Emergency Call Security: Security Threats and Requirements for Emergency Call Marking and Mapping

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.

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
   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.

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.

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.

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.

   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).

   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.

   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.

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.

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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