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Internet Engineering Task Force (IETF)                        J. SeedorfRequest for Comments: 6404                                  S. NiccoliniCategory: Informational                                              NECISSN: 2070-1721                                                  E. Chen                                                                     NTT                                                               H. Scholz                                                              VOIPFUTURE                                                           November 2011Session PEERing for Multimedia INTerconnect (SPEERMINT)Security Threats and Suggested CountermeasuresAbstract   The Session PEERing for Multimedia INTerconnect (SPEERMINT) working   group (WG) provides a peering framework that leverages the building   blocks of existing IETF-defined protocols such as SIP and ENUM for   the interconnection between SIP Service Providers (SSPs).  The   objective of this document is to identify and enumerate SPEERMINT-   specific threat vectors and to give guidance for implementers on   selecting appropriate countermeasures.  Security requirements for   SPEERMINT that have been derived from the threats detailed in this   document can be found inRFC 6271; this document provides concrete   countermeasures to meet those SPEERMINT security requirements.  In   this document, the different security threats related to SPEERMINT   are classified into threats to the Lookup Function (LUF), the   Location Routing Function (LRF), the Signaling Function (SF), and the   Media Function (MF) of a specific SIP Service Provider.  Various   instances of the threats are briefly introduced inside the   classification.  Finally, existing security solutions for SIP and   RTP/RTCP (Real-time Transport Control Protocol) are presented to   describe countermeasures currently available for such threats.  Each   SSP may have connections to one or more remote SSPs through peering   or transit contracts.  A potentially compromised remote SSP that   attacks other SSPs is out of the scope of this document; this   document focuses on attacks on an SSP from outside the trust domain   such an SSP may have with other SSPs.Seedorf, et al.               Informational                     [Page 1]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6404.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Seedorf, et al.               Informational                     [Page 2]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011Table of Contents1. Introduction ....................................................42. Security Threats Relevant to SPEERMINT ..........................52.1. Threats to the Lookup Function (LUF) .......................52.1.1. Threats to LUF Confidentiality ......................52.1.2. Threats to LUF Integrity ............................62.1.3. Threats to LUF Availability .........................62.2. Threats to the Location Routing Function (LRF) .............62.2.1. Threats to LRF Confidentiality ......................62.2.2. Threats to LRF Integrity ............................72.2.3. Threats to LRF Availability .........................72.3. Threats to the Signaling Function (SF) .....................72.3.1. Threats to SF Confidentiality .......................72.3.2. Threats to SF Integrity .............................82.3.3. Threats to SF Availability .........................102.4. Threats to the Media Function (MF) ........................102.4.1. Threats to MF Confidentiality ......................102.4.2. Threats to MF Integrity ............................102.4.3. Threats to MF Availability .........................113. Security Requirements ..........................................11      3.1. Security Requirements from SPEERMINT Requirements           Document ..................................................113.2. How to Fulfill the Security Requirements for SPEERMINT ....114. Suggested Countermeasures ......................................124.1. Database Security BCPs ....................................144.2. DNSSEC ....................................................144.3. DNS Replication ...........................................154.4. Cross-Domain Privacy Protection ...........................154.5. Secure Exchange of SIP Messages ...........................154.6. Ingress Filtering / Reverse-Path Filtering ................164.7. Strong Identity Assertion .................................164.8. Reliable Border Element Pooling ...........................174.9. Rate limit ................................................174.10. Topology Hiding ..........................................174.11. Border Element Hardening .................................174.12. Securing Session Establishment Data ......................184.13. Encryption and Integrity Protection of Media Stream ......185. Conclusions ....................................................186. Security Considerations ........................................187. Acknowledgements ...............................................198. Informative References .........................................19Seedorf, et al.               Informational                     [Page 3]

RFC 6404          SPEERMINT Threats and Countermeasures    November 20111.  Introduction   With Voice over IP (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 a compromise to 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., dual-tone multi-frequency (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, Denial-of-Service (DoS) attacks   [RFC4732], etc.).  Therefore, there is a need to investigate the   different security threats, to extract security-related requirements,   and to highlight potential solutions on how to protect against such   threats.   The Session PEERing for Multimedia INTerconnect (SPEERMINT) working   group provides a peering framework that leverages the building blocks   of existing IETF-defined protocols such as SIP and ENUM for the   interconnection between SIP servers [RFC5486].  The objective of this   document is to identify and enumerate SPEERMINT-specific threat   vectors and to give guidance for implementers on selecting   appropriate countermeasures.  Security requirements for SPEERMINT can   be found inRFC 6271 "Requirements for SIP-Based Session Peering"   [RFC6271].  These security requirements for SPEERMINT are derived   from the threats that are detailed in this document; they have been   moved from an earlier version of this document to the SPEERMINT   requirements document [RFC6271].  In addition to being the base for   those security requirements, this document provides to implementers   advice and examples for concrete countermeasures on how to meet these   security requirements for SPEERMINT with technical means.  The   SPEERMINT terminology outlined in [RFC5486] is used throughout this   document.   In this document, the different security threats related to SPEERMINT   are classified into threats to the Lookup Function (LUF), the   Location Routing Function (LRF), the Signaling Function (SF), and the   Media Function (MF) of a specific SIP Service Provider (SSP).   Various instances of the threats are briefly introduced inside theSeedorf, et al.               Informational                     [Page 4]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   classification.  Finally, existing security solutions for SIP and   RTP/RTCP are presented to describe countermeasures currently   available for such threats.  Each SSP may have connections to one or   more remote SSPs through peering or transit contracts.  A potentially   compromised remote SSP that attacks other SSPs is out of the scope of   this document; this document focuses on attacks on an SSP from   outside the trust domain such an SSP may have with other SSPs.2.  Security Threats Relevant to SPEERMINT   This section enumerates potential security threats relevant to   SPEERMINT.  A taxonomy of VoIP security threats is defined in   [VOIPSATAXONOMY].  This taxonomy is comprehensive and also takes into   account non-VoIP-specific threats (e.g., loss of power, etc.).   Threats relevant to the boundaries of Layer 5 SIP networks are   extracted from this taxonomy and mapped to the functions of the   SPEERMINT architecture as defined in [RFC6406].  Moreover, additional   threats for the SPEERMINT architecture are listed and detailed under   the same classification of SPEERMINT functions and according to the   CIA (Confidentiality, Integrity, and Availability) triad:   o  Lookup Function (LUF);   o  Location Routing Function (LRF);   o  Signaling Function (SF);   o  Media Function (MF).2.1.  Threats to the Lookup Function (LUF)   For a given request, the LUF provides a mechanism to determine 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 third party to provide the LUF functionality, confidentiality,   integrity, and authenticity of the responses are at risk.2.1.1.  Threats to LUF Confidentiality   For a given request, the Lookup Function (LUF) determines the target   domain to which the request should be routed.  The following attacks   are relevant with respect to eavesdropping on LUF messages:Seedorf, et al.               Informational                     [Page 5]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   o  SIP URI and peering domain harvesting - an attacker can exploit      this weakness if the underlying database has a weak authentication      system or if SIP messages are sent unencrypted, and then use the      gained knowledge to launch other kinds of attacks.   o  Third-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.2.1.2.  Threats to LUF Integrity   The underlying database or LUF messages could be vulnerable to input/   output message modification attacks:   o  Injection attack - an attacker could manipulate statements      performed on the database LUF messages sent to a third party.  A      specific version of this attack is known as "SQL injection".  An      SQL injection is a code insertion into the LUF due to incorrect      input validation.2.1.3.  Threats to LUF Availability   The underlying database or third party LUF service could be   vulnerable to:   o  Denial-of-Service attacks - For example, an attacker makes      incomplete requests causing the server to create an idle state for      each of them, which causes memory to be exhausted.2.2.  Threats 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   Similar to the LUF, the following attacks are related to   eavesdropping on LRF messages:   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.Seedorf, et al.               Informational                     [Page 6]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   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 modify messages, e.g., by feeding bogus information   to the LRF, if the routing data is not correctly validated or sent   unencrypted.  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 attacks - the attacker inserts or has already      inserted an unauthorized node in the signaling path modifying the      SED.  The result is that the attacker is then able to read,      insert, and modify the multimedia communications.   o  Incorrect destinations - the attacker redirects the calls to an      incorrect destination with the purpose of establishing fraud      communications like voice phishing or DoS attacks.2.2.3.  Threats to LRF Availability   The LRF can be the object of DoS attacks.  DoS attacks to the LRF can   be carried out by sending a large number of queries to the LRF or   LUF, 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)   The Signaling Function involves a great number of sensitive   information.  Through the Signaling Function, User Agents (UAs)   assert identities and operators authorize billable resources.   Correct and trusted operation 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:Seedorf, et al.               Informational                     [Page 7]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   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 Digest 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 a 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.2.3.2.1.  SIP Request Spoofing   Most SIP request spoofing attacks first require SIP message   eavesdropping.  However, some of these attacks can be also performed   by estimating certain fields in SIP headers (e.g., by exploiting the   fact that weak implementations may generate predictable SIP Dialog   parameters) or exploiting broken implementations that do not properly   verify the content of certain headers.  Threats in this category are   as follows:   o  session teardown - an attacker can send CANCEL/BYE messages in      order to tear down an existing call at the SIP layer; for such an      attack, the attacker either needs to know (e.g., by eavesdropping      a SIP INVITE message) the SIP Dialog of the call to be hijacked      (To-tag, From-tag, Call-ID) or alternatively may rely on SIP      implementations that do not properly authenticate requests based      on the SIP Dialog;   o  Billing fraud - the attacker can modify and replay an intercepted      INVITE request in order to bill a call to a victim UE and avoid      paying for the phone call;   o  User ID spoofing - SSPs are responsible for asserting the      legitimacy of a user ID; if an SSP fails to achieve the level of      identity assertion that the federation to which it belongs      expects, it may create an entry point for attackers to conduct      user ID spoofing attacks;Seedorf, et al.               Informational                     [Page 8]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   o  Unwanted requests - the attacker sends requests to interfere with      regular operation, e.g., by sending a REGISTER request in order to      hijack calls.  The SPEERMINT architecture as defined in [RFC6406]      does not require registrations between the Signaling Functions      (SFs) of the connected SSPs.  Hence, superfluous requests like      REGISTERs should be rejected.2.3.2.2.  SIP Reply Spoofing   Threats in this category are as follows:   o  Forged 199 Response - the attacker sends a forged 199 response to      terminate an early dialog.  The forged response will not terminate      the entire session but may alter the direction of the session;   o  Forged 200 Response - having seen the contents of an INVITE      request, an eavesdropper can inject a 200 response, affecting the      processing of the transaction of all proxies between the injection      point and the originating UA and at the originating UA itself.  In      the extreme case, this can result in a hijacked call.  In many      cases, however, such an attack will leave signaling artifacts that      may allow it to be detected (e.g., the element receiving the      forged 200 response may also receive other SIP reply messages from      the actual terminating UE);   o  Forged 302 Response - having seen the contents of an INVITE      request, an eavesdropper could also inject a forged "302 Moved      Temporarily" reply, affecting the processing of the transaction at      intermediate entities and the originating UA.  This may allow the      attacker to successfully redirect the call to any destination UE      of his choosing;   o  Forged 404 Response - having seen the contents of an INVITE      request, an eavesdropper could also inject a forged "404 Not      Found" reply, affecting the processing of the transaction at      intermediate entities and the originating UA.  Such an attack may      result in disrupting the call establishment.2.3.2.3.  SIP Message Tampering   This threat involves the alteration of important field values in a   SIP message or in the Session Description Protocol (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 a UE negotiate a poor quality   codec.Seedorf, et al.               Informational                     [Page 9]

RFC 6404          SPEERMINT Threats and Countermeasures    November 20112.3.3.  Threats to SF Availability   o  Flooding attack - a Signaling Path Border Element (SBE) is      susceptible to message flooding attacks that may come from      interconnected SSPs;   o  Session blackholing - 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 a 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 the media function, the UE can establish secure   communications and monitor the 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.,   PINs from DTMF tones).  Some SRTP key exchange mechanisms (e.g.,   [RFC4568]) are vulnerable to bid-down attacks, where an attacker   selectively changes key exchange protocol fields in order to enforce   the establishment of a less secure or even non-secure communication.2.4.2.  Threats to MF Integrity   Both RTP and RTCP are vulnerable to integrity violation in many ways:   o  Media injection - 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.  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.Seedorf, et al.               Informational                    [Page 10]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   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  Quality-of-Service (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 Data Path Border Element (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.3.  Security Requirements3.1.  Security Requirements from SPEERMINT Requirements Document   The security requirements for SPEERMINT have been moved from an   earlier version of this document to the SPEERMINT requirements   [RFC6271].  The security requirements for SPEERMINT are the   following, from [RFC6271]:   o  Requirement #15: The protocols used to query the Lookup and      Location Routing Functions SHOULD support mutual authentication.   o  Requirement #16: The protocols used to query the Lookup and      Location Routing Functions SHOULD 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.3.2.  How to Fulfill the Security Requirements for SPEERMINT   Requirements #15 and #16 state that the LUF and LRF should support   mutual authentication, data confidentiality, and integrity.  In   principle, these requirements can be fulfilled technically with   Transport Layer Security (TLS) or Datagram TLS (DTLS) [RFC5246]   [RFC4347] or IP layer security (IPsec) [RFC4301].  From a pureSeedorf, et al.               Informational                    [Page 11]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   security perspective both solutions fulfill the security requirements   for SPEERMINT, just on a different layer, and both solutions are   widely deployed.   However, from a more practical perspective, transport layer security   (i.e., TLS or DTLS) has the advantage that the application using it   is aware of whether or not security (or rather the corresponding   security features) is enabled.  For instance, using TLS has the   consequence that the connection fails if the corresponding connection   endpoint cannot authenticate properly.   While IPsec fulfills the same requirements from a security   perspective, IPsec is somewhat de-coupling security from the   application using it.  For instance, IPsec is often provided by   dedicated entities in such a way that from the application layer, it   cannot be recognized whether or not IPsec or certain security   features are turned on ("bump-in-the-wire").   In summary, TLS (or DTLS) has some notable advantages over IPsec for   addressing the SPEERMINT security requirements.  In particular,   transport layer security is preferable over IPsec for SPEERMINT   because with TLS (or DTLS) security is more closely coupled to the   LUF or LRF.  From a mere technical perspective, however, both   solutions (transport layer security or IPsec) fulfill the SPEERMINT   security requirements, and there may be particular cases where IPsec   is a preferable solution.4.  Suggested Countermeasures   This section describes implementer-specific countermeasures against   the threats described in the previous sections and for addressing the   SPEERMINT security requirements described in [RFC6271].  The   countermeasures listed in this section are not meant to be   exhaustive; rather, the suggested countermeasures are aimed to serve   as starting points and to give guidance for implementers that are   trying to select appropriate countermeasures against certain threats.   The following table provides a map of the relationships between   threats and countermeasures.  The suggested countermeasures are   discussed in detail in the subsequent subsections.Seedorf, et al.               Informational                    [Page 12]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   +-------+---------------+-------------------------------------------+   | Group | Threat        | Suggested Countermeasure                  |   +-------+---------------+-------------------------------------------+   |  LUF  | Unauthorized  | database security BCPs (Section 4.1),     |   |       | access        | Secure Exchange of SIP messages           |   |       |               | (Section 4.5)                             |   |       | SQL injection | database security BCPs (Section 4.1),     |   |       |               | Secure Exchange of SIP messages           |   |       |               | (Section 4.5)                             |   |       | DoS to LUF    | database security BCPs (Section 4.1),     |   |       |               | Secure Exchange of SIP messages           |   |       |               | (Section 4.5)                             |   |  LRF  | URI           | privacy protection (Section 4.4), Secure  |   |       | harvesting    | Exchange of SIP messages (Section 4.5)    |   |       | SIP equipment | privacy protection (Section 4.4), Secure  |   |       | enumeration   | Exchange of SIP messages (Section 4.5)    |   |       | MitM attack   | DNSSEC (Section 4.2), Secure Exchange of  |   |       |               | SIP messages (Section 4.5)                |   |       | Incorrect     | DNSSEC (Section 4.2), Secure Exchange of  |   |       | destinations  | SIP messages (Section 4.5)                |   |       | DoS to LRF    | DNS replication (Section 4.3)             |   |   SF  | Call pattern  | Secure Exchange of SIP messages           |   |       | analysis      | (Section 4.5), Securing Session           |   |       |               | Establishment Data (Section 4.12)         |   |       | Password      | Secure Exchange of SIP messages           |   |       | cracking      | (Section 4.5)                             |   |       | Network       | Securing Session Establishment Data       |   |       | discovery     | (Section 4.12), Topology Hiding           |   |       |               | (Section 4.10)                            |   |       | Session       | Secure Exchange of SIP messages           |   |       | teardown      | (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    | Secure Exchange of SIP messages           |   |       | Response      | (Section 4.5), ingress filtering          |   |       |               | (Section 4.6)                             |   |       | Forged 302    | Secure Exchange of SIP messages           |   |       | Response      | (Section 4.5), ingress filtering          |   |       |               | (Section 4.6)                             |   |       | Forged 404    | Secure Exchange of SIP messages           |   |       | Response      | (Section 4.5), ingress filtering          |   |       |               | (Section 4.6)                             |   |       | Flooding      | reliable border element pooling           |   |       | attack        | (Section 4.8), rate limit (Section 4.9)   |   |       | Session       | DNSSEC (Section 4.2)                      |   |       | blackholing   |                                           |Seedorf, et al.               Informational                    [Page 13]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   |       | SIP fuzzing   | border element hardening (Section 4.11)   |   |       | attack        |                                           |   |   MF  | Eavesdropping | Encryption and Integrity Protection of    |   |       |               | Media Stream (Section 4.13)               |   |       | Media         | Encryption and Integrity Protection of    |   |       | injection     | Media Stream (Section 4.13)               |   |       | Media session | Encryption and Integrity Protection of    |   |       | teardown      | Media Stream (Section 4.13)               |   |       | QoS           | Encryption and Integrity Protection of    |   |       | degradation   | Media Stream (Section 4.13)               |   |       | Malformed     | border element hardening (Section 4.11)   |   |       | messages      |                                           |   |       | Message       | rate limit (Section 4.9)                  |   |       | 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. [DBSEC] is one of many existing documents 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], and [RFC4035].  DNSSEC   has been designed to protect DNS against well-known attacks such as   DNS cache poisoning or Man-in-the-Middle (MitM) attacks on DNS   queries.  Essentially, DNSSEC is a set of public key cryptography   extensions to DNS that provide authentication of DNS data, integrity   protection for DNS entries, and authenticated denial of existence   regarding non-existing DNS entries.  In the context of SSP peering,   DNSSEC can provide authentication and integrity regarding the   location of a Signaling Function (SF) entity retrieved via DNS.   Using DNSSEC can thus help to defend against MitM attacks on DNS   queries invoked by the LRF, session blackholing and other attacks   that lead traffic to incorrect destinations.   DNSSEC has been deployed at the root level and in several top-level   domains (e.g., .com and .net).  Although, at the time of this   writing, DNSSEC is still not yet widely deployed on the Internet,   even limited deployment can add significant integrity protection andSeedorf, et al.               Informational                    [Page 14]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   authentication to the LRF for Signaling Function locations received   via DNS entries.  Neither end users nor terminals are involved in the   DNS resolution process of the LRF.  Hence, if a) the sending SSP uses   a DNS resolver that supports DNSSEC extensions, b) the receiving SSP   stores the location of its Signaling Function cryptographically   signed (using DNSSEC extensions) in the DNS, and c) the sending SSP   can obtain an authentication chain (i.e., a series of linked DS and   DNSKEY records) to the receiving SSP, the LRF can be secured with   DNSSEC.  In the context of SPEERMINT, all three of these requirements   can be fulfilled even in the case of partial DNSSEC deployment.  In   particular, even without Internet-wide deployment of DNSSEC, it may   be possible for a sending SSP to obtain a suitable trust anchor for   verifying the receiving SSP's public key.  For instance, a suitable   trust anchor could be configured for that specific SSP's top-level   domain or for the particular SSP's domain directly.  If the sending   and the receiving SSP use a common ENUM tree, DNSSEC use with the   ENUM tree's trust anchor is "straightforward".4.3.  DNS Replication   DNS replication is a very important countermeasure to mitigate DoS   attacks on the LRF.  Simultaneously bringing down multiple DNS   servers that support the 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.  Secure Exchange of SIP Messages   SIP can be used on top of UDP or TCP as transport protocol [RFC3261].   However, look-up and SED data should be exchanged securely (see   security requirements (Section 3.2)), e.g., to increase the   difficulty of performing session teardown and forging responses (200,   302, 404, etc).  If UDP is used to carry SIP messages, DTLS should be   used to secure SIP message exchange between SSPs.  If TCP is used as   a transport protocol, it can be secured with TLS.  Therefore,   depending on the underlying transport protocol, SSPs should use   either DTLS or TLS to secure SIP message delivery.Seedorf, et al.               Informational                    [Page 15]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   In general, encryption and integrity protection of signaling messages   can be achieved on the transport layer (with TLS or DTLS) or on the   network layer (with IPsec).  Both solutions are technically sound,   but transport layer security has some advantages.  Please refer to   the subsection on fulfilling the SPEERMINT security requirements   (Section 3.2) for a discussion on using TLS/DTLS or IPsec for   protecting the confidentiality and integrity of signaling messages.   Similar to strong identity assertion, a Public Key Infrastructure   (PKI) is assumed to be in place for TLS/DTLS (or IPsec) deployment so   that SSPs can obtain and trust the keys necessary to decrypt messages   and verify signatures sent by other SSPs.   Message-oriented protection such as [RFC3261] authentication does not   fulfill the SPEERMINT requirements (e.g., mutual authentication).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.  "Session   Initiation Protocol (SIP) Authenticated Identity Body (AIB) Format"   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 theSeedorf, et al.               Informational                    [Page 16]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   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 is assumed to   be in place.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 such mechanisms and protocols (see [RFC5351]   [RFC5352] [RFC5353] for details), a UE can effectively increase its   capacity in handling flooding attacks.4.9.  Rate limit   Flooding attacks on SFs and MFs can also be mitigated by limiting the   rate of incoming traffic through policing or queuing.  In this way,   legitimate clients can be denied 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.  Limitations related to NAT and mobility   issues apply and may result in false positives (i.e., source IP   addresses blocked) when multiple legitimate clients are located   behind the same NAT IP address.  It may be preferable to limit the   number of concurrent 'sessions', i.e., ongoing calls instead of the   messaging associated with it (since sessions 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 back-to-back user agent   (B2BUA) functionality.  The most common way is the use of a Session   Border Controller (SBC) as SBE.  Topology hiding is explained in   [RFC5853].4.11.  Border Element Hardening   To prevent attacks that 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 usedSeedorf, et al.               Informational                    [Page 17]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   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 [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.4.12.  Securing Session Establishment Data   Session Establishment Data (SED) contains critical information for   the routing of SIP sessions.  In order to prevent attacks such as   service hijacking and denial of service that exploit SED, SSPs should   adopt a secure transport protocol that provides authentication,   confidentiality and integrity to exchange SED among themselves.   Further details can be found in [DRINKS-SPPROV].4.13.  Encryption and Integrity Protection of Media Stream   The Secure Real-time Transport Protocol (SRTP) [RFC3711] prevents   eavesdropping on plain RTP by encrypting the data flow.  It uses AES   as the default cipher and defines two modes of operation (Segmented   Integer Counter Mode and f8-mode), which is agreed upon after   negotiation.  It also uses HMAC-SHA1 and index keeping to enable   message authentication/integrity and replay protection required to   prevent media injection 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.  The   choice of the external key management protocol is left to the   deployment, a PKI is necessary to implement the security requirements   of the SPEERMINT requirements document.5.  Conclusions   This document presented the different SPEERMINT security threats   classified in groups related to the LUF, LRF, SF, and MF,   respectively.  The multiple instances of the threats were presented   with a brief explanation.  Finally, suggested countermeasures for   SPEERMINT were outlined together with possible mitigation of the   existing threats by means of them.6.  Security Considerations   This document is entirely focused on the security threats for   SPEERMINT.Seedorf, et al.               Informational                    [Page 18]

RFC 6404          SPEERMINT Threats and Countermeasures    November 20117.  Acknowledgements   This document was originally inspired by the VOIPSA VoIP Security and   Privacy Threat Taxonomy.  The authors would like to thank VOIPSA for   having produced a comprehensive taxonomy as the starting point of   this document.  Additionally, the authors would like to thank Cullen   Jennings, Jon Peterson, David Schwartz, Hadriel Kaplan, Peter Koch,   Daryl Malas, Jason Livingood, and Robert Sparks for useful comments   to previous editions of this document on the mailing list as well as   during IETF meetings.   Jan Seedorf and Saverio Niccolini are partially supported by the   DEMONS project, a research project supported by the European   Commission under its 7th Framework Program (contract no. 257315).   The views and conclusions contained herein are those of the authors   and should not be interpreted as necessarily representing the   official policies or endorsements, either expressed or implied, of   the DEMONS project or the European Commission.8.  Informative References   [DBSEC]    Gertz, M. and S. Jajodia, "Handbook of Database Security:              Applications and Trends",  Springer, 2008.   [DRINKS-SPPROV]              Mule, J., Cartwright, K., Ali, S., and A. Mayrhofer,              "Session Peering Provisioning Protocol", Work in Progress,              September 2011.   [PROTOS]   Wieser, C., Laakso, M., and H. Schulzrinne, "SIP              Robustness Testing for Large-Scale Use",  First              International Workshop on Software Quality (SOQUA 2004),              September 2004.   [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.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and E.              Schooler, "SIP: Session Initiation Protocol",RFC 3261,              June 2002.   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session              Initiation Protocol (SIP)",RFC 3323, November 2002.   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted              Identity",RFC 3324, November 2002.Seedorf, et al.               Informational                    [Page 19]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   [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.   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.              Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC 3711, March 2004.   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)              Authenticated Identity Body (AIB) Format",RFC 3893,              September 2004.   [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.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer              Security",RFC 4347, April 2006.   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for              Authenticated Identity Management in the Session              Initiation Protocol (SIP)",RFC 4474, August 2006.   [RFC4475]  Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,              and H. Schulzrinne, "Session Initiation Protocol (SIP)              Torture Test Messages",RFC 4475, May 2006.   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session              Description Protocol (SDP) Security Descriptions for Media              Streams",RFC 4568, July 2006.   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-              Service Considerations",RFC 4732, December 2006.   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246, August 2008.Seedorf, et al.               Informational                    [Page 20]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011   [RFC5351]  Lei, P., Ong, L., Tuexen, M., and T. Dreibholz, "An              Overview of Reliable Server Pooling Protocols",RFC 5351,              September 2008.   [RFC5352]  Stewart, R., Xie, Q., Stillman, M., and M. Tuexen,              "Aggregate Server Access Protocol (ASAP)",RFC 5352,              September 2008.   [RFC5353]  Xie, Q., Stewart, R., Stillman, M., Tuexen, M., and A.              Silverton, "Endpoint Handlespace Redundancy Protocol              (ENRP)",RFC 5353, September 2008.   [RFC5486]  Malas, D. and D. Meyer, "Session Peering for Multimedia              Interconnect (SPEERMINT) Terminology",RFC 5486,              March 2009.   [RFC5853]  Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,              A., and M. Bhatia, "Requirements from Session Initiation              Protocol (SIP) Session Border Control (SBC) Deployments",RFC 5853, April 2010.   [RFC6271]  Mule, J-F., "Requirements for SIP-Based Session Peering",RFC 6271, June 2011.   [RFC6406]  Malas, D., Ed. and J. Livingood, Ed., "Session PEERing for              Multimedia INTerconnect (SPEERMINT) Architecture",RFC 6406, November 2011.   [VOIPSATAXONOMY]              Zar, J. and et al, "VOIPSA VoIP Security and Privacy              Threat Taxonomy, Public Release 1.0",http://www.voipsa.org/Activities/taxonomy.php,              October 2005.Seedorf, et al.               Informational                    [Page 21]

RFC 6404          SPEERMINT Threats and Countermeasures    November 2011Authors' Addresses   Jan Seedorf   NEC Laboratories Europe, NEC Europe,  Ltd.   Kurfuersten-Anlage 36   Heidelberg  69115   Germany   Phone: +49 (0) 6221 4342 221   EMail: jan.seedorf@neclab.eu   URI:http://www.neclab.eu   Saverio Niccolini   NEC Laboratories Europe, NEC Europe, Ltd.   Kurfuersten-Anlage 36   Heidelberg  69115   Germany   Phone: +49 (0) 6221 4342 118   EMail: saverio.niccolini@.neclab.eu   URI:http://www.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   Hendrik Scholz   VOIPFUTURE GmbH   Wendenstrasse 4   Hamburg  20097   Germany   Phone: +49 (0) 40 688 900 163   EMail: hendrik.scholz@voipfuture.com   URI:http://voipfuture.comSeedorf, et al.               Informational                    [Page 22]