ECRIT                                                          T. Taylor
Internet-Draft                                           (Editor) Nortel
Expires: October 19, December 27, 2006                                 H. Tschofenig
                                                          H. Schulzrinne
                                                             Columbia U.
                                                            M. Shanmugam
                                                          April 17,
                                                           June 25, 2006

Security Threats and Requirements for Emergency Call Marking and Mapping

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document reviews the security threats associated with the two
   current work items of the ECRIT Working Group.  The first is with:

   o  the marking of signalling messages to indicate that they are
      related to an emergency.  The second is emergency; and

   o  the process of mapping from locations to Universal Resource
      Identifiers (URIs) pointing 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 idnetified threats, this document establishes a set of
   security requirements for the the mapping protocol and for the
   handling of emergency-marked calls.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Marking, mapping, Mapping, and the emergency call routing process Emergency Call Routing Process . . .  5
   4.  Objectives of attackers Attackers  . . . . . . . . . . . . . . . . . . .  6
   5.  Potential attacks Attacks  . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Attacks involving Involving the emergency identifier Emergency Identifier . . . . . . . .  7
     5.2.  Attacks against Against or using Using the mapping process Mapping Process . . . . . . .  7
       5.2.1.  Attacks against Against the emergency response system Emergency Response System  . . . .  8  7
       5.2.2.  Attacks to prevent To Prevent a specific individual from
               receiving aid Specific Individual From
               Receiving Aid  . . . . . . . . . . . . . . . . . . . .  9
       5.2.3.  Attacks to gain information about To Gain Information About an emergency Emergency . . . .  9
   6.  Security requirements relating to ECRIT work items Requirements Relating To Emergency Marking and
       Mapping  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18

1.  Introduction

   Legacy telephone network users can summon help for emergency services
   such as ambulance, fire and 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.  As one of the
   steps to achieve this, an emergency marker must be defined that can
   be attached to call signalling to indicate that the call relates to
   an emergency.  In addition, a protocol must be developed allowing a
   client entity to submit a location and receive a URI pointing to the
   applicable PSAP for that location.

   Attacks against the PSTN (most often focusing on free calling) 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.  In view of the mandate of the ECRIT Working Group,  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

   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 ECRIT, 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",
   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

   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, Mapping, and the emergency call routing process

   The ECRIT Working Group has Emergency Call Routing Process

   This memo deals with two work items topics relating to the routing of emergency
   calls to their proper destination.  The first is the marking of call
   signalling to enable entities along the signalling path to recognize
   that a particular signalling message is associated with an emergency
   call.  The ECRIT
   Working Group is defining content that can be added to the signalling
   messages, an emergency identifier, for this purpose.  Signalling containing the emergency identifier may be given
   priority treatment, special processing, and/or special routing.

   The first 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 [I-D.ecrit-requirements],
   mapping, the second ECRIT work item,
   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

   Since mapping requires location information for input, when and where
   the location information is acquired imposes constraints upon when
   mapping can be done and which devices can act as mapping clients.
   The key distinction in "when" is before the emergency or during the
   emergency.  The key distinction in "where" is at the emergency
   caller's device or at another device in the signalling path between
   the emergency caller and the PSAP.  The mapping client can be the
   device that acquires the location information or any device
   downstream of that point.  It is even possible for a PSAP itself to
   initiate mapping, to determine whether an arriving call should be
   handled by a call taker at that PSAP or should be proxied to another

4.  Objectives of attackers 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

   o  to gain fraudulent use of services, by using an emergency
      identifier to bypass normal authentication, authorization, and
      accounting procedures.

   o  to divert emergency responders to non-emergency sites.  No attacks
      affecting the ECRIT Working Group's decisions on the emergency
      identifier and mapping protocol have been  This memo
      has not identified any attacks within its intended scope that
      achieve this objective objective, so it will not be mentioned further.

   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 Attacks

5.1.  Attacks involving Involving the emergency identifier Emergency Identifier

   The main attack possibility involving the emergency identifier is to
   use it to bypass 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 addresses emergency calls using signals the result of mapping
       based on 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

   d.  The service provider routes it 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.  Most probably
   in this case, the call signalling will not include any location

   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 Against or using Using the mapping process 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 Against the emergency response system 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 immediate effect is to
   increase emergency caller's device
   might not have the correct PSAP URI.  As a consequence, the
   probability that emergency calls are routed to the wrong
   PSAP.  This PSAP is
   increased.  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 a router through which the mapping queries
      and responses pass and using that control to block them.  An
      adversary may also attempt to modify the mapping protocol
      signaling 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.  Impersonation itself is an issue for
   mapping server discovery rather than for the mapping protocol
   directly.  However, the mapping protocol may help allow impersonation to protect against
   be detected, thereby preventing acceptance of responses from an
   impersonating entity. entity and possibly triggering a more secure discovery

   Corruption of the mapping database cannot be mitigated directly by
   mapping protocol design.  The mapping protocol may have a role to
   play in analysis of which records have been corrupted, once that
   corruption has been detected.

   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.

   The other possible attack is one where the 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 To Prevent a specific individual from receiving aid 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.
   It is more likely, if the attacker is using a mass attack but does
   not wish it to have too broad an effect, that it is used 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 a router through which the mapping queries and
      responses pass and 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 To Gain Information About an emergency 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).  The  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.,
   news reporter, 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, 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 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 ECRIT work items Requirements Relating To Emergency Marking and Mapping

   This section describes the security requirements which 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: fraudulent calls.

   Requirement: for calls which 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: use of emergency identifier to probe in order to identify
   emergency call routing entities.

   Requirement: topology hiding SHOULD be applied to call signalling
   returned to the emergency caller, so that the identity of
   intermediate routing entities is not disclosed.  The obvious
   exception is where these entities are already visible to the caller.
   Note that there is little point in hiding the PSAP itself.

   From Section 5.2.1:

   Attack: flooding attack on the mapping client, mapping server, or a
   third entity.

   Requirement: The mapping protocol MUST NOT create new opportunities
   for flooding attacks, including amplification attacks.

   Attack: insertion of interfering messages.

   Requirement: The protocol MUST permit the mapping client to verify
   that the response it receives is responding to the query it sent out.

   Attack: man-in-the-middle alteration of messages.

   Requirement: The protocol or the system within which it is
   implemented MUST maintain request and response integrity.

   Attack: impersonation of the mapping server.

   Requirement: the security considerations for any discussion of
   mapping server discovery MUST address measures to prevent
   impersonation of the mapping server.

   Requirement: the protocol or the system within which it is
   implemented MUST permit the mapping client to authenticate the source
   of mapping responses.

   Attack: corruption of the mapping database.

   Requirement: the security considerations for the mapping protocol
   MUST address measures to prevent database corruption by an attacker.

   Requirement: to provide an audit trail, the protocol SHOULD allow the
   inclusion of an identifier in its response that indicates which
   database records were used in preparing the response.  This
   identifier SHOULD be encrypted along with randomizing information
   such as date/time, to minimize the information provided to an
   attacker in mapping responses.

   From Section 5.2.2: no new requirements.

   From Section 5.2.3:

   Attack: snooping of location and other information.

   Requirement: the protocol or the system within which it is
   implemented MUST maintain confidentiality of the request and

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 and
   Kmaran Aquil have also provided detailed reviews of this document at
   various stages.  The authors thank them.

9.  IANA Considerations

   This document does not require actions by the IANA.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3693]  Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
              J. Polk, "Geopriv Requirements", RFC 3693, February 2004.

10.2.  Informative References

              Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              March 2006.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

Authors' Addresses

   Tom Taylor
   1852 Lorraine Ave
   Ottawa, Ontario  K1H 6Z8


   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bayern  81739


   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7042

   Murugaraj Shanmugam
   Otto-Hahn-Ring 6
   Munich, Bayern  81739


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