Secure Telephone Identity Threat Model
draft-ietf-stir-threats-01
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| Document | Type | Active Internet-Draft (stir WG) | |
|---|---|---|---|
| Author | Jon Peterson | ||
| Last updated | 2014-02-04 | ||
| Replaces | draft-peterson-stir-threats | ||
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draft-ietf-stir-threats-01
Network Working Group J. Peterson
Internet-Draft NeuStar, Inc.
Intended status: Informational February 5, 2014
Expires: August 9, 2014
Secure Telephone Identity Threat Model
draft-ietf-stir-threats-01.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
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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 August 9, 2014.
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
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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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Scope . . . . . . . . . . . . . . . . . . . 2
2. Actors . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Endpoints . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Intermediaries . . . . . . . . . . . . . . . . . . . . . 4
2.3. Attackers . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Voicemail Hacking via Impersonation . . . . . . . . . . . 6
3.2. Unsolicited Commercial Calling from Impersonated Numbers 7
3.3. Telephony Denial-of-Service Attacks . . . . . . . . . . . 8
4. Attack Scenarios . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Solution-Specific Attacks . . . . . . . . . . . . . . . . 10
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 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
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, 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
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 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
identity.
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
document.
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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
security.
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
US
Email: jon.peterson@neustar.biz
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