Internet DRAFT - draft-peterson-stir-threats
draft-peterson-stir-threats
Network Working Group J. Peterson
Internet-Draft NeuStar, Inc.
Intended status: Informational September 04, 2013
Expires: March 08, 2014
Secure Telephone Identity Threat Model
draft-peterson-stir-threats-00.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 powers available to and used by attackers, and
describing scenarios in which those powers are exercised.
Status of This Memo
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This Internet-Draft will expire on March 08, 2014.
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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 6
3.3. Attack Scenarios . . . . . . . . . . . . . . . . . . . . 7
3.4. Solution-Specific Attacks . . . . . . . . . . . . . . . . 8
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Informative References . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and Scope
As is discussed in the STIR problem statemnt [9], the primary enabler
of robocalling, vishing and related attacks is the capability to
impersonate a calling party number. The most stark 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 prefereable. 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 an end 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 particular chosen number, one that the
attacker does not have the authority to call from, in order for that
number to be 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 calls that contain attacker-chosen
calling telephone numbers in their signaling. The solution space
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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.
Rendering an authenticated calling party number during 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. In those end-to-end IP environments where such an assurance
is possible, it is highly desirable, but in the threat model
considered in this document, the threat of impersonation does not
extend to impersonating an authorized listener after a call has been
completed. Attackers that could impersonate an authorized listener
require powers 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 protocols like SIP, call signaling can be renegotiated after the
call has been completed, and through various transfer mechanisms
common in telephone systems, callees can easily be connected to, or
conferenced in with, telephone numbers other than the original
calling number once a call has been set up. 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 explored
in this document focuses only on the calling party number, though
presenting a forged calling party number can let the attacker cause a
forged 'Caller ID' name to be rendered to the user as well.
Providing a verifiable calling party number therefore does improve
the security of Caller ID systems, but this threat model does not
consider attacks specific to Caller ID, such as attacks on the
databases consulted by the terminating side of a call to provide
Caller ID, or impersonators choosing to forge a particular calling
party number in order to present a misleading Caller ID to the user.
Finally, the scope of impersonation in this threat model does not
consider simple anonymity as a threat. The ability to place
anonymous calls has always been a feature of the PSTN, and users of
the PSTN today have the capability to reject anonymous calls should
they wish to.
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2. Actors
2.1. Endpoints
There are two main categories of end-user terminals, 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 show only a limited
number of characters (typically, enough for a telephone number and
an accompanying name, sometimes less). These devices are
controlled by service providers in the network.
Smart devices are general purpose computers with some degree of
programmability and the capacity to access the Internet, along
with a rich display. This includes smart phones, telephone
applications on desktop and laptop computers, IP private branch
exchanges, and so on.
There are also various hybrid devices, such as terminal adapters
which attach dumb devices to a VoIP service, but which may in turn
use auxiliary screens as displays for rich information (for example,
some cable deployments use the television screen to render caller
ID). These devices expose little programmability to end users.
There is a further category of automated terminals without an end
user. These include systems like voicemail services that consume the
calling party number without rendering it to a human. 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
We assume that a call between two endpoints traverses 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
signaling that an intermediary can introduce.
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Gateways translate 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, and that this
is an ordinary condition in call routing. This is significant to the
solution space, however, because it limits the ability of the
originator to anticipate what the telephone number of the respondent
will be.
Furthermore, we assume that some intermediaries or gateways may, due
to their capabilities or policies, discard calling party number
information, as a 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 that
the gateway happens to use 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 limit 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. This is not however indicative of an attack or other
security failure.
2.3. Attackers
We assume that an attacker has the following powers:
The attacker can create telephone calls at will, originating them
on either the PSTN or over IP, and can supply an arbitrary calling
party number.
The attacker can capture and replay signaling previously received.
[TBD: should this include a passive attacker that can capture
signaling that isn't directly sent to it? Not a factor for
robocalling, but perhaps for voicemail hacking, say.]
The 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 many potential threats 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 threats are
not considered in this threat model, though secure design of systems
to prevent these sorts of attacks is necessary for any of these
countermeasures to work.
This threat model also does not consider a case in which the
operators of intermediaries or gateways are themselves adversaries
who intentionally suppress identity or send falsified identity with
their own credentials.
3. Attacks
3.1. Voicemail Hacking via Impersonation
A voicemail service allows users calling from their mobile phones
access to their voicemail boxes on the basis of the calling party
number. An attacker wants to access the voicemail of a particular
target. The attacker therefore impersonates the calling party number
using one of the scenarios described below.
In all cases, the countermeasure to this threat is for the voicemail
service to have an expectation that calls to its service will supply
an authenticated identity, and in the absence of that identity, for
it to adopt a different policy (perhaps requiring a shared secret to
be dialed as a PIN). Authenticated identity alone provides a
positive confirmation only when an identity is claimed legitimately;
the absence of authenticated identity here is not evidence of malice,
just of uncertainty.
If the voicemail service could know ahead of time that it should
always expect authenticated identity from a particular number, that
would enable the voicemail service to adopt different policies for
handling a request without authenticated identity. Since users
contact a voicemail service repeatedly, this is something that a
voicemail server could learn, for example, the first time that a user
contacts it. Alternatively, it could access a directory of some kind
that informs verifiers that they should expect identity from
particular numbers.
3.2. Unsolicited Commercial Calling from Impersonated Numbers
The unsolicited commercial calling, or for short robocalling, threat
is similar to the voicemail threat, except in so far as the
robocaller does not need to impersonate any specific number, merely a
plausible number. A robocaller may impersonate a number that is not
a valid number (for example, in the United States, a number beginning
with 0), or an unassigned number. The robocaller may change numbers
every time a new call is placed, even selecting numbers randomly.
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The countermeasures to robocalling are similar to the voicemail
example, but there are significant differences. One important
potential countermeasure is simply to verify that the calling party
number is in fact valid and assigned. Unlike voicemail services, end
users typically have never been contacted by the number used by a
robocaller before, so they can't rely on past association to know
whether or not the calling party number should always supply
authenticated identity. If there were a directory that could inform
the terminating side of that fact, however, that would help in the
robocalling case.
When alerting a human is involved, the time frame for executing these
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 operations and network operations must be minimized for
these countermeasures to be practical.
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 to use authenticated
identity for numbers traceable to them, which would then allow for
other forms of redress.
3.3. Attack Scenarios
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 CPN field of an IAM. When the
IAM reaches the endpoint terminal, the terminal renders the
attacker's chosen calling party number as the calling identity.
Countermeasure: out-of-band authenticated identity
Impersonation, PSTN-PSTN
An attacker with a traditional PBX (connected to the PSTN through an
ISDN PRI) sends a Q.931 SETUP request with a chosen calling party
number which a service provider inserts into the corresponding SS7
CPN field of an IAM. When the IAM reaches the endpoint terminal, the
terminal renders the attacker's chosen calling party number as the
calling identity.
Countermeasure: out-of-band authenticated identity
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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.
Countermeasure: in-band authenticated 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 CPN field 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.
Countermeasure: out-of-band authenticated identity
3.4. Solution-Specific Attacks
[TBD: This is just forward-looking notes]
Threats Against In-band
Token replay
Removal of in-band signaling features
Threats Against Out-of-Band
Provisioning Gargbage CPRs
Data Mining
Threats Against Either Approach
Attack on directories/services that say whether you should expect
authenticated identity or not
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Canonicalization attack
4. Acknowledgments
Henning Schulzrinne, Hannes Tschofenig, Cullen Jennings and Eric
Rescorla provided key input to the discussions leading to this
document.
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
This document provides a threat model and is thus entirely about
security.
7. Informative References
[1] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[2] 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.
[3] 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.
[4] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[5] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[6] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[7] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
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[8] Cooper, A., Tschofenig, H., Peterson, J., and B. Aboba,
"Secure Call Origin Identification", draft-cooper-iab-
secure-origin-00 (work in progress), November 2012.
[9] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Origin Identification: Problem Statement, Threat Model,
Requirements, and Roadmap", draft-peterson-secure-origin-
ps-01 (work in progress), July 2013.
[10] Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
[11] Rosenberg, J., "Concerns around the Applicability of RFC
4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
progress), February 2008.
[12] Kaplan, H. and V. Pascual, "Loop Detection Mechanisms for
Session Initiation Protocol (SIP) Back-to- Back User
Agents (B2BUAs)", draft-ietf-straw-b2bua-loop-detection-01
(work in progress), August 2013.
[13] Barnes, M., Jennings, C., Rosenberg, J., and M. Petit-
Huguenin, "Verification Involving PSTN Reachability:
Requirements and Architecture Overview", draft-jennings-
vipr-overview-04 (work in progress), February 2013.
[14] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.
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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