Network Working Group                                        J. Peterson
Internet-Draft                                             NeuStar, Inc.
Intended status: Informational                          February 5, 2014
Expires: August 9, 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

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   This Internet-Draft will expire on August 9, 2014.

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   include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction and Scope

   As is discussed in the STIR problem statement [2], the primary
   enabler of robocalling, vishing, swatting and related attacks is the
   capability to impersonate a calling party number.  The starkest
   example 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 caller identity, callees are less
   likely to pick up calls from blocked identities, and therefore
   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 specific 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

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   attacker from creating calls 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 set-up time
   does not entail anything 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 an
   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, such 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,
   impersonating a reached number to the originator of a call is outside
   the scope of this threat model.

   In much of the PSTN, there exists a supplemental service that
   translates calling party numbers into regular names, including the
   proper names of people and businesses, for rendering to the called
   user.  These services (frequently termed '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 'Caller
   ID' name to be rendered to the user as well.  Providing a verifiable
   calling party number therefore improves the security of Caller ID
   systems, but this threat model does not consider attacks specific to
   Caller ID.  Such attacks may be carried out against the databases
   consulted by the terminating side of a call to provide Caller ID, or
   by impersonators forging a particular calling party number in order
   to present a misleading Caller ID to the user.

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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, enough for a telephone number and
      an accompanying name, sometimes less).  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, and so on.

   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 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,' to automatically transcribe voicemail
   to email, and so on).

2.2.  Intermediaries

   The endpoints of a traditional telephone call connect through
   numerous intermediary switches 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 may also appear on a call path.

      Intermediaries forward call signaling to the next entity 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

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      signaling that an intermediary can introduce (which is consistent
      with the observed behavior of such devices).

      Gateways translate 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 an
   common condition in call routing.  This 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 [3]).

   Furthermore, we assume that some intermediaries or gateways may, due
   to their capabilities or policies, discard calling party number
   information, in whole or 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.  A call might also gateway to a multifrequency network
   where only a limited number of digits of automatic numbering
   identification (ANI) data are signaled, for example.  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 identity 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.

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

   There are attack scenarios in which an attacker compromises
   intermediaries in the call path, or captures credentials that allow

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   the attacker to impersonate a target.  Those system-level attacks are
   not considered in this threat model, though secure design and
   operation of systems to prevent these sorts of attacks is 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 using their own
   credentials.  The design of the credential system will however limit
   the scope of the credentials issued to carriers or national
   authorities to those numbers that fall under their purview.

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 their 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 allows 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 below.

   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 would also be analogous to an attack where 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 identity
   differently from other calls.  In the absence of identity, for
   example, a voicemail service might enforce some other caller
   authentication policy (perhaps requiring a PIN for caller
   authentication).  Authenticated identity alone provides a positive
   confirmation only when an identity is claimed legitimately; the

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   absence of authenticated identity here may not be evidence of malice,
   just of uncertainty.

   If the voicemail service could learn ahead of time that it should
   expect authenticated identity from a particular number, that would
   enable the voicemail service to adopt stricter policies for handling
   a request without authenticated identity.  Since users typically
   contact a voicemail service repeatedly, the service could for example
   remember which users usually sign their requests and require further
   authentication mechanisms when signatures are absent.  Alternatively,
   issuers of credentials or other authorities could provide a service
   that informs verifiers that they should expect identity signatures 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, even
   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 identity.  If there were a service that
   could inform the terminating side of that it should expect an
   identity signature in calls or texts from that number, however, that
   would also help in the robocalling case.

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   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 their calls or messages, or to force
   robocallers to use authenticated identity for 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
   constant telephone calls, an attacker renders a target number
   unreachable by legitimate callers.  These attaacks might target a
   business, an individual or a public resource like emergency
   responders; the attack may intend to extort the target or have other
   motivations.  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
   authenticated identity 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

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   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 identity 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 an authenticated identity signature, or
   even the presence of an invalid signature, in many cases will not
   remove that obligation.

4.  Attack Scenarios

   The examples that follow rely on Internet protocols including SIP [1]
   and WebRTC.

   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 call setup message (the CPN
   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 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 calling
   party number (CPN) 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-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, IP-PSTN-IP

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   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 CPN 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 puts takes the
   calling party number in the IAM CPN 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.
   Some of the attacks that should be considered in the future include
   the following:

   Attacks Against In-band

      Token replay

      Removal of in-band signaling features

   Attacks Against Out-of-Band

      Provisioning Garbage CPRs

      Data Mining

   Attacks Against Either Approach

      Attack on directories/services that say whether you should expect
      authenticated identity or not

      Canonicalization attacks

5.  Acknowledgments

   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

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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

   [1]        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.

   [2]        Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement", draft-ietf-stir-
              problem-statement-01 (work in progress), December 2013.

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

Author's Address

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


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