then
   allow for other forms of redress.

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3.3.  Telephony Denial-of-Service Attacks

   In the case of telephony denial-of-service (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; 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
   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.

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4.  Attack Scenarios

   The examples that follow rely on Internet protocols including SIP
   [RFC3261] and WebRTC [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 Private Branch Exchange (PBX)
      (connected to the PSTN through ISDN) sends a Q.931 SETUP request
      [Q931] with a chosen calling party number, which a service
      provider inserts into the corresponding SS7 [Q764] calling party
      number (CgPN) field of a call setup message (Initial Address
      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 [Q764] call
      setup message (the CgPN field of an IAM).  When the call setup
      message reaches the terminating telephone switch, the terminal
      renders the attacker's chosen calling party number as the calling

   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

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      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 identity.

4.1.  Solution-Specific Attacks

   Solution-specific attacks are outside the scope of this document,
   though two sorts of solutions are anticipated by the STIR problem
   statement: in-band and out-of-band solutions (see [RFC7340]).  There
   are a few points that future work on solution-specific threats must
   acknowledge.  The design of the credential system envisioned as a
   solution to these 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:

   o  Attacks against in-band solutions

      *  Replaying parts of messages used by the solution

      *  Using a SIP REFER request to induce a party with access to
         credentials to place a call to a chosen number

      *  Removing parts of messages used by the solution

   o  Attacks against out-of-band solutions

      *  Provisioning false or malformed data reflecting a placed call
         into any datastores that are part of the out-of-band mechanism

      *  Mining any datastores that are part of the out-of-band

   o  Attacks against either approach

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

      *  Canonicalization attacks

5.  Security Considerations

   This document provides a threat model and is thus entirely about

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6.  Informative References

   [OMTP-VV]  Open Mobile Terminal Platform, "Visual Voice Mail
              Interface Specification", Version 1.3, June 2010,

   [Q764]     ITU, "Signalling System No. 7 - ISDN User Part signalling
              procedures", Recommendation ITU-T Q.764, December 1999,

   [Q931]     ITU, "ISDN user-network interface layer 3 specification
              for basic call control", Recommendation ITU-T Q.931,
              May 1998, <http://www.itu.int/rec/T-REC-Q.931/>.

   [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, <http://www.rfc-editor.org/rfc/rfc3261.txt>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, September 2014,

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", Work in Progress,
              draft-ietf-rtcweb-overview-12, October 2014.

              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements", Work in Progress,
              draft-peterson-sipping-retarget-00, February 2005.


   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.

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Author's Address

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

   EMail: jon.peterson@neustar.biz

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