Internet DRAFT - draft-ietf-stir-threats
draft-ietf-stir-threats
Network Working Group J. Peterson
Internet-Draft NeuStar, Inc.
Intended status: Informational August 12, 2014
Expires: February 13, 2015
Secure Telephone Identity Threat Model
draft-ietf-stir-threats-04.txt
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 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 February 13, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Peterson Expires February 13, 2015 [Page 1]
Internet-Draft STIR Threats August 2014
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
Peterson Expires February 13, 2015 [Page 2]
Internet-Draft STIR Threats August 2014
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 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: 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
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
Peterson Expires February 13, 2015 [Page 3]
Internet-Draft STIR Threats August 2014
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.
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.
Peterson Expires February 13, 2015 [Page 4]
Internet-Draft STIR Threats August 2014
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
[I-D.peterson-sipping-retarget]).
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.
Peterson Expires February 13, 2015 [Page 5]
Internet-Draft STIR Threats August 2014
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:
it is remedying this problem that 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.
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.
Peterson Expires February 13, 2015 [Page 6]
Internet-Draft STIR Threats August 2014
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
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 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.
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. 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, increasingly,
broadband residential services support text messaging as well. The
originators of these messages typically impersonate a calling party
Peterson Expires February 13, 2015 [Page 7]
Internet-Draft STIR Threats August 2014
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 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.
Peterson Expires February 13, 2015 [Page 8]
Internet-Draft STIR Threats August 2014
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 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
Peterson Expires February 13, 2015 [Page 9]
Internet-Draft STIR Threats August 2014
(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
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
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
[I-D.ietf-stir-problem-statement]). 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 Solutions
Peterson Expires February 13, 2015 [Page 10]
Internet-Draft STIR Threats August 2014
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
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 mechansim
Mining any datastores that are part of the out-of-band mechanism
Attacks Against Either Approach
Attack on any directories/services that report 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
security.
8. Informative References
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-10
(work in progress), June 2014.
Peterson Expires February 13, 2015 [Page 11]
Internet-Draft STIR Threats August 2014
[I-D.ietf-stir-problem-statement]
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.
[I-D.peterson-sipping-retarget]
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.
[refs.OMTP-VV]
OMTP, , "Visual Voice Mail Interface Specification", URL:
http://www.gsma.com/newsroom/wp-content/uploads/2012/07/
OMTP_VVM_Specification_1_3.pdf, May 1998.
[refs.Q764]
ITU-T, , "Signaling System No. 7; ISDN User Part Signaling
procedure", ITU-T URL:
http://www.itu.int/rec/T-REC-Q.764/_page.print, September
1997.
[refs.Q931]
ITU-T, , "ISDN user-network interface layer 3
specification for basic call control", ITU-T URL:
http://www.itu.int/rec/T-REC-Q.931-199805-I/en, May 1998.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
Peterson Expires February 13, 2015 [Page 12]
