Network Working Group T. Taylor, Ed.
Request for Comments: 5069 Nortel
Category: Informational H. Tschofenig
Nokia Siemens Networks
H. Schulzrinne
Columbia University
M. Shanmugam
Detecon
January 2008
Security Threats and Requirements for
Emergency Call Marking and Mapping
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document reviews the security threats associated with the
marking of signalling messages to indicate that they are related to
an emergency, and with the process of mapping locations to Universal
Resource Identifiers (URIs) that point to Public Safety Answering
Points (PSAPs). This mapping occurs as part of the process of
routing emergency calls through the IP network.
Based on the identified threats, this document establishes a set of
security requirements for the mapping protocol and for the handling
of emergency-marked calls.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Marking, Mapping, and the Emergency Call Routing Process . . . 3
3.1. Call Marking . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Objectives of Attackers . . . . . . . . . . . . . . . . . . . 4
5. Potential Attacks . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Attacks Involving the Emergency Identifier . . . . . . . . 5
5.2. Attacks Against or Using the Mapping Process . . . . . . . 5
5.2.1. Attacks Against the Emergency Response System . . . . 6
5.2.2. Attacks to Prevent a Specific Individual from
Receiving Aid . . . . . . . . . . . . . . . . . . . . 7
5.2.3. Attacks to Gain Information about an Emergency . . . . 7
6. Security Requirements Relating to Emergency Marking and
Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
1. Introduction
Legacy telephone network users can summon help for emergency services
(such as an ambulance, the fire department, and the police) using a
well known number (e.g., 911 in North America, 112 in Europe). A key
factor in the handling of such calls is the ability of the system to
determine caller location and to route the call to the appropriate
Public Safety Answering Point (PSAP) based on that location. With
the introduction of IP-based telephony and multimedia services,
support for emergency calling via the Internet also has to be
provided. Two core components of IP-based emergency calling include
an emergency service identifier and a mapping protocol. The
emergency service identifier indicates that the call signaling
establishes an emergency call, while the mapping protocol translates
the emergency service identifier and the caller's geographic location
into an appropriate PSAP URL.
Attacks against the Public Switched Telephone Network (PSTN) have
taken place for decades. The Internet is seen as an even more
hostile environment. Thus, it is important to understand the types
of attacks that might be mounted against the infrastructure providing
emergency services and to develop security mechanisms to counter
those attacks. While this can be a broad topic, the present document
restricts itself to attacks on the mapping of locations to PSAP URIs
and attacks based on emergency marking. Verification by the PSAP
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operator of the truthfulness of a reported incident and various other
attacks against the PSAP infrastructure related to the usage of faked
location information are outside the scope of the document.
This document is organized as follows: Section 2 describes basic
terminology. Section 3 briefly describes how emergency marking and
mapping fit within the process of routing emergency calls. Section 4
describes some motivations of attackers in the context of emergency
calling, Section 5 describes and illustrates the attacks that might
be used, and Section 6 lists the security-related requirements that
must be met if these attacks are to be mitigated.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119], with the
qualification that unless otherwise stated, they apply to the design
of the mapping protocol, not its implementation or application.
The terms "call taker", "mapping service", "emergency caller",
"emergency identifier", "mapping", "mapping client", "mapping
server", "mapping protocol", and "Public Safety Answering Point
(PSAP)" are taken from [RFC5012].
The term "location information" is taken from RFC 3693 [RFC3693].
The term "emergency caller's device" designates the IP host closest
to the emergency caller in the signalling path between the emergency
caller and the PSAP. Examples include an IP phone running SIP,
H.323, or a proprietary signalling protocol, a PC running a soft
client or an analogue terminal adapter, or a residential gateway
controlled by a softswitch.
3. Marking, Mapping, and the Emergency Call Routing Process
This memo deals with two topics relating to the routing of emergency
calls to their proper destination: call marking and mapping.
3.1. Call Marking
Marking of call signalling enables entities along the signalling path
to recognize that a particular signalling message is associated with
an emergency call. Signalling containing the emergency identifier
may be given priority treatment, special processing, and/or special
routing.
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3.2. Mapping
An important goal of emergency call routing is to ensure that any
emergency call is routed to a PSAP. Preferably, the call is routed
to the PSAP responsible for the caller's location, since misrouting
consumes valuable time while the call taker locates and forwards the
call to the right PSAP. As described in [RFC5012], mapping is part
of the process of achieving this preferable outcome.
In brief, mapping involves a mapping client, a mapping server, and
the protocol that passes between them. The protocol allows the
client to pass location information to the mapping server and to
receive back a URI, which can be used to direct call signalling to a
PSAP.
4. Objectives of Attackers
Attackers may direct their efforts either against a portion of the
emergency response system or against an individual. Attacks against
the emergency response system have three possible objectives:
o to deny system services to all users in a given area. The
motivation may range from thoughtless vandalism, to wide-scale
criminality, to terrorism. One interesting variant on this
motivation is the case where a victim of a large emergency hopes
to gain faster service by blocking others' competing calls for
help.
o to gain fraudulent use of services, by using an emergency
identifier to bypass normal authentication, authorization, and
accounting procedures.
o to divert emergency calls to non-emergency sites. This is a form
of a denial-of-service attack similar to the first item, but quite
likely more confusing for the caller himself or herself since the
caller expects to talk to a PSAP operator but instead gets
connected to someone else.
Attacks against an individual fall into two classes:
o attacks to prevent an individual from receiving aid.
o attacks to gain information about an emergency that can be applied
either against an individual involved in that emergency or to the
profit of the attacker.
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5. Potential Attacks
5.1. Attacks Involving the Emergency Identifier
The main possibility of attack involves use of the emergency
identifier to bypass the normal procedures in order to achieve
fraudulent use of services. An attack of this sort is possible only
if the following conditions are true:
a. The attacker is the emergency caller.
b. The call routing system assumes that the emergency caller's
device signals the correct PSAP URI for the caller's location.
c. The call enters the domain of a service provider, which accepts
it without applying normal procedures for authentication and
authorization because the signalling carries the emergency
identifier.
d. The service provider routes the call according to the called
address (e.g., SIP Request-URI), without verifying that this is
the address of a PSAP (noting that a URI by itself does not
indicate the nature of the entity it is pointing to).
If these conditions are satisfied, the attacker can bypass normal
service provider authorization procedures for arbitrary destinations,
simply by reprogramming the emergency caller's device to add the
emergency identifier to non-emergency call signalling. In this case,
the call signalling most likely will not include any location
information, or there could be location information, but it is false.
An attacker wishing to disrupt the emergency call routing system may
use a similar technique to target components of that system for a
denial-of-service attack. The attacker will find this attractive to
reach components that handle emergency calls only. Flooding attacks
are the most likely application of the technique, but it may also be
used to identify target components for other attacks by analyzing the
content of responses to the original signalling messages.
5.2. Attacks Against or Using the Mapping Process
This section describes classes of attacks involving the mapping
process that could be used to achieve the attacker goals described in
Section 4.
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5.2.1. Attacks Against the Emergency Response System
This section considers attacks intended to reduce the effectiveness
of the emergency response system for all callers in a given area. If
the mapping operation is disabled, then the emergency caller's device
might not have the correct PSAP URI. As a consequence, the
probability that emergency calls will be routed to the wrong PSAP
increases. In the worst case, the emergency caller's device might
not be able to obtain a PSAP URI at all. Routing to the wrong PSAP
has a double consequence: emergency response to the affected calls is
delayed, and PSAP call taker resources outside the immediate area of
the emergency are consumed due to the extra effort required to
redirect the calls. Alternatively, attacks that cause the client to
receive a URI that does not lead to a PSAP have the immediate effect
of causing emergency calls to fail.
Three basic attacks on the mapping process can be identified: denial
of service, impersonation of the mapping server, or corruption of the
mapping database. Denial of service can be achieved in several ways:
o by a flooding attack on the mapping server;
o by taking control of the mapping server and either preventing it
from responding or causing it to send incorrect responses; or
o by taking control of any intermediary node (for example, a router)
through which the mapping queries and responses pass, and then
using that control to block them. An adversary may also attempt
to modify the mapping protocol signalling messages. Additionally,
the adversary may be able to replay past communication exchanges
to fool an emergency caller by returning incorrect results.
In an impersonation attack, the attacker induces the mapping client
to direct its queries to a host under the attacker's control rather
than the real mapping server, or the attacker suppresses the response
from the real mapping server and sends a spoofed response.
The former type of impersonation attack itself is an issue of mapping
server discovery rather than the mapping protocol directly. However,
the mapping protocol may allow impersonation to be detected, thereby
preventing acceptance of responses from an impersonating entity and
possibly triggering a more secure discovery procedure.
Corruption of the mapping database cannot be mitigated directly by
mapping protocol design. Once corruption has been detected, the
mapping protocol may have a role to play in determining which records
have been corrupted.
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Beyond these attacks on the mapping operation itself, it is possible
to use mapping to attack other entities. One possibility is that
mapping clients are misled into sending mapping queries to the target
of the attack instead of the mapping server. Prevention of such an
attack is an operational issue rather than one of protocol design.
Another possible attack is where the mapping server is tricked into
sending responses to the target of the attack through spoofing of the
source address in the query.
5.2.2. Attacks to Prevent a Specific Individual from Receiving Aid
If an attacker wishes to deny emergency service to a specific
individual, the mass attacks described in Section 5.2.1 will
obviously work provided that the target individual is within the
affected population. Except for the flooding attack on the mapping
server, the attacker can in theory limit these attacks to the target,
but this requires extra effort that the attacker is unlikely to
expend. If the attacker is using a mass attack but does not wish to
have too broad an effect, it is more likely to attack for a carefully
limited period of time.
If the attacker wants to be selective, however, it may make more
sense to attack the mapping client rather than the mapping server.
This is particularly so if the mapping client is the emergency
caller's device. The choices available to the attacker are similar
to those for denial of service on the server side:
o a flooding attack on the mapping client;
o taking control of any intermediary node (for example, a router)
through which the mapping queries and responses pass, and then
using that control to block or modify them.
Taking control of the mapping client is also a logical possibility,
but raises no issues for the mapping protocol.
5.2.3. Attacks to Gain Information about an Emergency
This section discusses attacks used to gain information about an
emergency. The attacker may be seeking the location of the caller
(e.g., to effect a criminal attack). Alternatively, the attacker may
be seeking information that could be used to link an individual (the
caller or someone else involved in the emergency) with embarrassing
information related to the emergency (e.g., "Who did the police take
away just now?"). Finally, the attacker could be seeking to profit
from the emergency, perhaps by offering his or her services (e.g., a
news reporter, or a lawyer aggressively seeking new business).
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The primary information that interceptions of mapping requests and
responses will reveal are a location, a URI identifying a PSAP, the
emergency service identifier, and the addresses of the mapping client
and server. The location information can be directly useful to an
attacker if the attacker has high assurance that the observed query
is related to an emergency involving the target. The type of
emergency (fire, police, or ambulance) might also be revealed by the
emergency service identifier in the mapping query. The other pieces
of information may provide the basis for further attacks on emergency
call routing, but because of the time factor, are unlikely to be
applicable to the routing of the current call. However, if the
mapping client is the emergency caller's device, the attacker may
gain information that allows for interference with the call after it
has been set up or for interception of the media stream between the
caller and the PSAP.
6. Security Requirements Relating to Emergency Marking and Mapping
This section describes the security requirements that must be
fulfilled to prevent or reduce the effectiveness of the attacks
described in Section 5. The requirements are presented in the same
order as the attacks.
From Section 5.1:
Attack A1: fraudulent calls.
Requirement R1: For calls that meet conditions a) to c) of
Section 5.1, the service provider's call routing entity MUST verify
that the destination address (e.g., SIP Request-URI) presented in the
call signalling is that of a PSAP.
Attack A2: Use of emergency identifier to probe in order to identify
emergency call routing entities for attack by other means.
Requirement: None identified, beyond the ordinary operational
requirement to defend emergency call routing entities by means such
as firewalls and, where possible, authentication and authorization.
From Section 5.2.1:
Attack A3: Flooding attack on the mapping client, mapping server, or
a third entity.
Requirement R2: The mapping protocol MUST NOT create new
opportunities for flooding attacks, including amplification attacks.
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Attack A4: Insertion of interfering messages.
Requirement R3: The protocol MUST permit the mapping client to verify
that the response it receives is responding to the query it sent out.
Attack A5: Man-in-the-middle modification of messages.
Requirement R4: The mapping protocol MUST provide integrity
protection of requests and responses.
Requirement R5: The mapping protocol or the system within which the
protocol is implemented MUST permit the mapping client to
authenticate the source of mapping responses.
Attack A6: Impersonation of the mapping server.
Requirement R6: The security considerations for any discussion of
mapping server discovery MUST address measures to prevent
impersonation of the mapping server.
Requirement R5 also follows from this attack.
Attack A7: Corruption of the mapping database.
Requirement R7: The security considerations for the mapping protocol
MUST address measures to prevent database corruption by an attacker.
Requirement R8: The protocol SHOULD include information in the
response that allows subsequent correlation of that response with
internal logs that may be kept on the mapping server, to allow
debugging of mis-directed calls.
From Section 5.2.2: No new requirements.
From Section 5.2.3:
Attack A8: Snooping of location and other information.
Requirement R9: The protocol and the system within which it is
implemented MUST maintain confidentiality of the request and
response.
7. Security Considerations
This document addresses security threats and security requirements.
Therefore, security is considered throughout this document.
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8. Acknowledgements
The writing of this document has been a task made difficult by the
temptation to consider the security concerns of the entire personal
emergency calling system, not just the specific pieces of work within
the scope of the ECRIT Working Group. Hannes Tschofenig performed
the initial security analysis for ECRIT, but it has been shaped since
then by the comments and judgement of the ECRIT WG at large. At an
earlier stage in the evolution of this document, Stephen Kent of the
Security Directorate was asked to review it and provided extensive
comments, which led to a complete rewriting of it. Brian Rosen,
Roger Marshall, Andrew Newton, and most recently, Spencer Dawkins,
Kamran Aquil, and Ron Watro have also provided detailed reviews of
this document at various stages. The authors thank them.
We would like to thank Donald Eastlake for his review on behalf of
the Security Area Directorate and Christian Vogt for his review as
part of the General Area Review Team.
Finally, we would like to thank Jari Arkko, Jon Peterson, and Russ
Housley for their IETF Last Call comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC5012] Schulzrinne, H. and R. Marshall, Ed., "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, January 2008.
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Authors' Addresses
Tom Taylor (editor)
Nortel
1852 Lorraine Ave
Ottawa, Ontario K1H 6Z8
Canada
EMail: tom.taylor@rogers.com
Hannes Tschofenig
Nokia Siemens Networks
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
EMail: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
EMail: hgs+ecrit@cs.columbia.edu
URI: http://www.cs.columbia.edu
Murugaraj Shanmugam
Detecon International GmbH
Oberkasseler str 2
Bonn, NRW 53227
Germany
EMail: murugaraj.shanmugam@detecon.com
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RFC, FYI, BCP