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
Abstract
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
http://www.rfc-editor.org/info/rfc7375.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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
device:
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
failure.
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
numbers.
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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
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
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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
mechanism
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
security.
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6. Informative References
[OMTP-VV] Open Mobile Terminal Platform, "Visual Voice Mail
Interface Specification", Version 1.3, June 2010,
<http://www.gsma.com/newsroom/wp-content/uploads/2012/07/
OMTP_VVM_Specification_1_3.pdf>.
[Q764] ITU, "Signalling System No. 7 - ISDN User Part signalling
procedures", Recommendation ITU-T Q.764, December 1999,
<http://www.itu.int/rec/T-REC-Q.764/>.
[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,
<http://www.rfc-editor.org/info/rfc7340>.
[RTCWEB-OVERVIEW]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", Work in Progress,
draft-ietf-rtcweb-overview-12, October 2014.
[SIP-SECURITY]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", Work in Progress,
draft-peterson-sipping-retarget-00, February 2005.
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.
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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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