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Versions: (draft-niccolini-speermint-voipthreats) 00 01 02 03 04 05 06 07 08 09 RFC 6404

SPEERMINT Working Group                                     S. Niccolini
Internet-Draft                                                       NEC
Intended status: Informational                                   E. Chen
Expires: January 14, 2010                                            NTT
                                                              J. Seedorf
                                                                     NEC
                                                               H. Scholz
                                                                 freenet
                                                           July 13, 2009


        SPEERMINT Security Threats and Suggested Countermeasures
                  draft-ietf-speermint-voipthreats-01

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on January 14, 2010.

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Abstract

   This memo presents the different security threats related to
   SPEERMINT, classifying them into threats to the Lookup Function
   (LUF), Location Routing Function (LRF), to the Signaling Function
   (SF) and to the Media Function (MF).  The different instances of the
   threats are briefly introduced inside the classification.  Finally,
   the existing security solutions for SIP and RTP/RTCP are presented to
   describe the countermeasures currently available for such threats.
   Security requirements for SPEERMINT can be found in
   draft-ietf-speermint-requirements.  The objective of this document is
   to identify and enumerate SPEERMINT-specific threat vectors and to
   give guidance for implementers on selecting appropriate
   countermeasures.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Security Threats relevant to SPEERMINT . . . . . . . . . . . .  5
     2.1.  Threats Relevant to the Look-Up Function (LUF) . . . . . .  5
       2.1.1.  Threats to LUF Confidentiality . . . . . . . . . . . .  5
       2.1.2.  Threats to LUF Integrity . . . . . . . . . . . . . . .  6
       2.1.3.  Threats to LUF Availability  . . . . . . . . . . . . .  6
     2.2.  Threats Relevant to the Location Routing Function (LRF)  .  6
       2.2.1.  Threats to LRF Confidentiality . . . . . . . . . . . .  6
       2.2.2.  Threats to LRF Integrity . . . . . . . . . . . . . . .  6
       2.2.3.  Threats to LRF Availability  . . . . . . . . . . . . .  7
     2.3.  Threats to the Signaling Function (SF) . . . . . . . . . .  7
       2.3.1.  Threats to SF Confidentiality  . . . . . . . . . . . .  7
       2.3.2.  Threats to SF Integrity  . . . . . . . . . . . . . . .  7
       2.3.3.  Threats to SF Availability . . . . . . . . . . . . . .  9
     2.4.  Threats to the Media Function (MF) . . . . . . . . . . . .  9
       2.4.1.  Threats to MF Confidentiality  . . . . . . . . . . . .  9
       2.4.2.  Threats to MF Integrity  . . . . . . . . . . . . . . .  9
       2.4.3.  Threats to MF Availability . . . . . . . . . . . . . . 10
   3.  Security Requirements  . . . . . . . . . . . . . . . . . . . . 11
   4.  Suggested Countermeasures  . . . . . . . . . . . . . . . . . . 12
     4.1.  Database Security BCPs . . . . . . . . . . . . . . . . . . 14
     4.2.  DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  DNS Replication  . . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Cross-Domain Privacy Protection  . . . . . . . . . . . . . 14
     4.5.  Use TCP instead of UDP to deliver SIP messages . . . . . . 14
     4.6.  Ingress Filtering / Reverse-Path Filtering . . . . . . . . 15
     4.7.  Strong Identity Assertion  . . . . . . . . . . . . . . . . 15
     4.8.  Reliable Border Element Pooling  . . . . . . . . . . . . . 16
     4.9.  Rate limit . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.10. Topology Hiding  . . . . . . . . . . . . . . . . . . . . . 16
     4.11. Border Element Hardening . . . . . . . . . . . . . . . . . 16
     4.12. Minimization of Session Establishment Data . . . . . . . . 17
     4.13. Encyrption and Integrity Protection of Signalling
           Messages . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.14. Encyrption and Integrity Protection of Media Stream  . . . 17
   5.  Current Deployment of Countermeasures  . . . . . . . . . . . . 18
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24








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1.  Introduction

   With VoIP, the need for security is compounded because there is the
   need to protect both the control plane and the data plane.  In a
   legacy telephone system, security is a more valid assumption.
   Intercepting conversations requires either physical access to
   telephone lines or to compromise the Public Switched Telephone
   Network (PSTN) nodes or the office Private Branch eXchanges (PBXs).
   Only particularly security-sensitive organizations bother to encrypt
   voice traffic over traditional telephone lines.  In contrast, the
   risk of sending unencrypted data across the Internet is more
   significant (e.g.  DTMF tones corresponding to the credit card
   number).  An additional security threat to Internet Telephony comes
   from the fact that the signaling devices may be addressed directly by
   attackers as they use the same underlying networking technology as
   the multimedia data; traditional telephone systems have the signaling
   network separated from the data network.  This is an increased
   security threat since a hacker could attack the signaling network and
   its servers with increased damage potential (call hijacking, call
   drop, DoS attacks, etc.).  Therefore there is the need of
   investigating the different security threats, to extract security-
   related requirements and to highlight the solutions how to protect
   from such threats.

   The objective of this document is to identify and enumerate
   SPEERMINT-specific threat vectors and to give guidance for
   implementers on selecting appropriate countermeasures.  The SPEERMINT
   terminology outlined in [RFC5486] is used throughout this document.























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2.  Security Threats relevant to SPEERMINT

   This section enumerates potential security threats relevant to
   SPEERMINT.  A taxonomy of VoIP security threats is defined in
   [refs.voipsataxonomy].  Such a taxonomy is really comprehensive and
   takes into account also non-VoIP-specific threats (e.g. loss of
   power, etc.).  Threats relevant to the boundaries of layer-5 SIP
   networks are extracted from such a taxonomy and mapped to the
   classification relevant for the SPEERMINT architecture as defined in
   [refs.speermintarch], moreover additional threats for the SPEERMINT
   architecture are listed and detailed under the same classification
   and according the CIA (Confidentiality, Integrity and Availability)
   triad:

   o  Look-Up Function (LUF);

   o  Location Routing Function (LRF);

   o  Signaling Function (SF);

   o  Media Function (MF).

2.1.  Threats Relevant to the Look-Up Function (LUF)

   The LUF provides a mechanism for determining for a given request the
   identity of the requested resource on the terminating domain.  The
   returned identity can be used to look up Session Establishment Data
   (SED) using the Location Routing Function (LRF).  In direct peerings
   the LUF is usually hosted locally whereas in a federation context
   this function may be offered by a third party.

   If the LUF is hosted locally it is vulnerable to the same threats
   that affect database systems in general.  If the SSP relies on a
   remote 3rd party to provide the LUF functionality both integrity and
   authenticity of the responses are at risk.

2.1.1.  Threats to LUF Confidentiality

   o  SIP URIs and peering domains harvesting - an attacker can exploit
      this weakness if the underlying database has a weak authentication
      system, and then use the gained knowledge to launch other kind of
      attacks.

   o  3rd party information - a LUF providing information to multiple
      companies / third parties can be attacked to obtain information
      about third party peering configurations and possible contracts.





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2.1.2.  Threats to LUF Integrity

   The underlying database could be vulnerable to:

   o  Injection attack - an attacker could manipulate statements
      performed on the database by the end user.

2.1.3.  Threats to LUF Availability

   The underlying database could be vulnerable to:

   o  Denial of Service attacks - e.g. an attacker makes incomplete
      requests causing the server to create an idle state for each of
      them causing memory to be exhausted.

2.2.  Threats Relevant to the Location Routing Function (LRF)

   The LRF determines the location of the Signaling Function (SF) for
   the target domain of a given request.  Optionally it may return
   additional SED.

2.2.1.  Threats to LRF Confidentiality

   o  URI harvesting - the attacker harvests URIs and IP addresses of
      the existing User Endpoints (UEs) by issuing a multitude of
      location requests.  Direct intrusion against vulnerable UEs or
      telemarketing are possible attack scenarios that would use the
      gained knowledge.

   o  SIP device enumeration - the attacker discovers the IP address of
      each intermediate signaling device by looking at the Via and
      Record-Route headers of a SIP message.  Targeting the discovered
      devices with subsequent attacks is a possible attack scenario.

2.2.2.  Threats to LRF Integrity

   An attacker may feed bogus information to the LRF if the routing data
   is not correctly validated.  Dynamic call routing discovery and
   establishment, as in the scope of SPEERMINT, introduce opportunities
   for attacks such as the following.

   o  Man-in-the-Middle attack - the attacker has already or inserts an
      unauthorized node in the signaling path modifying the SED.  The
      results is that the attacker is then able to read, insert and
      modify the multimedia communications.

   o  Incorrect destinations - the attacker redirect the calls to a
      incorrect destination with the purpose of establishing fraud



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      communications like voice phishing or DoS attacks.

2.2.3.  Threats to LRF Availability

   The LRF can be object of DoS attacks.  DoS attacks to the LRF can be
   carried out by sending a large number of queries to the LS or Session
   Manager, SM, with the result of preventing an originating SSP from
   looking up call routing data of any URI outside its administrative
   domain.  As an alternative the attacker could target the DNS to
   disable resolution of SIP addresses.

2.3.  Threats to the Signaling Function (SF)

   Signaling function involves a great number of sensitive information.
   Through signaling function, user agents (UA) assert identities and
   VSP operators authorize billable resources.  Correct and trusted
   operations of signaling function is essential for service providers.
   This section discusses potential security threats to the signaling
   function to detail the possible attack vectors.

2.3.1.  Threats to SF Confidentiality

   SF traffic is vulnerable to eavesdropping, in particular when the
   data is moved across multiple SSPs having different levels of
   security policies.  Threats for the SF confidentiality are listed
   here:

   o  call pattern analysis - the attacker tracks the call patterns of
      the users violating his/her privacy (e.g. revealing the social
      network of various users, the daily phone usage, etc.), also rival
      SSPs may infer information about the customer base of other SSPs
      in this way;

   o  Password cracking - the challenge-response authentication
      mechanism of SIP can be attacked with offline dictionary attacks.
      With such attacks, an attacker tries to exploit weak passwords
      that are used by incautious users.

   o  Network discovery - the attacker may learn information about the
      internal network structure of peering partner that is directly or
      indirectly connected by looking at SIP routing information (i.e
      Record-Route, Via or Contact headers).

2.3.2.  Threats to SF Integrity

   The integrity of the SF can be violated using SIP request spoofing,
   SIP reply spoofing and SIP message tampering.




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2.3.2.1.  SIP Request Spoofing

   Most SIP request spoofing require first a SIP message eavesdropping
   but some of the them could be also performed by guessing or
   exploiting broken implementations.  Threats in this category are:

      session teardown - the attacker uses CANCEL/BYE messages in order
      to tear down an existing call at SIP layer, it is needed that the
      attacker replicates the proper SIP header for the hijacking to be
      successful (To, From, Call-ID, CSeq);

      Billing fraud - the attacker alters an INVITE request to bill a
      call to a victim UE and avoid paying for the phone call.

      user ID spoofing - SSPs are responsible for asserting the
      legitimacy of user ID; if an SSP fails to achieve the level of
      identity assertion that the federation it belongs expects, it may
      create an entry point for attackers to conduct user ID spoofing
      attacks.

      Unwanted requests - the attacker sends requests to interfere with
      regular operation, i.e. sends a REGISTER request to hijack calls.
      The SPEERMINT architecture as defined in [refs.speermintarch] does
      not require registrations between the signaling functions (SF) of
      the connected SSPs.  Superfluous requests like REGISTERs should be
      rejected.

2.3.2.2.  SIP Reply Spoofing

   Threats in this category are:

      Forged 200 Response - the attacker sends a forged CANCEL request
      to terminate a call in progress tricking the terminating UE to
      believe that the originating UE actually sent it, and successfully
      hijacks a call sending a forged 200 OK message to the originating
      UE communicating the address of the rogue UE under the attacker's
      control;

      Forged 302 Response - the attacker sends a forged "302 Moved
      Temporarily" reply instead of a 200 OK, this enables the attack to
      hijack the call and to redirect it to any destination UE of his
      choosing;

      Forged 404 Response - the attacker sends a forged "404 Not Found"
      reply instead of a 200 OK, this enables the attack to disrupt the
      call establishment;





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2.3.2.3.  SIP Message Tampering

   This threat involves the alternation of important field values in a
   SIP message or in the SDP body.  Examples of this threat could be the
   dropping or modification of handshake packets in order to avoid the
   establishment of a secure RTP session (SRTP).  The same approach
   could be used to degrade the quality of media session by letting UE
   negotiate a poor quality codec.

2.3.3.  Threats to SF Availability

   o  Flooding attack - a SBE is susceptible to message flooding attack
      that may come from interconnected SSPs;

   o  Session Black Holing - the attacker (assumed to be able to make
      Man-in-the-Middle attacks) intentionally drops essential packets,
      e.g.  INVITEs, to prevent certain calls from being established;

   o  SIP Fuzzing attack - fuzzing tests and software can be used by
      attackers to discover and exploit vulnerabilities of a SIP entity,
      this attack may result in crashing SIP entity.

2.4.  Threats to the Media Function (MF)

   The Media function (MF) is responsible for the actual delivery of
   multimedia communication between the users and carries sensitive
   information.  Through media function, UE can establish secure
   communications and monitor quality of conversations.  Correct and
   trusted operations of MF is essential for privacy and service
   assurance issues.  This section discusses potential security threats
   to the MF to detail the possible attack vectors.

2.4.1.  Threats to MF Confidentiality

   The MF is vulnerable to eavesdropping in which the attacker may
   reconstruct the voice conversation or sensitive information (e.g.
   PIN numbers from DTMF tones).  SRTP and ZRTP are vulnerable to bid-
   down attacks, i.e. by selectively dropping key exchange protocol
   packets may result in the establishment of a non-secure
   communications.

2.4.2.  Threats to MF Integrity

   Both RTP and RTCP are vulnerable to integrity violation in many ways:

   o  Media Hijack - if an attacker can somehow detect an ongoing media
      session and eavesdrop a few RTP packets, he can start sending
      bogus RTP packets to one of the UEs involved using the same codec.



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      As illustrated in Fig. 8, if the bogus RTP packets have
      consistently greater timestamps and sequence numbers (but within
      the acceptable range) than the legitimate RTP packets, the
      recipient UE may accept the bogus RTP packets and discard the
      legitimate ones.

   o  Media Session Teardown - the attacker sends bogus RTCP BYE
      messages to a target UE signaling to tear down the media
      communication, please note that RTCP messages are normally not
      authenticated.

   o  QoS degradation - the attacker sends wrong RTCP reports
      advertising more packet loss or more jitter than actually
      experimented resulting in the usage of a poor quality codec
      degrading the overall quality of the call experience.

2.4.3.  Threats to MF Availability

   o  Malformed messages - the attacker tries to cause a crash or a
      reboot of the DBE/UE by sending RTP/RTCP malformed messages;

   o  Messages flooding - the attacker tries to exhaust the resources of
      the DBE/UE by sending many RTP/RTCP messages.




























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3.  Security Requirements

   The security requirements for SPEERMINT have been moved from an
   earlier version of this draft to the SPEERMINT requirements draft
   [I-D.ietf-speermint-requirements].  These security requirements are
   the following [I-D.ietf-speermint-requirements]:

   o  Requirement #15: The protocols used to query the Lookup and
      Location Routing Functions MUST support mutual authentication.

   o  Requirement #16: The protocols used to query the Lookup and
      Location Routing Functions MUST provide support for data
      confidentiality and integrity.

   o  Requirement #17: The protocols used to enable session peering MUST
      NOT interfere with the exchanges of media security attributes in
      SDP.  Media attribute lines that are not understood by SBEs MUST
      be ignored and passed along the signaling path untouched.

   The security requirements are currently being finalized and this
   creates a dependency for this draft.  As soon as they will be mature
   and stable enough this section will provide a mapping of concrete
   solutions and protocols to meet them.




























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4.  Suggested Countermeasures

   This section describes implementer-specific countermeasures against
   the threats described in the previous section to supplement the
   security requirements described in [I-D.ietf-speermint-requirements].

      The following table provides a map of the relationships between
      threats and countermeasures.  The suggested countermeasures are
            discussed in detail in the subsequent subsections.

   +-------+---------------+-------------------------------------------+
   | Group | Threat        | Suggested Countermeasure                  |
   +-------+---------------+-------------------------------------------+
   |  LUF  | Unauthorized  | database security BCPs (Section 4.1)      |
   |       | access        |                                           |
   |       |               |                                           |
   |       | SQL injection | database security BCPs                    |
   |       |               |                                           |
   |       | DoS to LUF    | database security BCPs                    |
   |       |               |                                           |
   |       |               |                                           |
   |  LRF  | URI           | DNSSEC (Section 4.2)                      |
   |       | harvesting    |                                           |
   |       |               |                                           |
   |       | SIP equipment | DNSSEC, privacy protection (Section 4.4)  |
   |       | enumeration   |                                           |
   |       |               |                                           |
   |       | MitM attack   | DNSSEC                                    |
   |       |               |                                           |
   |       | Incorrect     | DNSSEC                                    |
   |       | destinations  |                                           |
   |       |               |                                           |
   |       | DoS to LRF    | DNS replication (Section 4.3)             |
   |       |               |                                           |
   |       |               |                                           |
   |   SF  | Call pattern  | Encyrption and Integrity Protection of    |
   |       | analysis      | Signalling Messages (Section 4.13),       |
   |       |               | Minimization of Session Establishment     |
   |       |               | Data (Section 4.12)                       |
   |       |               |                                           |
   |       | Password      | Encyrption and Integrity Protection of    |
   |       | cracking      | Signalling Messages, Minimization of      |
   |       |               | Session Establishment Data                |
   |       |               |                                           |
   |       | Network       | Minimization of Session Establishment     |
   |       | discovery     | Data, Topology Hiding (Section 4.10)      |
   |       |               |                                           |




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   |       | Session       | Encyrption and Integrity Protection of    |
   |       | teardown      | Signalling Messages, TCP (Section 4.5),   |
   |       |               | ingress filtering (Section 4.6)           |
   |       |               |                                           |
   |       | Billing fraud | strong identity assertion (Section 4.7)   |
   |       |               |                                           |
   |       | User ID       | strong identity assertion (Section 4.7)   |
   |       | spoofing      |                                           |
   |       |               |                                           |
   |       | Forged 200    | Encyrption and Integrity Protection of    |
   |       | Response      | Signalling Messages, TCP, ingress         |
   |       |               | filtering                                 |
   |       |               |                                           |
   |       | Forged 302    | Encyrption and Integrity Protection of    |
   |       | Response      | Signalling Messages, TCP, ingress         |
   |       |               | filtering                                 |
   |       |               |                                           |
   |       | Forged 404    | Encyrption and Integrity Protection of    |
   |       | Response      | Signalling Messages, TCP, ingress         |
   |       |               | filtering                                 |
   |       |               |                                           |
   |       | Flooding      | reliable border element pooling           |
   |       | attack        | (Section 4.8), rate limit (Section 4.9)   |
   |       |               |                                           |
   |       | Session black | DNSSEC                                    |
   |       | holing        |                                           |
   |       |               |                                           |
   |       | SIP fuzzing   | border element hardening (Section 4.11)   |
   |       | attack        |                                           |
   |       |               |                                           |
   |       |               |                                           |
   |   MF  | Eavesdropping | Encyrption and Integrity Protection of    |
   |       |               | Media Stream (Section 4.14)               |
   |       |               |                                           |
   |       | Media hijack  | Encyrption and Integrity Protection of    |
   |       |               | Media Stream                              |
   |       |               |                                           |
   |       | Media session | Encyrption and Integrity Protection of    |
   |       | teardown      | Media Stream                              |
   |       |               |                                           |
   |       | QoS           | Encyrption and Integrity Protection of    |
   |       | degradation   | Media Stream                              |
   |       |               |                                           |
   |       | Malformed     | border element hardening                  |
   |       | messages      |                                           |
   |       |               |                                           |





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   |       | Message       | rate limit                                |
   |       | flooding      |                                           |
   +-------+---------------+-------------------------------------------+

4.1.  Database Security BCPs

   Adequate security measures must be applied to the LUF to prevent it
   from being a target of attacks often seen on common database systems.
   Common security Best Current Practices (BCPs) for database systems
   include the use of strong passwords to prevent unauthorized access,
   parameterized statements to prevent SQL injections and server
   replication to prevent any database from being a single point of
   failure. [refs.dbsec] is one of many existing literatures that
   describe BCPs in this area.

4.2.  DNSSEC

   If DNS is used by the LRF, it is recommended to deploy the recent
   version of Domain Name System Security Extensions (informally called
   "DNSSEC-bis") defined by [RFC4033][RFC4034][RFC4035] to enhance the
   security of DNS data using strong cryptography.  DNSSEC provides
   authentication to defend against URI harvesting, SIP equipment
   enumeration, as well as integrity checking to defend against MitM
   attacks, session blackholing and other attacks that lead traffic to
   incorrect destinations.

4.3.  DNS Replication

   DNS replication is a very important countermeasure to mitigate DoS
   attacks on LRF.  Simultaneously bringing down multiple DNS servers
   that support LRF is much more challenging than attacking a sole DNS
   server (single point of failure).

4.4.  Cross-Domain Privacy Protection

   Stripping Via and Record-Route headers, replacing the Contact header,
   and even changing Call-IDs are the mechanisms described in [RFC3323]
   to protect SIP privacy.  This practice allows an SSP to hide its SIP
   network topology, prevents intermediate signaling equipment from
   becoming the target of DoS attacks, as well as protects the privacy
   of UEs according to their preferences.  This practice is effective in
   preventing SIP equipment enumeration that exploits LRF.

4.5.  Use TCP instead of UDP to deliver SIP messages

   SIP clients need to stay connected with the server on a persistent
   basis (differently from HTTP clients).  Scalability requirements are
   therefore much more stringent for a SIP server than for a web server.



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   This leads to the choice of UDP as protocol used between SSPs to
   carry SIP messages (especially for providers with a large user
   community).  New improvements in the Linux kernel
   [refs.tcp-scalability] show a big increase of the scalability of TCP
   in handling large number of persistent (but idle) connections.
   Therefore SSP operators still using UDP for their SIP network should
   consider switching to TCP.  This would significantly increase the
   difficulty of performing session teardown and forging responses (200,
   302, 404 etc).  Since look-up and SED data should be exchanged
   securely (see security requirements), it is further recommended to
   not only use TCP but TLS for messages exchanged between SSPs.

4.6.  Ingress Filtering / Reverse-Path Filtering

   Ingress filtering, i.e., blocking all traffic coming from a host that
   has a source address different than the addresses that have been
   assigned to that host (see [RFC2827]) can effectively prevent UEs
   from sending packets with a spoofed source IP address.  This can be
   achieved by reverse-path filtering, i.e., only accepting ingress
   traffic if responses would take the same path.  This practice is
   effective in preventing session teardown and forged SIP replies (200,
   302, 404 etc), if the recipient correctly verifies the source IP
   address for the authenticity of each incoming SIP message.

4.7.  Strong Identity Assertion

   "Caller ID spoofing" can be achieved thanks to the weak identity
   assertion on the From URI of an INVITE request.  In a single SSP
   domain, strong identity assertion can be easily achieved by
   authenticating each INVITE request.  However, in the context of
   SPEERMINT, only the originating SSP is able to verify the identity
   directly.  In order to overcome this problem there are currently only
   two major approaches: transitive trust and cryptographic signature.
   The transitive trust approach builds a chain of trust among different
   SSP domains.  One example of this approach is a combined mechanism
   specified in [RFC3324] and [RFC3325].  Using this approach in a
   transit peering network scenario, the terminating SSP must establish
   a trust relationship with all SSP domains on the path, which can be
   seen as an underlying weakness.  The use of cryptographic signatures
   is an alternative approach.  "SIP Authenticated Identity Body (AIB)"
   is specified in [RFC3893].  [RFC4474] introduces two new header
   fields IDENTITY and IDENTITY-INFO that allow a SIP server in the
   originating SSP to digitally sign an INVITE request after
   authenticating the sending UE.  The terminating SSP can verify if the
   INVITE request is signed by a trusted SSP domain.  Although this
   approach does not require the terminating SSP to establish a trust
   relationship with all transit SSPs on the path, a PKI infrastructure
   is assumed to be in place.



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4.8.  Reliable Border Element Pooling

   It is advisable to implement reliable pooling on border elements.  An
   architecture and protocols for the management of server pools
   supporting mission-critical applications are addressed in the
   RSERPOOL WG.  Using this mechanism (see [RFC3237] for requirements),
   a UE can effectively increase its capacity in handling flooding
   attacks.

4.9.  Rate limit

   Flooding attacks on SF and MF can also be mitigated by limiting the
   rate of incoming traffic through policing or queuing.  In this way
   legitimate clients can be denied of the service since their traffic
   may be discarded.  Rate limiting can also be applied on a
   per-source-IP basis under the assumption that the source IP of each
   attack packet is not spoofed dynamically and will all the limitations
   related to NAT and mobility issues.  It may be preferable to limit
   the number of concurrent 'sessions', i.e., ongoing calls instead of
   the messaging associated with it (since session use more resources on
   backend-systems).  When calculating rate limits all entities along
   the session path should be taken into account.  SIP entities on the
   receiving end of a call may be the limiting factor (e.g., the number
   of ISDN channels on PSTN gateways) rather than the ingress limiting
   device.

4.10.  Topology Hiding

   Topology hiding applies to both the signaling and media plane and
   consists of limiting the amount of topology information exposed to
   peering partners.  Topology hiding requires B2BUA functionality.  The
   most common way is the use of a Session Border Controller (SBC) as
   SBE.  Topology hiding is explained in [refs.sbcfuncs]

4.11.  Border Element Hardening

   To prevent attacks which exploit vulnerabilities (such as buffer
   overflows, format string vulnerabilities, etc.) in SPEERMINT border
   elements these implementations should be security hardened.  For
   instance, fuzz testing is a common black box testing technique used
   in software engineering.  Also, security vulnerability tests can be
   carried out preventively to assure a UE/SBE/DBE can handle unexpected
   data correctly without crashing.  [RFC4475] and [refs.protos] are
   examples of torture test cases specific for SIP devices and freely
   available security testing tools, respectively.  These type of tests
   needs to be carried out before product release and in addition
   throughout the product life cycle.




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4.12.  Minimization of Session Establishment Data

   In order to give attackers as few chances as possible for
   eavesdropping, session hijacking, and other attacks, SSPs should try
   to minimize session establishment data.  Unneccesary data exchange
   also increases the risk of an implementation vulnerability that could
   be exploited by attackers.  In addition. unnecessary data exchange
   among SSPs can increase the risk of call patterns analysis or
   discovery of some SSPs interior topology.

4.13.  Encyrption and Integrity Protection of Signalling Messages

   Encryption of signalling messages can be achieved with TLS or IPSec.
   Similar to strong identity assertion, a PKI infrastructure is assumed
   to be in place for TLS (or IPSec) deployment so that SSPs can obtain
   and trust the keys necessary to decrypt messages and verify
   signatures sent by other SSPs.

4.14.  Encyrption and Integrity Protection of Media Stream

   The Secure Real-time Transport Protocol (SRTP) [RFC3711] adds
   security features to plain RTP by mainly providing encryption using
   AES to prevent eavesdropping.  It also uses HMAC-SHA1 and index
   keeping to enable message authentication/integrity and replay
   protection required to prevent media hijack attacks.  Secure RTCP
   (SRTCP) provides the same security-related features to RTCP as SRTP
   does for RTP.  SRTCP is described in [RFC3711] as optional.  In order
   to prevent media session teardown, it is recommended to turn this
   feature on.






















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5.  Current Deployment of Countermeasures

   At the time of writing this document not all suggested
   countermeasures are widely deployed.  In particular, the following
   measures to prevent attacks suggested in section Section 4 have not
   seen wide deployment:

   o  DNSSEC

   Nevertheless, these protocols and solutions can provide effective
   means for preventing some of the attacks with respect to the
   SPEERMINT architecture described in this document.  It is envisioned
   that these countermeasures will be more widely deployed in the
   future.  Therefore, these mechanisms are listed in this document even
   though they are not widely deployed today.




































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6.  Conclusions

   This memo presented the different SPEERMINT security threats
   classified in groups related to the LUF, LRF, SF and MF respectively.
   The multiple instances of the threats are presented with a brief
   explanation.  Afterwards the suggested countermeasures for SPEERMINT
   were outlined together with possible mitigation of the existing
   threats by means of them.











































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7.  Security Considerations

   This memo is entirely focused on the security threats for SPEERMINT.
















































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8.  Acknowledgements

   This memo takes inspiration from VOIPSA VoIP Security and Privacy
   Threat Taxonomy.  The authors would like to thank VOIPSA for having
   produced such a comprehensive taxonomy which is the starting point of
   this draft.  The authors would also like to thank Cullen Jennings for
   the useful slides presented at the VoIP Management and Security
   workshop in Vancouver, and for his comments on previous editions of
   this draft on the mailing list.










































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9.  Informative References

   [refs.voipsataxonomy]
              Zar, J. and et al, "VOIPSA VoIP Security and Privacy
              Threat Taxonomy", October 2005.

   [refs.speermintarch]
              Uzelac, A., "SPEERMINT Peering Architecture",
              draft-ietf-speermint-architecture-08.txt (work in
              progress), March 2009.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC5486]  Malas, D. and D. Meyer, "Session Peering for Multimedia
              Interconnect (SPEERMINT) Terminology", RFC 5486,
              March 2009.

   [I-D.ietf-speermint-requirements]
              Mule, J., "SPEERMINT Requirements for SIP-based Session
              Peering", draft-ietf-speermint-requirements-07 (work in
              progress), October 2008.

   [refs.dbsec]
              Gertz, M. and S. Jajodia, "Handbook of Database Security".

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.

   [refs.tcp-scalability]
              Shemyak, K., "Scalability of TCP Servers, Handling



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              Persistent Connections".

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", RFC 3324, November 2002.

   [RFC3325]  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.

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              September 2004.

   [RFC3237]  Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L.,
              Loughney, J., and M. Stillman, "Requirements for Reliable
              Server Pooling", RFC 3237, January 2002.

   [refs.protos]
              Wieser, C., "SIP Robustness Testing for Large-Scale Use".

   [RFC4475]  Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
              and H. Schulzrinne, "Session Initiation Protocol (SIP)
              Torture Test Messages", RFC 4475, May 2006.

   [refs.sbcfuncs]
              Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
              A., and M. Bhatia, "Requirements from SIP (Session
              Initiation Protocol) Session Border Control Deployments",
              draft-ietf-sipping-sbc-funcs-08.txt (work in progress),
              January 2009.
















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Authors' Addresses

   Saverio Niccolini
   NEC Laboratories Europe, NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 (0) 6221 4342 118
   Email: saverio.niccolini@nw.neclab.eu
   URI:   http://www.nw.neclab.eu


   Eric Chen
   Information Sharing Platform Laboratories, NTT
   3-9-11 Midori-cho
   Musashino, Tokyo  180-8585
   Japan

   Email: eric.chen@lab.ntt.co.jp
   URI:   http://www.ntt.co.jp/index_e.html


   Jan Seedorf
   NEC Laboratories Europe, NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 (0) 6221 4342 221
   Email: jan.seedorf@nw.neclab.eu
   URI:   http://www.nw.neclab.eu


   Hendrik Scholz
   freenet Cityline GmbH
   Am Germaniahafen 1-7
   Kiel  24143
   Germany

   Phone: +49 (0) 431 9020 552
   Email: hendrik.scholz@freenet.ag
   URI:   http://freenet.ag








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